Condensing Boilers Evaluation: 

Retrofit and New Construction 

Applications

Dylan Cutler

Jesse Dean

Jason Acosta

Dennis Jones

June 2014 

Prepared for the U.S. General Services Administration

By the National Renewable Energy Laboratory
The Green Proving Ground program leverages GSA’s real estate portfolio 

to evaluate innovative sustainable building technologies and practices. 

Findings are used to support the development of GSA performance 

specifications and inform decision-making within GSA, other federal 

agencies, and the real estate industry. The program aims to drive 

innovation in environmental performance in federal buildings and help 

lead market transformation through deployment of new technologies.
Condensing Boiler Evaluation – Retrofit and New Construction Page i 

Disclaimer 

This document was prepared as an account of work sponsored by the United States Government. While this 

document is believed to contain correct information, neither the United States Government nor any agency 

thereof, nor the National Renewable Energy Laboratory, nor any of their employees, makes any warranty, 

express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any 

information, apparatus, product, or process disclosed, or represents that its use would not infringe privately 

owned rights. Reference herein to any specific commercial product, process, or service by its trade name, 

trademark, manufacturer, or otherwise, does not constitute or imply its endorsement, recommendation, or 

favoring by the United States Government or any agency thereof, or the U.S. Department of Energy’s National 

Renewable Energy Laboratory. The views and opinions of authors expressed herein do not necessarily state or 

reflect those of the United States Government or any agency thereof or the National Renewable Energy 

Laboratory. 

The work described in this report was funded by the U.S. General Services Administration under Contract No. 

IAG-11-01815 and Task No. WFK91003. 

This report was prepared by NREL, a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Acknowledgements

United States General Services Administration (GSA) Region 8: Silas Campbell, James Shull, Andrew Bond, 

Douglas Rothgeb, and Jessica Higgins

National Renewable Energy Laboratory: Otto VanGeet, Shanti Pless 

Group14 Engineering Inc.: Sue Reilly, Brody Wilson

For more information contact:

Energy and Environment Team

Facilities Management and Services Programs 

Public Buildings Service - Rocky Mountain Region

U.S. General Services Administration

One Denver Federal Center 

P.O. Box 25546 (8P2PM)

Denver, CO 80225-0546

Phone: (303)236 - 8000
Condensing Boiler Evaluation – Retrofit and New Construction Page ii

Table of Contents 

Page 

I. Executive Summary............................................................................................................................ 1

A. Background ................................................................................................................................................ 1

B. What is the Technology?............................................................................................................................ 1

C. Study Design and Objectives...................................................................................................................... 2

D. Project Results/Findings............................................................................................................................. 2

E. Conclusions ................................................................................................................................................ 5

II. Background ........................................................................................................................................ 7

A. Introduction ............................................................................................................................................... 7

B. Opportunity................................................................................................................................................ 7

C. Demonstration Project Locations .............................................................................................................. 8

III. Technology Description and Objectives ............................................................................................ 9

A. Technical Objectives................................................................................................................................... 9

B. Condensing Boiler Technology Description .............................................................................................10

IV. Assessment ...................................................................................................................................... 15

A. Facility Description...................................................................................................................................15

B. Technology Specification .........................................................................................................................21

C. Technology Deployment ..........................................................................................................................22

D. Test Plan...................................................................................................................................................23

E. Instrumentation Plan ...............................................................................................................................23

V. Results.............................................................................................................................................. 24

A. Utility Bill Analysis....................................................................................................................................24

B. Metered Data Analysis.............................................................................................................................29

C. Hot Water Distribution Effects on Boiler Operation................................................................................33

VI. Summary Findings and Conclusions ................................................................................................ 35

A. Overall Technology Assessment at Demonstration Facility.....................................................................35

B. Best Practices...........................................................................................................................................36

C. Barriers and Enablers to Adoption...........................................................................................................45

D. Market Potential within the GSA Portfolio ..............................................................................................46

E. Recommendations for Installation and Commissioning ..........................................................................48

Appendices................................................................................................................................................ 50

A. Utility Regression Analysis .......................................................................................................................50

B. Efficiency Analysis with Metered Data.....................................................................................................53

C. Calculation of Average Firing Rate in Building 25 ....................................................................................56

D. Glossary....................................................................................................................................................56
Condensing Boiler Evaluation – Retrofit and New Construction Page 1 

I. Executive Summary

A. Background 

The U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) evaluated five condensing boiler 

plants on behalf of General Services Administration (GSA) Rocky Mountain Region (Region 8). Condensing boilers 

can increase the efficiency of a building’s steady-state heating efficiency from 6% to 18% over standard boiler 

systems, depending on how the condensing boilers are designed, operated, and the efficiency of the existing 

boilers. GSA Region 8 has jurisdiction, custody, or control over approximately 615 federally owned and leased

buildings throughout the region. Region 8 is generally a heating-dominated climate and the significant efficiency 

improvement available through correctly designed condensing boiler systems could achieve significant savings in 

operating costs and reduce energy consumption in buildings across the region. 

This technology evaluation was carried out through observation, measurement, and verification (OM&V) of the 

performance of five different condensing boiler installations that had been deployed in various GSA buildings at 

the Denver Federal Center (DFC) in Lakewood, Colorado. The buildings evaluated were buildings: 25, 45, 54, 

710a, and 810. The condensing boilers retrofits were completed in the summer of 2010. This report evaluates 

the design characteristics, operational efficiency, and life cycle cost effectiveness of the installations. 

Nationwide, GSA owns and leases over 354 million square feet (ft2

) of space in 9,600 buildingsi. This 

demonstration project was hosted by Region 8, yet it can serve to inform both the region and GSA nationwide 

on the appropriate use of condensing boiler installations. By evaluating the technical and economic feasibility of 

new ideas and technologies, such as energy efficient condensing boiler technology, GSA will be in a better 

position to select appropriate technology and drive innovation in environmental performance across the 

agency’s portfolio. 

GSA is a leader among federal agencies in aggressively pursuing energy efficiency opportunities for its facilities 

and installing renewable energy systems to heat and power these facilities. This is especially true in Region 8. 

The Energy Independence and Security Act of 2007 (EISA) established energy reduction goals for federal 

facilities, and mandated energy intensity reductions of 3% per fiscal year (FY) relative to a 2003 baseline. 

B. What is the Technology?

Condensing boilers achieve higher efficiencies by condensing water vapor contained in flue gases. When natural 

gas is combusted in a boiler, approximately 90% of the energy contained in the fuel is converted into sensible 

heat (heat that causes a change in temperature) and approximately 10% is converted into latent heat (heat 

involved in phase change of a material). This latent heat is stored in the water vapor that is a byproduct of the 

combustion process. 

In traditional boiler technology, the latent heat contained in the water vapor is allowed to escape in the flue 

gases. Condensing boilers reclaim that latent heat by condensing the water vapor and transferring that heat 

back into the return water. This process of condensing the water vapor can increase the steady-state efficiency 

of an average boiler from ≈80% to between 86% and 98% (depending on boiler operation)ii. For existing boilers, 

their operational efficiency will often be much lower than 80% due to pre- and post-purge during cycling and the 

high mass of older boilers. In 2001, the Consortium for Energy Efficiency produced a market analysis of 

condensing boilersiii that notes that condensing boilers achieve the high efficiencies discussed above and have 

high market penetration in Europe due to this, as well as other factors such as increased incentives and higher 

energy costs. 

These potential efficiency gains can only be attained if the boiler is operating in “condensing” mode. Condensing 

mode is achieved when the temperature of the return water is below the dew point of the water vapor in the 

flue gas. For natural gas boilers in Denver’s climate, the dew point is approximately 130°F. The dew point of the 
Condensing Boiler Evaluation – Retrofit and New Construction Page 2 

water vapor in the exhaust is dependent on the altitude, temperature, and type of fuel. Generally, if the return 

water temperature is below approximately 130°F, the water vapor will start to condense and the system will 

achieve a higher efficiency. The lower the return water temperature, the more condensation occurs and higher 

efficiency is achieved. To reduce the return water temperature it is important to consider: 

• The type of coils used in the system – higher delta T (ΔT) coils and coils designed to operate at lower

supply water temperature will result in lower return water temperature. 

• The piping design and valve selection – less bypass and mixing of supply and return water, as well as 

selection of two-way valves instead of three-way valves, will result in lower return water temperature. 

Two way control valves when combined with variable flow pumping will result in lower return water 

temperatures and less pumping energy use.

• Supply water temperature control – lowering the supply water temperature when the outdoor 

temperature is higher (outdoor air reset control) will still meet the building load, yet reduce return 

water temperature. 

Condensing boilers offer the potential to reduce operating costs and save energy through the higher efficiencies 

attainable with the flue gas condensing technology. These savings are only realistically attained through 

appropriate system design and operation. 

C. Study Design and Objectives

This study evaluates the performance of five separate condensing boiler installations at the DFC. The buildings 

evaluated were buildings: 25, 45, 54, 710a, and 810. Each of these buildings received a condensing boiler retrofit 

during the summer of 2010. 

The objective of this demonstration is to: 

• Evaluate the installed performance of the condensing boilers at the five buildings listed above. 

• Identify best practices for condensing boiler installations.

• Develop a methodology for evaluating future designs and installations. 

• Assess the economic performance of condensing boiler technology based on life cycle costing (LCC). 

• Identify the potential to adopt condensing boilers within GSA region 8.

The overall goal of this report is to provide technical information and best practice guidance with regards to

condensing boiler installation and operation. The field demonstrations will provide the necessary data to 

evaluate the technology’s performance and will allow accurate comparison with non-condensing boilers, nearcondensing boilers, and other available technologies.

The main parameters that will be evaluated in this study will be the installed performance of the boilers and the 

cost-effectiveness of the retrofits. The variable that has the largest single impact on the installed performance of 

the boilers is the configuration of the distributed heating system that the boiler is supplying. Therefore, the 

different heating systems in each building will be discussed and evaluated for effective pairing with the 

condensing boiler technology.

D. Project Results/Findings

Analysis of the utility bill data showed that all of the condensing boiler installations are realizing significant 

savings. These savings ranged from 16% to 41% reduction in energy use. This analysis included weather 

normalization to compare pre- and post-retrofit gas usage on equal grounds. The total estimated savings for the 

boiler projects was approximately 10,100 million British thermal units (MMBtus) in 2011. 
Condensing Boiler Evaluation – Retrofit and New Construction Page 3 

The capital costs in this analysis were normalized to provide an economic assessment with typical installation 

costs (providing an average installed cost that was verified against available data sources, see pg. 46). The 

economic analysis was also completed for actual installation costs at the DFC, and can be viewed in the Utility 

Bill Analysis subsection located in the Results section (see pg. 24). The economic analysis demonstrated that all 

five of the projects had positive net present value (NPV) (using a 25-year lifetime) for the case where a boiler 

replacement was necessary. The standard replacement boiler in this analysis was assumed to be an 84% 

efficient, near-condensing boiler that would comply with federal mandates for energy efficient heating systems.

Boiler replacements were necessary for four of the five installations. These four buildings all had existing boilers 

that were more than 20 years old. The American Society of Heating, Refrigerating, and Air Conditioning 

Engineers (ASHRAE) maintains a database on equipment lifetimes and the replacement age for boilers and 

suggests replacement after 18 to 25 years (depending on exact boiler type)iv. Building 810 was the only building 

that made the replacement specifically for improved efficiency. The high initial cost of condensing boilers made 

the technology not cost effective for the one case where a boiler replacement was not deemed necessary. The 

economic analysis for each building is shown in Table 1. 

Table 1 – Calculated simple payback and NPV for the five sites for condensing boilers using normalized 

initial costs

Building

Initial Cost 

Replacement 

Boiler

Initial Cost

Condensing 

Boiler

Energy Cost 

Savings

Simple 

Payback Period 

(Years)

Net Present 

Value

25 $629,084 $697,406 $15,326 4.46 $254,452

45 $183,836 $203,801 $6,361 3.141 $114,001

54 $524,236 $581,171 $9,161 6.21 $136,004

710A $122,557 $135,868 $2,344 5.68 $36,046

810 $629,0842 $697,406 $16,368 42.61 -$352,695 1

The energy savings for building 45 are increased due to a steam to hot water conversion that occurred during the retrofit. 

This cost is not reflected in the normalized initial cost and results in an enhanced economic evaluation for that building.

2

The replacement cost for building 810 is shown here, yet the economics shown are for efficiency upgrade (not an 

incremental cost analysis) due to the fact that building 810’s boilers were not due for replacement.

One of the criteria set out in the objectives section was achieving condensing mode operation for greater than 

20% of the annual operating hours. Analysis of the building automation system (BAS) trend data showed that 

only two of the five boiler plants achieved this goal during the monitored period. Figure 1 shows the percent of 

monitored hours that return water temperature was recorded in 10°F bins. 
Condensing Boiler Evaluation – Retrofit and New Construction Page 4 

Figure 1 – Measured return water temperatures over monitoring period, separated into 10°F bins

The boilers’ efficiencies are a function of the firing rate, as well as the return water temperature. The higher 

return water temperatures received by the boilers led to calculated annual thermal efficiencies ranging from 

87% to 93% depending on the firing rate. When the boiler staging control sequence was taken into account (to 

determine approximate firing rate), it was estimated that those efficiencies were in the 88% to 90% range 

depending on the building.

These efficiencies could be improved if the return water temperature to the boilers was brought down further in 

to the condensing zone (less than 130°

F) for a larger portion of the year. This could be achieved through retrofit 

of some of the hot water distribution systems, specifically the valving, piping, and coil design. 

A cost/benefit analysis for retrofitting the hot water system in building 25 was conducted and demonstrated 

simple payback periods ranging from 11 to 33 years, depending on the actual return water temperature 

achieved by the retrofit. The analysis of retrofitting the hot water delivery system was only completed for 

building 25 due to the detailed information available on that building. In building 25, one potential solution to 

eliminating the problem of mixing supply and return water, which raises the temperature of the return water,

would be to close the balancing valve on the air handler heating coil bypass leg and turn off the booster pumps. 

If the booster pumps need to operate to overcome the static pressure within the unit then the site can explore 

the option of installing a variable frequency drive (VFD) on each pump and modulating the booster pump flow 

rate based on hot water valve position. The installation costs associated with this estimate include the cost of a 

VFD on each booster pump, as well as the cost of a new control point. 

This is a very building-specific type of analysis that should be investigated in other buildings and in proposed 

installations. Other low-cost or no-cost solutions are more aggressive outdoor air reset controls and valve 

adjustments on bypass piping.
Condensing Boiler Evaluation – Retrofit and New Construction Page 5 

E. Conclusions

Condensing boilers are an effective technology for reducing the energy consumed for space heating if best 

practices are followed during installation. Condensing boilers should first be considered for retrofit in buildings 

that already return water temperatures below 130o

F throughout much of the year. The building automation 

system (BAS) in many buildings can provide this information. In buildings where it is impossible or difficult to 

avoid return water temperatures above 130o

F, high efficiency near-condensing boilers, with thermal efficiencies 

above 84%, may be more cost effective. It is important to note that basic steps, like a more aggressive outdoor 

air temperature reset and balancing of the three-way valves, should be taken into account before a conclusion 

can be made regarding the return water temperature. Comprehensive retrofits, including replacement of threeway valves with two-way valves and installation of VFDs on the pumps, should be considered during the 

evaluation process.

After two way valves have been installed and mixing has been eliminated the site should implement the most 

aggressive outside air reset schedule that they can for the given facility. The facilities analyzed in this study 

could have potentially implemented a more aggressive outside air reset that would have resulted in more hours 

in condensing mode. As an example, the supply water temperature could be reset from 170 °F (at or below 20 

°F outside air temperature) down to 130 °F at or above 60 °F. The exact outdoor air reset formula will be specific 

to climate and building characteristics. In addition to optimizing the outdoor air reset schedule, mild climates 

can operate in condensing mode for a higher percentage of time than more extreme climates given the same 

system design and outside air reset.

The boiler plants installed at the DFC were installed correctly as modular systems with the proper controls to 

ensure that the boilers maintained a low firing rate throughout most of the year. The installation of outdoor air 

reset controls reduced the supply hot water temperature on warmer days, which in turn reduced the return 

water temperature to allow the boilers to operate in condensing mode more often. The installation of a 

condensate treatment system in the boilers at DFC reduces the environmental impact of the acidic condensate 

water. 

Installing a modular condensing boiler plant alone does not guarantee savings beyond what near-condensing 

boilers can achieve. Near-condensing boilers are boilers that attain the highest efficiency possible without

recapturing latent heat loss with condensing operation. Their steady-state efficiencies are between 85% and 

88%. If the return hot water temperature never falls below 130o

F, the threshold for condensing operation, the 

more expensive condensing boilers will only operate as efficiently as near-condensing boilers (typical cost 

premium for condensing boilers range from 10% to 15% more than near-condensing boilers).

Condensing boilers have the potential to reduce the cost and environmental impact of heating GSA’s buildings 

throughout region 8. Typical candidates include, but are not limited to, buildings in moderate climates, buildings 

with low temperature heating systems (such as in-floor radiant heating), and buildings with hot water loops that 

can easily be retrofitted to avoid coil bypasses and three-way valves. Region 8 should consider condensing 

boilers during each boiler replacement, but a building-by-building analysis is necessary to determine the most 

cost effective boiler replacement in each building. 

To ensure the maximum amount of savings is achieved from installation of condensing boilers, a review of the 

hot water distribution system must be undertaken with an aim of providing the lowest possible return water 

temperature to the boilers. Improvements can include the removal of bypasses and three- way valves, along 

with the installation of VFDs on pumps. This will reduce mixing and lower the return water temperature. Further 

improvements can include the installation of high ∆T coils in air handlers and variable air volume (VAV) boxes 
Condensing Boiler Evaluation – Retrofit and New Construction Page 6 

that still can meet heating loads at lower supply water temperatures. The valving measures are generally lower 

cost solutions and should be addressed prior to coil retrofits. The calculated simple payback ranged from 11 to 

33 years for installing VFDs on the small AHU booster pumps and closing the balancing valve in the by-pass loop 

for building 25 (depending on the assumed improvement in return water temperature). For this particular 

building the installed costs would not have been reduced if this was implemented when the boiler plant was 

installed since the modifications need to be made at the AHUs themselves. 

Finally, it should be noted that after the writing of this report, the Federal Energy Management Program (FEMP) 

updated the minimum efficiency requirements associated with natural gas fired boilers to require 94% or 

greater thermal efficiency from these systems.v This essentially requires all federal agencies to install 

condensing boilers for all boiler replacements. This mandate makes the recommendations and best practices 

described in this report even more important; in order to obtain the high efficiency operation that is the 

motivation for such a mandate, it will be crucial to install these systems with the correct controls and hydronic 

configuration to ensure the maximum amount of operating time in condensing mode.
Condensing Boiler Evaluation – Retrofit and New Construction Page 7 

II. Background 

A. Introduction

Heating energy use accounts for 26% of the total energy use in commercial buildings in the United States. It is 

the largest single energy end-use in buildings. Space heating in commercial buildings is provided by boilers in 

32% of commercial buildings, making boilers the most common heating technology in the commercial sector.

vi

Installing condensing boilers will reduce onsite thermal energy use and help GSA move one step closer to 

achieving the energy use intensity reduction requirements that are mandated in EISA. This report will help 

provide guidance on how condensing boilers can contribute to significant reduction in the energy that space 

heating accounts for. 

Condensing boiler technology has the capability of increasing the steady-state efficiency between 6% and 18% 

over traditional boiler technology. Steady-state efficiency is the efficiency of the boiler once it has reached

normal, specified operating temperatures and does not include standby losses. The average gas-fired boiler in 

the United States in 2010 had a rated (steady state) efficiency of 80%.

vii Older boilers can operate at a 

significantly lower efficiency. The efficiency of older equipment depends on its condition, as well as the mixture 

of combustion fuel in the boiler. Actual boiler efficiency can be determined with a combustion analyzer. 

Condensing boilers have a range of efficiencies, depending on the operating conditions, that span from 

approximately 86-98%. If appropriately installed and operated, condensing boiler technology offers GSA the 

ability to significantly reduce space heating energy consumption.

The main barriers to achieving these increased efficiencies are how the boilers are integrated with the hot water 

distribution system and how the boilers are operated. In new construction, condensing boilers are paired with 

hot water distributions systems with a large temperature differential from supply to return. These systems often 

have specific, high ∆T coils (40° to 50°F) that enable the required temperature differential for the boilers to 

operate in condensing mode for large portions of the year. Heated radiant floor, pool heating, and snow melt 

are other systems that allow boilers to operate in condensing mode. It can be difficult to achieve these types of 

operations in a retrofit scenario. 

B. Opportunity

The most common technology used for space heating in domestic commercial buildings are boilers (accounting 

for 32% of all heated floor area), followed by furnaces and packaged units (Table 2). The vast majority of boilers 

are traditional, natural gas fired boilers that generate hot water or steam. These boilers had an average rated 

efficiency of 80%, as of 2010. Condensing boilers have steady-state efficiencies that range from 86% to 98% 

depending on boiler operation. This presents a significant opportunity for GSA to reduce its energy consumption 

and reduce annual utility costs region wide. Due to the heating-dominated climate of Region 8 (which includes 

ASHRAE climate zones 5, 6, and 7), condensing boilers represent an even greater opportunity for this region if 

correctly deployed. Denver is located in climate zone 5. Climate zones 6 and 7 are colder than climate zone 5. 
Condensing Boiler Evaluation – Retrofit and New Construction Page 8 

Table 2 - Type of heating equipment used in US commercial buildings, 2003 (CBECS)* 

Heating Equipment 2003

Boilers 32%

Furnaces 30%

Packaged Heating Units 28%

Individual Space Heaters 19%

Heat Pumps 14%

District Heat 8%

Other 5%

* Heating and cooling equipment percentages of floor space total 

more than 100% because equipment share floor space

Condensing boilers offer the potential to reduce operating costs and save energy through the higher efficiencies 

attainable with the technology. These savings are only realistically attained through appropriate system design 

and operation. 

The technology evaluated at the five buildings that were monitored at the DFC were Cleaver Brooks Clear Fire 

Condensing boilers (CFC 2,500 MBH and CFC 1,500 MBH models). Similar boilers are currently available from 

other manufacturers and have been available in the marketplace for approximately 30 years. The first 

generation condensing boilers were manufactured in the early 1980s. These boilers generally had single-stage 

firing rates and lacked the modulating capabilities that the newer condensing boilers offer. The second 

generation of condensing boilers had fully modulating burners that offer improved efficiencies at lower firing 

rates and improved staging opportunities. These came into production in the early 1990s.iv Condensing boilers 

have been deployed to a limited extent across the United States and to a larger extent across Europe (where 

governmental standards or incentives and higher energy costs have encouraged more widespread adoption). As 

of 2001, condensing boilers represented approximately 2% of total boiler sales in the United States.

iii

 

The low level of market penetration demonstrates a significant opportunity for increased deployment of this 

technology. GSA has already started to lead this effort and has deployed condensing boilers in many of its

buildings in an attempt to reduce energy consumption and operating costs. 

C. Demonstration Project Locations

The five buildings selected for this study were chosen due to the fact that they had received condensing boiler 

retrofits during the summer and early fall of 2010 and were now fully operational. All five buildings are located 

at the DFC in Denver, CO. The five sites are: buildings 25, 45, 54, 710a, and 810. The retrofits in buildings 25, 45, 

and 54 all replaced boilers that were approximately 20 years old. Building 810 replaced boilers from 1998. 

Building 710a did not have data on the pre-existing boiler age, yet the low-rated efficiency listed (66%) implies 

an older vintage.

The buildings range in size from 37,000 ft2 to 787,000 ft2

. They provide hot water to a wide variety of air-side 

systems, terminal units, and perimeter baseboard units. One of the retrofits (building 45), included a steam to 

hot water retrofit during the boiler switch over. The air handling unit types include hot deck/cold deck, constant 

volume (CV), and VAV. The wide variety in the distribution systems was important to the study in that it allowed 

for comparison across numerous types of retrofit applications. For more detailed descriptions of the individual 

buildings, refer to subsection IV-A. Facility Description (page 15).
Condensing Boiler Evaluation – Retrofit and New Construction Page 9 

This study was completed during the spring of 2012. The trend data acquired from the BAS system was collected 

from February 11 to April 16, 2012. The utility data was collected from October 2008 until October 2011. This 

range provided two years of utility data prior to the condensing boiler retrofits and one year of data postretrofit.

III. Technology Description and Objectives 

A. Technical Objectives 

The ability to achieve the upper end of the potential efficiency improvement relies on appropriate system design 

and operation. It is the purpose of this report to study the operation of the five installations at the DFC, quantify 

the installed performance of those installations, and report on the actual savings achieved by these systems. The 

condensing boiler technology will be compared to both the pre-existing boilers and to near-condensing boilers. 

The operation and performance of the systems will be evaluated through:

• Fifteen minute trend data from the BAS, collected throughout the winter-early spring of 2012.

• Utility bill analysis - using regression analysis on pre- and post-retrofit utility data to quantify actual 

savings due to the boiler retrofits.

These two analysis methods will be used to evaluate actual and predicted annual efficiencies. Using these 

results, the actual energy savings will be quantified. Specifically, this study will evaluate the different types of 

heat distribution systems in the five different buildings at the DFC and will evaluate the effects that the different 

system characteristics, such as piping configuration and coil type, had on the condensing boiler operation and 

annual efficiency. 

The report also will use the Condensing Boiler Design Model (CBDM), developed by Group 14 Engineering 

through a subcontract with NREL, to further support and verify the best practice recommendations on both new 

and retrofit condensing boiler installations. 

Finally, this study will evaluate the economic viability of the technology and its applicability for widespread 

adoption across the GSA building portfolio. Table 3 lists some of the specific objectives, metrics, and success 

criteria defined for this study. 
Condensing Boiler Evaluation – Retrofit and New Construction Page 10

Table 3 – Performance metrics developed at outset of study

Performance 

Objective Metric Data Requirements Success Criteria

Quantitative Performance Objectives

Condensing Mode 

Operation

Percent of time 

operating in 

condensing mode

Supply Water Temperature (F)

Return Water Temperature (F)

Operating in condensing 

mode > 20% of the time

Cost Effectiveness Life cycle cost 

effectiveness

Installation costs relative to 

standard high efficiency boiler

Natural gas rates for GSA 

facilities in each region

Exceeds GSA's life cycle 

cost criteria for energy 

efficiency projects versus 

standard high efficiency 

boiler

Qualitative Performance Objectives

Operation and 

Maintenance

(O&M) Costs

Heating, ventilating, 

and air conditioning 

(HVAC)

maintenance costs 

per year

O&M costs of pre retrofit steam 

boiler

O&M costs of pre retrofit hot 

water boiler

Reduction in O&M costs 

relative to steam boiler and 

existing hot water boiler

It is important to note that one of the main objectives of this report is to provide analysis and recommendations 

for appropriate integration with the hot water distribution system in the building. Appropriate integration will 

increase the percent of time spent in condensing mode and improve the cost-effectiveness of the installations.

B. Condensing Boiler Technology Description

Condensing boilers have been manufactured since the 1980s. The first generation boilers were typically single 

stage boilers, with the first fully modulating models coming into production in the mid-1990s.

viii Condensing 

boilers are a fully mature technology that is in line with DOE/FEMP’s TRL 9 classification.

ix These systems have 

been deployed throughout the United States and the world and are currently supported with various state and 

utility incentives. 

Condensing boilers achieve their higher efficiencies by condensing water vapor in the flue gases. When natural 

gas is combusted in a boiler, approximately 90% of the energy contained in the fuel is converted into sensible 

heat (heat that causes a change in temperature) and approximately 10% is converted into latent heat (heat 

involved in phase change). This latent heat is stored in the water vapor that is a byproduct of the combustion 

process. One can see from the simplified combustion equation for methane, the principal component of natural 

gas, that two molecules of water are produced for every molecule of methane combusted: 

CH4 + 2O2 → CO2 + 2H2O + Heat

It takes energy to evaporate these water molecules into the flue gases. It is this latent energy contained in the 

water vapor that condensing boilers recapture.

In traditional boiler technologies, the latent heat contained in the water vapor is allowed to escape in the flue 

gases. Condensing boilers reclaim that latent heat by condensing the water vapor and transferring that heat 

back into the return water. According to the manufacturer of the boilers in this case study, this process of 

condensing the water vapor can increase the steady-state efficiency of the boiler from between 80% and 87% to

between 88% and 98% (depending on boiler operation).

x

 
Condensing Boiler Evaluation – Retrofit and New Construction Page 11

These potential efficiency gains can only be attained if the boiler is operating in “condensing” mode. Condensing 

mode is achieved when the temperature of the return water is below the dew point of the water vapor in the 

flue gas. For natural gas the dew point is approximately 130°F. Therefore, if the return water temperature is 

below 130°F, the water vapor will start to condense and the system will achieve a higher efficiency. The lower 

the return water temperature, the more condensation occurs and higher efficiency is achieved. 

The design characteristics of the condensing boiler plant and the hot water systems that the boiler plant serves

affect both the operational efficiency and return water temperature of the boilers. The design characteristics of 

the boiler plant, piping system, heating system, and control system need to be documented and understood to 

accurately design and operate a condensing boiler plant.

Boiler Plant

Boiler Sizing – Condensing boilers operate at higher efficiencies at lower firing rates. Most condensing boilers 

can modulate to very low firing rates (≈10% of full fire), and achieve higher efficiency at these low rates. 

Appropriate sizing of boiler plants also limits cycling losses at low loads, further improving the overall efficiency 

of the plant. The first step in properly designing a condensing boiler plant is to design the system to include 

multiple smaller boilers to meet a given load, rather than using one or two large boilers to meet a given load. 

This will help achieve higher operating efficiencies and avoid excess initial cost by matching the capacity more 

closely to the building load. 

Boiler Turndown Ratio – The turndown ratio of a boiler defines the range of firing rates at which a boiler can 

continuously operate. Below the minimum turndown ratio, the boiler will start short-cycling, introducing losses 

associated with pre- and post- burner purging. This impacts the part-load efficiency of the boiler; the higher the 

turn turndown ratio the better the part-load performance. For example a turn down ratio of 10:1 would allow 

for operation down to 10% of the rated output, where as a 5:1 turn down ratio would only allow for operation 

down to 20% of the rated output.

Boiler Minimum Water Flow – Some condensing boilers have a minimum water side flow rate at minimum and 

maximum firing rates, to protect the heat exchanger. This minimum flow rate can affect the sequencing of the 

boilers, the operation of the boiler plant, and ultimately the supply and return water temperatures. The lower 

the minimum flow rate the better the part-load performance of the plant.

Boiler Efficiency – The efficiency of a condensing boiler is a function of the hot water return temperature and the 

percent firing rate. The efficiency of the boiler increases at lower return water temperature and lower firing 

rates. Conventional boilers operate at lower efficiencies when firing rates are lower due to the increased 

amount of excess air in the exhaust and the high exhaust air temperature. Condensing boilers are able to 

operate at high efficiencies with increased amounts of excess air (at lower firing rates) due to the low exhaust 

air temperatures. 

The increase in efficiency at low firing rates holds true until the boiler needs to operate below the low end of the 

turndown ratio. Efficiency then decreases significantly due to short-cycling. The part-load performance of the 

units needs to be understood and accounted for when selecting and sizing the boilers. 

Piping and Pumping System

Primary versus Primary/Secondary – A primary hot water loop is designed with one water loop that circulates 

water through boilers and the heating coils within the facility. This loop can be CV or variable volume depending 

on the requirements of the boiler (Figure 2). 
Condensing Boiler Evaluation – Retrofit and New Construction Page 12

Figure 2 – Constant Volume Primary Hot Water Loop

A primary / secondary loop is typically set up with a CV primary loop and either a CV secondary loop or variable 

volume secondary loop that pulls hot water off of the main primary loop through the boiler plant and supplies 

water to the coils within the facility (Figure 3). In systems designed with distributed booster pumps, the small 

booster pump loops can serve as the secondary loop.

Figure 3 – Primary/Secondary Hot Water Loop
Condensing Boiler Evaluation – Retrofit and New Construction Page 13

Secondary Flow Rate and Control Valve Selection – The flow rate through the heating coils is a function of the 

heating coil design and the loads at any given time. The flow rate through the entire system is a function of the 

piping and control valve design with additional water pumped and mixing if there is a three-way valve. Older 

facilities were designed with three-way control valves and CV water loops because engineers were encouraged 

to provide constant flow through the boiler, which is no longer necessary with modulating boilers. With this type 

of configuration, much of the supply water can bypass the coil (especially at lower loads). The bypass stream 

then mixes with the water leaving the coil, increasing the return water temperature. An image of a heating coil 

with a three-way control valve is provided below (Figure 4). 

Figure 4 – Three Way Control Valve 

Image Credit: Jesse Dean (NREL)

If the coils are designed with two way control valves, the water flow rate through the coils is modulated and the 

mixing problem is eliminated. This type of design requires the use of variable frequency drives on the pumps 

(Figure 5). 

Figure 5 – Two Way Control Valve 

Image Credit: Jesse Dean (NREL)

Some systems are designed with a bypass between the supply and return water at the main boiler plant that 

maintains a minimum flow rate through the plant. Other boilers have this included in an internal circulator. All 

of these factors need to be accounted for in the design phase.
Condensing Boiler Evaluation – Retrofit and New Construction Page 14

Heating System and Heating Coil Design

Heating Coil Design ∆T – Most air handling unit and terminal unit heating coils are designed for a 20° or 30°F 

temperature drop across the coil. For example, a typical coil will be designed for 180°F supply water 

temperature and 150°F return water temperatures during peak conditions, which would prohibit the system 

from operating in condensing mode with a 180°F supply water temperature. Heating coils can be designed with 

higher ∆T’s, on the order of 50 to 60°F, which allow for much cooler return water temperatures given a supply 

water temperature of 180°F. Ideally, the coil will be designed for a high ∆T and a lower supply water 

temperature. This extra step will ensure the water leaving the coil is below 130°F for a greater percentage of the 

time than a high ∆T coil alone.

Radiant/Convective Baseboard Heating Systems – Perimeter baseboard radiant heaters are designed to operate 

with higher supply water temperatures, on the order of 180°F. If a space is heated with primarily perimeter 

baseboard/radiators then the supply water temperature can be decreased by increasing the heater surface area

on the baseboard/radiator system. If area increase is not feasible, the supply water temperature might need to 

remain high to properly heat the space.

Control Sequences

Boiler Staging – The boiler operation can be staged so that multiple boilers are on at a low firing rate instead of a 

single boiler operating at close to full capacity. For example, three boilers could be operating at 30% of capacity 

instead of a single boiler operating at 90%. The only caveat with this operation strategy is that it is important to 

not have the boilers on so low that short cycling occurs. 

Outside Air or Load Based Reset – The supply water temperature can be reset based on the outside air 

temperature or the heating coil valve positions. The reduced supply water temperature allows for cooler return 

water temperatures and some type of supply water reset should be implemented to reduce the return water 

temperature. 

Material Requirements

It is important to note that the acidic nature of the flue gas condensate requires highly corrosion-resistant 

materials to be used in condensing boiler heat exchanger and venting construction. Traditional boilers do not 

use the corrosion resistant material and therefore must ensure operation above the condensing temperature. 

This effectively puts a cap on the efficiency achievable with traditional boiler technology. It also must be noted 

that the corrosion resistant materials used in the condensing boilers come at a much higher cost. A condensate 

treatment system may be required to treat the acidic condensate before it enters the drain. All of the boiler 

installations at the DFC included condensate treatment. The increased cost for installation of this piece of 

equipment increases the installation costs compared to a standard boiler. 

Condensing boilers offer the potential to reduce operating costs and save energy through the higher efficiencies 

attainable with the technology. These savings are only realistically attained through appropriate system design 

and operation.
Condensing Boiler Evaluation – Retrofit and New Construction Page 15

IV. Assessment

A. Facility Description

The DFC buildings were evaluated through numerous site visits during the 2011-2012 heating season. DFC 

building managers and mechanical technicians provided access and information on the different buildings’ HVAC

systems. The site visits provided information on the boiler plants and HVAC systems within each facility. The 

following information was collected during the site visits:

• Boiler Plant Design – The number of boilers, boiler name plate capacity, and minimum water-side flow 

rate. 

• Pump and Piping Design – The pump name plate data and variable frequency drive minimum set point 

data, as well as information on the primary/secondary configuration.

• Air Handling Units, Terminal Units, and Baseboard Heaters - The number and type of air handling units

(AHUs). The heating coil ∆T, when this information was available, as well as control valve configuration 

and secondary pumping system information.

• Controls – The control sequence for the boiler plant and the heating loads on the boiler plant were 

noted and trended where applicable.

While it was not possible to evaluate every AHU and terminal unit in the building, representative units were 

evaluated and documented. The following building descriptions do not attempt to document every aspect of the 

heating system for each building; instead they provide an overview of the different types of systems, coils, and 

valving found in each building. These aspects of the heating system have a significant impact on the return water

temperature, and, therefore, performance of the condensing boilers. 

The following table gives a summary of the boilers, primary pumps, and coils/valving for each of the five 

buildings evaluated in this study. This is followed by an overview of each specific building.
Condensing Boiler Evaluation – Retrofit and New Construction Page 16

Table 4 – Facility Boiler Plant and HVAC System Description

Building 25 45 54 710A 810

Number of boilers 6 3 (Plus 2 old backup 

steam boilers)

5 (Plus an old 

backup boiler) 2 6 (Plus an oldbackup boiler)

Max Output per 

boiler (MBH) 2,500 1,500 - new boilers, 1,339 - old boilers

2,500 - new 

boilers 10,400 - 

old boiler

1,500 2,500

Primary pumps, 

Rated Flow (GPM) 2 @ 480 3 @ 225 2,000 2 @ 40 3 @ 475

VFDs on Pumps Yes Yes No Yes Yes

Air Handling Units 28 AHUs 6 AHUs 20 AHUs hot deck/ 

cold deck - 30 AHUs - not all have heating coils

Control Valve 

Design 

2 way valves

with heating 

coil bypass

3 way valves 3 way or 2 way 

valves with bypass - 3 way valves

Terminal Units 360 VAV boxes 8 Unit heaters - Terminal units - 

Control Valve 

Design 2 way valves 2 way valves - 2 way valves - 

Baseboard Units 200 feet 

baseboard

1.2 MBH 

baseboards - - 300 ft. baseboard

Control Valve 

Design 2 way valves 2 way valves - - 2 way valves

Other 

6 laboratory 

AHUs - 100% 

outdoor Air 

4 Fan coil Units 7 additional AHUs - - 

Control Valve 

Design 

2 way valves 

with AHU 

bypass

2 way valves 3 way or 2 way 

valves with bypass - - 

Building 25:

The National Enforcement Investigations Center occupies building 25 at the DFC and provides technical support 

for U.S. Environmental Protection Agency (EPA) enforcement and compliance assurance programs nationwide. 

The total facility area is 101,735 ft2

, with an estimated 137 EPA personnel occupying the facility. The interior 

space is a mixture of office space and laboratory space. The energy use within the facility is driven by the 

ventilation loads and energy requirements of the laboratories.xi

Building 25’s entire HVAC system was upgraded in 2002, with the exception of the boilers, which were installed 

in 2010. A one line diagram of the boiler plant and associated HVAC system components is provided in the 

following graphic (Figure 6). 
Condensing Boiler Evaluation – Retrofit and New Construction Page 17

Figure 6 – EPA Building Heating System One Line Diagram

The six 2,500 MBH boilers provide hot water to AHUs, VAV box re-heat coils, and perimeter baseboard units. 

The plant is set up with a variable flow primary loop and two variable speed pumps. Each pump is rated at 480 

gpm and only one pump operates at a time. The AHUs are the largest thermal heating load on the system and 

are set up with CV booster pumps, two-way control valves, and a hot water bypass at each AHU. The office AHUs 

are VAV and the six laboratory AHUs are 100% outdoor air units. Each lab is equipped with a phoenix valve that 

can reduce air flow to 50% during unoccupied hours. The VAV boxes have hot water reheat coils and there is 

approximately 200 ft. of baseboard radiators. The VAV box re-heat coils and baseboard heaters are connected to 

the variable flow primary loop and have two-way control valves.

The three main hydronic loops (AHUs, VAV reheat, and baseboards) are operated in parallel and mix return 

water before re-entering the boiler plant. The booster pumps in the AHUs are required to overcome the 

pressure drop in the coils, based on the fact that the system has been retrofitted over time and the additional 

AHUs that have been added to the system have increased the pressure drop through the system. The CV booster 

pumps and AHU bypass mix hot supply water with cooler return water, which has the effect of increasing the 

return water temperatures. The new boilers at the EPA building have an outdoor air lockout that shuts them off 

when the outside air temperature is above 72o

F. 
Condensing Boiler Evaluation – Retrofit and New Construction Page 18

Building 45:

The boiler plant in building 45 contains three new 1500 MBH condensing boilers, as well as two old steam 

boilers. The old steam boilers act as a backup system and transfer the steam to hot water through a heat 

exchanger. This heating plant serves both building 45 (≈78,000 ft2

) and building 46 (≈21,400 ft2). It is 

approximately 50% high bay and 50% office space. As part of the boiler retrofit, building 45 was converted from 

a steam to a hot water distribution system. The steam to hot water conversion was pursued both to improve 

heating system efficiency and to remove a potential health liability due to asbestos-encased piping systems in 

the old steam installation.

There are five to six “multizone” air handlers that are equipped with three-way valves. There are eight unit 

heaters that serve the high bay garage area. These unit heaters are rated at 180 MBH, 200o

F entering water 

temperature, and 180o

F return water temperature. Due to the high temperature requirements and low ∆T, 

these units are not ideal for a condensing boiler system. A small number of baseboard radiators provide heating 

as well. Four fan coil units are located in the garage that are controlled by two-way valves. Three primary pumps 

circulate hot water; each equipped with variable speed drives and rated at 225 gpm. 

Figure 7– Building 45 Heating System One Line Diagram
Condensing Boiler Evaluation – Retrofit and New Construction Page 19

Building 54

The boilers located in building 54 serve buildings 52, 53, and 55. Building 54 is a small facility with two main 

rooms, one room houses the condensing boiler plant, and the other serves as a small storage room. Five new 

condensing boilers were installed. Each boiler has a rated capacity of 2,500 MBH. One of the existing boilers was 

left in place to serve as a backup. The backup boiler has a rated capacity of 10,400 MBH and a steady state 

efficiency of 80%. The 2,000 gpm pumps are CV, which essentially prohibits the installation of two-way valves (it 

can still follow the pump curve, with reduced efficiency and unstable operation). Without any two-way valves, 

the return water temperature will be higher, as coil bypasses are necessary to prohibit increasing the pump 

head when the valves begin to close. There are approximately 27 AHUs served by the boilers, 20 of which are 

hot deck/cold deck units. In hot deck/cold deck units, air is separately heated and cooled and then mixed to 

achieve the desired temperature. This is most commonly a CV system design that results in significant mixing of 

supply water into the return water from the coils. All of the main air handler coils had small booster pumps to 

maintain a certain flow through the coil. All of the heating coils in the AHUs are equipped with three-way valves.

A one-line diagram of the heating system is presented below (Figure 8). 

Figure 8 – Building 54 Heating System One Line Diagram
Condensing Boiler Evaluation – Retrofit and New Construction Page 20

Building 710A

Building 710A’s heating is served entirely by terminal units controlled by two-way valves. The building is 

approximately 37,000 ft2 of office space. There is one central AHU that contains only a cooling coil, with the 

terminal units providing the reheat needed. Two-way control valves control the terminal reheat. Since all of the 

heating coils are controlled by two-way valves and all the hot water pumps are already equipped with variable 

speed drives, this facility is an ideal candidate for control adjustments that can lower the return water 

temperature without significant capital investment. There are two new condensing boilers that heat the entire 

building. Each of these boilers has a 1,500 MBH capacity. The primary hot water pumps consist of two 40 gpm 

pumps installed in parallel and equipped with variable speed drives. A one-line diagram of the heating system is 

presented below (Figure 9). 

Figure 9 – Building 710A Heating System One Line Diagram

Building 810

Building 810 is approximately 787,000 ft2 and contains six new condensing boilers (2,500 MBH each) as well as a 

large, older boiler. The building is half warehouse space and half offices. There are approximately 30 air

handlers, although not all of them contain heating coils. They are hot deck/cold deck, constant volume systems.

All of the heating coils in the air handlers have three-way valves. There are also approximately 70 unit heaters. 

The unit heaters are ceiling mounted fans that contain a hot water coil and are prevalent in high bay areas.

There is also approximately 300 ft of baseboard heating in building 810. The baseboard radiators and the unit 

heaters require relatively hot entering water temperatures, which makes it difficult for the return water 

temperatures to be below the 130o

F required for the boilers to operate in condensing mode. There are three 

primary hot water pumps rated at 475 gpm and equipped with variable speed drives. There are three CV booster 

pumps in the mechanical room and several more located throughout the building.
Condensing Boiler Evaluation – Retrofit and New Construction Page 21

B. Technology Specification

The condensing boiler technology is a system of technologies, consisting of the boiler itself, the flue stack, and 

the condensate collection system. A schematic of the Cleaver Brooks boilers installed at the DFC is presented in

Figure 10. There are many other manufacturers that supply condensing boilers that demonstrate similar 

performance. The salient characteristics of the condensing boiler are the stainless steel, high-efficiency heat 

exchangers and fully modulating burners. The flue stack must be manufactured of corrosion resistant material 

(such as AL294-C), instead of the traditional B-Vent due to the flue gases condensing in the stack. Condensing 

boilers also need a condensate collection system to deal with the acidic condensate in an environmentally safe 

manner. This is usually accomplished through a corrosion resistant collection system (often PVC) that feeds into 

a holding tank that contains some sort of neutralizing agent. 

Figure 10 - Diagram of the Cleaver Brooks Clear Fire boiler

Image Credit: Cleaver Brooks

It should be noted that while the actual condensing boiler technology system ends here, the

operation/performance of the technology depends on its integration with the building’s heating system. The 

manner in which the system is integrated with the hot water distribution system will determine how efficiently 

the boilers operate and will impact both energy usage and cost savings. It is, therefore, recommended to specify 

not only the technology itself, but also how it should be integrated with the building’s heating system.
Condensing Boiler Evaluation – Retrofit and New Construction Page 22

C. Technology Deployment

The existing boilers at the site consisted of natural gas fired boilers of the non-condensing type. In all of the 

buildings except for building 45, these boilers provided hot water to the building that was used in AHU heating 

coils, baseboard, and terminal units to provide heat to the building. In building 45, the existing system was a 

steam system. The existing boiler data is provided in Table 5, and was provided by GSA. 

Table 5 - Existing boiler data

Building Manufacturer Model Year 

Built

Max 

Input 

(MBH) 

Min 

Input 

(MBH) 

Max 

Output 

(MBH) 

Min 

Output 

(MBH) 

Rated 

Efficiency 

(Input/ 

Output)

25 York Shipley SPLV-250-N 1990 10468 

CFH

8369 

CFH 80%

25 York Shipley SPLV-250-N 1990 10468 

CFH

8369 

CFH 80%

25 Bryan CL120-S-15-

FDG 1990 900 450 720 80%

45 Bryan RV350-S-15-

FDG 1990 3110 1555 2488 80%

45 Bryan RV350-S-15-

FDG 1990 3110 1555 2488 80%

54 Bryan RV250-W-FDG 1990 2125 1063 1700 80%

54 Bryan RV250-W-FDG 1990 2125 1063 1700 80%

710A Multitemp MOP-1540 1540 1014 66%

710A Multitemp MOP-1540 1540 1014 66%

810 Bryan RW850-W-FDG 1998 8500 3386 6800 2709 80%

810 Bryan RW1050-WFDG 1998 10500 3888 8400 3111 80%

The condensing boilers were deployed in each of the five buildings evaluated during the summer-fall of 2010. 

This consisted of removing the existing boiler(s) and installing the condensing boilers in the existing boiler room. 

The boilers were all installed in parallel, allowing for staging of the boilers as required by the load. This implies 

that a single boiler can be on and receiving all of the flow or numerous boilers can be on with each receiving a 

portion of the flow. This enables staging of the boilers as programmed into the controller. It also allows for equal 

runtime of the different boilers with appropriate sequencing. 

In all cases except for building 54, the new pumps were matched with variable frequency drives (VFDs) to enable 

variable flow to the hydronic loop. When matched with the appropriate heating system, this allows for reduced 

flow at lower loads, reducing pump power consumption and maintaining a larger temperature differential 

between supply and return. The pumps also were installed in parallel. 

In general, the boiler retrofit did not include any retrofit to the rest of the HVAC system. There was no swap out 

of coils or changeover from CV systems to VAV systems. Whatever system was in place in each building was left, 

and was simply supplied by the condensing boilers in place of the old boilers. The one exception to this was 
Condensing Boiler Evaluation – Retrofit and New Construction Page 23

building 45. In building 45, the entire heating system was switched over from steam to hot water at the same 

time as the boiler retrofit.

D. Test Plan

The testing plan consisted of evaluating the performance of the condensing boilers through the use of BAS trend 

data, utility bills, and site assessments. 

Utility data was provided by GSA for the period October 2008 through September 2011. This provided 

approximately two years of pre-retrofit data, and one year of post-retrofit data. These data were normalized to 

outside air temperatures to provide accurate comparison between pre and post-retrofit data. 

The 15 minute trend data was gathered from the BAS from February 7, 2012 through April 15, 2012. These data 

were provided to NREL by the head of advanced metering at the DFC. This data was provided in weekly 

installments for each building under observation. These data were then compiled and processed using Python 

scripts that reformatted the data into comma separated files that allowed for efficient data analysis. Python is 

an open source programming language that provides efficient data processing capabilities.

Finally, each building was evaluated during site visits to the DFC campus. These sites visits provided name plate 

data on all of the relevant HVAC systems within the buildings. 

E. Instrumentation Plan

The BAS points that were monitored for each building consisted of: outdoor air temperature [°F], supply water 

temperature [°F], return water temperature [°F], pump enabled [ON/OFF], VFD speed [%], and coil position for 

the first two and last two coils in the hydronic loop [%]. In some cases the coil position points were not available 

(either due to pneumatic control or lack of direct digital control (DDC) data). In other cases, the VFD speed was 

not provided. All of these points were recorded every 15 minutes for the two-month data collection period. The 

exact data points collected are listed in Table 6. 

Table 6 - Monitored Points for the each building

In building 54, pump status and speed were not recorded due to the fact that it still contained CV pumps. It is 

also worth noting that the pump speeds for all three pumps in building 810 were recorded as 80% for the entire 

period of observation and these three control points should be commissioned to correct the fault associated 

with the pump operating at the same flow rate. The pump speed should modulate based on the heating load 

and vary throughout the winter. 
Condensing Boiler Evaluation – Retrofit and New Construction Page 24

V. Results

The results section is broken up into three main sections. The first section describes the utility data analysis and 

results. The second section evaluates the BAS trend data that were collected and looks at predicted annual 

efficiencies, estimated from the data collection period. The last section evaluates results on how the different 

hot water distribution systems were interacting with the condensing boilers in terms of return water 

temperature and firing rates. 

A. Utility Bill Analysis

Monthly natural gas consumption data were provided by Region 8 for the period October 2008 through 

September 2011. The condensing boiler retrofits occurred during late summer/early fall of 2010. Therefore, the 

utility data provided contains approximately two years of pre-retrofit data, and one year of post-retrofit data.

The function and schedule of the buildings did not change over the period of the analysis.

To accurately quantify the savings due to the condensing boiler retrofit, it was necessary to normalize the utility 

data with respect to weather. Ideally, one would be able to compare the gas consumption between a winter 

month before the retrofit and a winter month after, yet a simple comparison is not accurate due to the fact that 

one month might have experienced warmer or colder weather than the other. Therefore a method called 

steady-state inverse modeling was used to compare the pre- and post-retrofit data. This method is described in

detail in the 2009 ASHRAE Handbook – Fundamentalsxii, and an overview of the process is described below. 

First, daily average outdoor dry-bulb temperature data were gathered from Weather Underground for the three 

year period for which the utility data was providedxiii. These data were used, along with the building’s “balance 

temperature,” to approximate the heating demand for that time period. “Balance temperature” is defined as

the temperature below which active heating is required. A balance temperature is a building-specific 

temperature that is a function of the building’s construction and internal loads. The heating demand for the 

building is then quantified in terms of heating-degree days using the outdoor air temperature and the building’s 

balance temperature. Heating-degree days quantify how far and how often the outdoor temperature differs 

from the balance temperature of the building. Therefore, they quantify the heating demand on the building.

A linear regression was then performed on the correlation between heating-degree days and daily energy use. 

An example regression is shown in Figure 11, the rest of the figures are included in Appendix A. This was done 

twice for each individual building, once for the pre-retrofit gas consumption and once for the post-retrofit gas 

consumption. It can be seen that the regressions had very high R2

 values across all of the five buildings, with all 

but one regression having R2 between 94% and 99% (other value at 86%). 

These two correlations describe how much natural gas was used by the building before and after the condensing 

boiler retrofit for any given outdoor air temperature. This makes it possible to compare the gas consumption on 

an equal basis regardless of the weather data for that specific year. 
Condensing Boiler Evaluation – Retrofit and New Construction Page 25

Figure 11 – Regression on gas usage as a function of heating degree-days (Building 45)

The pre- and post-correlations were used to estimate the annual savings due to the boiler retrofits. The energy 

savings are presented in Figure 12. The first pair of bars for each building are the actual gas consumption values 

for 2009 and 2011 (pre- and post-retrofit), and the blue, second set of bars is the weather normalized gas 

consumption for the same year. 

Figure 12 – Natural gas savings from condensing boilers (actual and weather normalized savings)
Condensing Boiler Evaluation – Retrofit and New Construction Page 26

The estimated annual cost savings are presented in Table 7. These cost savings are again based on the weather 

normalized savings for 2011. The assumed utility rate for this example is the average natural gas rate for January 

to September 2011. That average cost was calculated at $7.10/Dekatherm. Changes in natural gas rates will 

affect the estimated savings and resulting paybacks. This is shown in Figure 13 . 

Table 7 – Estimated savings for 2011 for each observed building

Building Year Estimated 

Savings

Percent 

Savings

Bldg 25 2011 $25,544 19%

Bldg 45 2011 $10,602 28%

Bldg 54 2011 $15,269 16%

Bldg 710A 2011 $3,906 41%

Bldg 810 2011 $16,368 16%

The total estimated energy savings from the utility data analysis that was performed was 100,097 therms of 

natural gas, and $71,690 of annual utility bill savings. 

The estimated savings are larger than what would be expected from a change in steady-state efficiency alone. 

This can be attributed to the switch from the old “high mass” boilers, which require more energy to heat the 

boiler to the operating temperature. Condensing boilers are “low mass” systems that contain less water and 

lower mass heat exchangers, which can reduce cycling losses. In the case of building 45, this retrofit also 

included a conversion from steam to hot water. This change to the hydronic system also would impact the 

monitored savings.

Table 17 shows the economic evaluation of replacing the existing boilers with condensing boilers for each of the 

five sites at the DFC. The initial costs used for the condensing boiler systems were based on the normalized cost 

of $48.18/MBH, established in this report. 

Initial cost estimates were generated from a number of sources including capital cost values for the condensing 

projects provided by GSA. These values were combined with research from a full market-research effort to 

generate a $/MBH installed cost estimate for standard boilers, near-condensing and condensing boilers 

($26.21/MBH for standard, $43.46/MBH for near-condensing, and $48.17/MBH for condensing). For discussion 

of the market research, refer to subsection VI.D: Market Potential within the GSA Portfolio.

Table 8 and Table 9 show the results of this economic analysis. Table 8 presents the economic analysis with the 

“average” installed cost generated from market research. Table 9 shows the same analysis with the actual 

installed costs of the DFC installations. The cost of the standard replacement boilers were estimated using the 

86% efficient boiler costs from the market cost research performed for this report (see Table 15). The standard 

replacement boiler costs were used in calculating the incremental cost when a replacement is necessary;

therefore it was necessary to use a system cost that would reflect the typical replacement option. It was 

assumed that GSA would replace older boilers with at least a near-condensing system in order to pursue energy 

reduction on-site and stay compliant with federal requirements. When calculating the incremental energy cost 

savings of the condensing boiler over the near-condensing replacement option, the calculated energy cost 

savings were multiplied by the ratio of the rated efficiency improvement ((95−86)

(95−80)

 or 60% of the savings achieved 
Condensing Boiler Evaluation – Retrofit and New Construction Page 27

by the condensing boilers) to account for the incremental energy savings that would be achieved by the 

condensing option.

Table 8 – Calculated simple payback and net present value for the five sites for condensing boilers 

with normalized initial costs

Replacement Necessary Replacement Optional

Building

Initial Cost 

Replacement 

Boiler 

Initial Cost 

Condensing 

Boiler 

Incremental 

Energy Cost 

Savings 

Simple 

Payback 

Period 

(Years) 

Net 

Present 

Value

Energy 

Cost 

Savings 

Simple 

Payback 

Period 

(Years) 

Net Present 

Value

25 $629,084 $697,406 $15,326 4.46 $254,452 $25,544 27.30 -$159,449

45 $183,836 $203,801 $6,361 3.14 $114,001 $10,602 19.22 $19,477

54 $524,236 $581,171 $9,161 6.21 $136,004 $15,269 38.06 -$259,606

710A $122,557 $135,868 $2,344 5.68 $36,046 $3,906 34.78 -$53,607

810 $629,084 $697,406 $9,821 6.96 $138,504 $16,368 42.61 -$352,695

Table 9 – Calculated simple payback and net present value for the five sites for condensing boilers 

with actual initial costs

Replacement Necessary Replacement Optional

Building

Initial Cost 

Replacement 

Boiler 

Initial Cost 

Condensing 

Boiler 

Incremental 

Energy Cost 

Savings 

Simple 

Payback 

Period 

(Years)1 

Net Present 

Value

Energy 

Cost 

Savings 

Simple 

Payback 

Period 

(Years) 

Net Present 

Value

25 $629,084 $618,057 $15,326 NA $333,800 $25,544 24.2 -$80,100

45 $183,836 $1,458,4692 $6,361 200.4 -$1,140,666 $10,602 137.6 -$1,235,191

54 $524,236 $506,389 $9,161 NA $210,786 $15,269 33.2 -$184,824

710A $122,557 $361,347 $2,344 101.9 -$189,434 $3,906 92.5 -$279,087

810 $629,084 $570,748 $9,821 NA $265,162 $16,368 34.9 -$226,038 1

Certain installations had lower actual installed costs than the estimated cost of a near-condensing replacement. This 

makes the incremental replacement payback immediate. These are denoted “NA”.

2 Note that the high initial cost associated with this retrofit included a full steam-to-hot water conversion and asbestos 

abatement costs.

With the actual initial costs used in calculations, as can be seen in Table 9, the retrofits at buildings 45 and 710A 

no longer have positive net present values. The variation between the average installed cost and actual installed 

cost calculations demonstrates the fact that actual installed costs can vary significantly due to a variety of 

factors including: boiler room size/access, access for flue stack venting, complexity of boiler room piping, and 

various other factors. The retrofit at Building 45 included a full steam to hot water conversion which included 

the installation of hot water piping throughout the building, asbestos removal and installation of necessary hot 

water pumps and controls. Although GSA does not have itemized costs for this building the installed costs for 

the condensing boiler made up a small percentage of the overall retrofit costs for this building.
Condensing Boiler Evaluation – Retrofit and New Construction Page 28

A sensitivity analysis on the cost of natural gas can be seen in Figure 13 and Figure 14. This analysis utilizes the 

average (not actual) installed boiler costs. Figure 13 shows simple payback for varying natural gas rates when a 

boiler replacement is necessary (end-of-life replacement). Figure 14 shows the same data for the case where a 

boiler replacement is not required (efficiency upgrade only). As expected the payback period is shorter for 

higher natural gas costs and it is also significantly shorter payback when a boiler replacement is necessary. It can 

be noted that for higher gas rates, the “efficiency only” replacement for can be cost effective.

0.0

5.0

10.0

15.0

20.0

25.0

4 5 6 7 8 9 10 11 12 13 14

Simple Payback Period (Years)

Natural Gas Cost ($/Dekatherm)

Building 25

Building 45

Building 54

Building 710A

Building 810

Figure 13 – Simple Payback Period vs. Natural Gas Cost for Normalized Boiler Costs When a Boiler 

Replacement is Necessary
Condensing Boiler Evaluation – Retrofit and New Construction Page 29

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

4 5 6 7 8 9 10 11 12 13 14

Simple Payback Period (Years)

Natural Gas Cost ($/Dekatherm)

Building 25

Building 45

Building 54

Building 710A

Building 810

Figure 14 - Simple Payback Period vs. Natural Gas Cost for Normalized Boiler Costs When a Boiler 

Replacement is not Required (efficiency upgrade only)

Net present value was calculated in this report according to the National Institute of Standards and Technology 

(NIST) Handbook 135xiv with an assumed boiler life of 25 years, a discount rate of 3% and natural gas as a fuel for 

a commercial building located in Colorado. The life expectancy of 25 years is the estimate provided by the 

Federal Energy Management Program’s Energy Efficient Product Procurement guide.

xv The replacement 

necessary category considers the difference in costs of a standard versus condensing boiler, while the 

replacement optional category considers only the cost of a new condensing boiler. 

B. Metered Data Analysis

The BAS data was gathered from February 7, 2012 through April 15, 2012. These data were provided to NREL by 

the head of advanced metering at the DFC. The BAS data was used to analyze the actual performance of the 

boiler systems in each of the five buildings. This section looks directly at the interaction of the boiler plant, the 

staging controls for the plant, and the hot water distribution system.

The effect of the return water temperature and firing rate on the boiler efficiency for the boilers that were 

installed at the DFC is shown in Table 10. In general, the lower the return water temperature and firing rate, the 

higher the efficiency.

Table 10 – Efficiency values at various firing rates and return water temperatures for boilers at DFC

Firing Rate Return Water Temperature °F (°C)

[%] 68 (20) 80 (27) 105 (38) 120 (49) 130 (55) 140 (60) 160 (72)

20 99 99 98 97 95.5 93.7 88

50 98.25 99 97 93 91.5 89.5 87.5

75 98 97 93 89 88 87.5 86.5

100 97.2 96.5 91 88.25 87.5 87 86.25
Condensing Boiler Evaluation – Retrofit and New Construction Page 30

The efficiency is affected by the staging of the boilers, the boiler firing rate and the return water temperature. 

The annual operational efficiency of the each boiler plant was estimated and compared to the efficiencies of the 

old boilers and the results of the utility analysis section.

Outside Air Reset

The design supply water temperature for the boiler plants that were monitored is between 180 °F and 200 °F 

and the hot water supply temperature is reset based on outside air temperature. A lower supply water 

temperature can allow for lower firing rates and also will lead to lower return water temperatures to the boilers. 

Both of these effects will increase the operating efficiency of the boiler plant. 

The outside air reset control directs the control system to reduce the supply water temperature when the 

outdoor temperature rises, essentially allowing the supply water temperature to be lower when there are lower 

loads on the system. The 15-minute supply water temperature data for each building was compared to the 15-

minute outdoor air temperature (OAT) data. The relationship between the supply water temperature and OAT is 

shown for building 25 in Figure 15. Building 25 showed very high supply water temperature for some of the 

coldest OATs (close to 200°F) and also did not appear to be turning off appropriately at the stated 72°F outdoor 

air cutoff temperature. Inspection of the remaining buildings demonstrated that the outdoor air reset control 

had been implemented in each building. There did not appear to be a cutoff temperature for any of buildings 

(except for building 810). Supply water temperatures for the other four buildings remained around 120°F with 

OATs reaching as high as 90°F. It is important to note that ensuring desired operation with the outdoor air cutoff 

could provide significant savings to GSA with minimal upfront costs and should be investigated on a building-bybuilding basis. It is difficult to quantify the exact savings that a correctly implemented cutoff temperature would 

provide, yet eliminating boiler operation when outside air temperatures are above 72°F will reducing total 

operating hours and contribute to overall savings.

100

110

120

130

140

150

160

170

180

190

200

0 10 20 30 40 50 60 70 80 90 100

Supply Water Temperature [F]

Outdoor Air Temperature [F]

Trend Data From Building 25 - OAT vs SWT

 HWST

Figure 15 – Demonstration of outdoor air reset control (building 25)
Condensing Boiler Evaluation – Retrofit and New Construction Page 31

Boiler Staging/Sequencing Controls

The firing rate of the boilers also has a large impact on the efficiency of the boilers. Condensing boilers have a 

different relationship with part load operation than traditional boilers, as they operate at a higher efficiency at 

lower partial loads. The staging of the boiler plants was investigated through discussion with the distributor who 

was responsible for the boiler installations at the DFC.

xvi The staging control that was implemented for the DFC 

boiler plants was a lead/lag staging, where a certain boiler is designated as the lead boiler and it controls when 

the other boilers are turned on. The specific boiler that is the lead boiler can be alternated to achieve equal 

runtime. The control sequence ramps the lead boiler up to a 70% firing rate and then turns on a second boiler. 

Once the second boiler is turned on the two boilers are controlled to ramp up at the same firing rate, for 

example, instead of one boiler operating at 80%, both would fire at 40%. These two boilers would ramp up to 

70% and then a third boiler would be turned on, and so on down the line. It was stated that the 70% threshold 

was selected for two reasons:

• To allow for adequate warm-up time for the lag boiler and still be able to meet an increasing load; and

• To eliminate any cycling (even with a 5:1 turn down ratio there is the possibility for cycling at low loads, 

which reduces overall efficiency).

This parallel staging of the boilers is an effective way to reduce firing rates and increase efficiency. The exact 

threshold for lead/lag staging is difficult to determine and the initial programming of 70% could potentially be 

evaluated and optimized for maximum operation time at low firing rates while avoiding cycling, yet this is 

generally the appropriate control for boiler plants with multiple condensing boilers.

Return Water Temperature Analysis

The return water temperature is the variable that has the largest impact on condensing boiler efficiency. The 

return water temperature for each building was binned into ten bins of 10°F each. The bins ranged from 90°F to 

180°F, with each bin value representing the frequency with which the return water temperature was monitored 

in that range of temperatures. The value of each bin represents the upper limit of the bin (for example, the 

130°F bin contains all instances where the return water temperature was between 120°F and 130°F). 

Figure 16 – Histogram showing percent of monitored hours that return water temperature was in 

various temperature bins
Condensing Boiler Evaluation – Retrofit and New Construction Page 32

For the boiler to be in condensing mode, the return water temperature needs to be less than approximately 

130°F. It can be noted that the return water temperature was often higher than this 130°F for all of the 

buildings. The exact percentage of time that the return water temperature was monitored at or below 130°F is 

shown in Table 11. One performance metric that was established at the beginning of the study was a 

requirement that the system would operate in condensing mode for at least 20% of the time. It can be seen that 

only buildings 710A and 810 achieved that goal. It should also be noted that the monitoring period consisted of 

relatively warm temperatures for the Denver area (due to both a warmer than average winter and the fact that 

the period stretched into the middle of April), and that the percent of time on an annual basis for future years 

might be lower than those noted below. 

Table 11 – Percent of time in condensing mode (assuming 130°F) 

Building

25 45 54 710A 810

0.7% 12.4% 14.9% 31.4% 32.5%

To accurately compare the installed performance of the boilers to the incumbent technology, it was necessary 

to estimate an annual efficiency for the condensing boilers. This was done by correlating the return water 

temperature to the OAT during the monitoring period by means of a linear regression. Typical meteorological 

year (TMY) data for outdoor dry bulb was then gathered for the Denver/Golden location. Using the regression 

correlation, return water temperature was estimated for a typical year. The operating efficiency was then 

calculated for each hour that was below 72°F (the stated cut off temperature for the boilers at select facilities). 

Due to the difficulty in estimating firing rate, the efficiencies were calculated for three different firing rates. The 

annual efficiencies for each of the different buildings are presented in Table 12. 

Table 12 – Estimated annual efficiency for each building (for firing rates of 20-75%)

Building

Firing Rate 25 45 54 710A 810

20% 90.1% 90.7% 90.8% 93.4% 92.3%

50% 88.2% 88.6% 88.7% 90.6% 90.1%

75% 86.7% 86.9% 87.0% 87.9% 87.9%

From the control logic for boiler staging, it can be determined that the boilers would be firing between 35% and 

70% for the vast majority of the year. The only times when a firing rate would be below 35% is at very low loads 

when only the lead boiler is on and is below 35%. The only time when a firing rate would be above 70% is at very 

high loads when all of the boilers have been called on and then need to fire at over 70% to meet the load. 

Therefore, an average of the boilers would be in the 50% firing rate range for the majority of the year. 

To accurately compare the calculated efficiencies of the condensing boilers (presented above in Table 12) with 

the pre-existing boilers, it was necessary to calculate an installed efficiency for the old boilers. This was done by 

utilizing the weather normalized utility data to calculate the load on each building. This load was divided by the 

pre-retrofit energy usage (again weather normalized) to estimate an annual efficiency. These results are 
Condensing Boiler Evaluation – Retrofit and New Construction Page 33

presented in Table 13. It can be noted that the calculated annual efficiencies are significantly lower than the 

reported steady state efficiencies listed in Table 5. These were listed as 80% for all buildings except 710A, which

was listed at 66%. 

Table 13 – Calculated efficiencies of the pre-existing boilers

Building

Pre Retrofit

Energy Use 

(therms)

Actual 

Load 

(therms)

Original 

Annual 

Efficiency

25 192,576 138,120 72%

45 53,224 33,927 64%

54 136,567 102,059 75%

710A 13,576 7,315 54%

810 147,831 112,425 76%

Ideally, firing rates would have been calculated from the load in the building (calculated using the formula Q̇ =

ṁ Cp∆T = 500.4 × GPM × (SWT − RWT)). The supply water temperature and return water temperature was 

gathered from the metered data. The GPM could be calculated by multiplying the VFD speed (in percent) by the

rated GPM for the pump (valid due to the variable flow aspect of the pump). A correlation between load and 

OAT could then be calculated and firing rates could be estimated for the TMY data, allowing a more accurate 

estimate of annual efficiency. This was generally not possible due to the fact that:

• Building 54 did not report VFD speed. 

• Building 810 reported 80% VFD speed for all of its pumps for the entire monitoring period. 

• Building 45 ran the pumps at approximately constant speed of 49-52% making any correlation between 

load and outdoor air temperature very weak. 

• Building 710A ran at such low loads (calculated at less than 19% of a single boiler capacity for the entire 

monitoring period) that it was not necessary to calculate. 

• Building 25 presented the best data and this calculation is documented in the Appendix. The calculated 

average firing rate of 49.4% compared very well with the 50% estimated rate based on sequencing 

control logic. 

Though the data limitations listed above limited the ability to calculate an exact annual efficiency for each boiler 

combination, they did provide some interesting insight on how the different buildings were operating. These will 

be evaluated in the last section of the results.

C. Hot Water Distribution Effects on Boiler Operation 

Heating Coil Design

As previously mentioned, condensing boilers are best matched with coils specifically designed for a large 

temperature differential or for low supply water temperatures, or both. This allows for low return water 

temperatures, both at design conditions (where a 50°F ΔT coil can take supply water temperature at 180°F and 

provide 130°F return water temperature) and at lower load conditions where low supply water temperature

coils maintain significant temperature differentials at low supply water temperatures. In a retrofit scenario, it is 

often difficult and costly to retrofit the heating coils. The condensing boiler retrofits at the DFC did not include 

coil retrofits. This limited the ability of the boilers to achieve the upper end of their efficiency potential due to 
Condensing Boiler Evaluation – Retrofit and New Construction Page 34

the consistently higher return water temperatures. The impact that the pre-existing hot water distribution 

system has on return water temperatures can be seen in Figure 17 for building 45. It is clear that the hydronic 

system was designed for a 20°F temperature differential at near-design conditions. This temperature differential 

drops to only a few degrees at lower supply water temperatures. With higher ∆T coils, and low supply water 

temperature coils, the return water temperature points would most likely have seen much larger separation 

from supply water temperature points, allowing for more return water temperatures in the condensing range 

for the boilers.

Figure 17 – Supply and return temperatures for Building 45

Control Valve/Piping Design

The type of control valve and piping design also can drive higher return water temperatures. The use of three 

way control valves mixes return water with supply water and increases the return water temperature.

In buildings 25, 54, and 810, there are a number of large AHUs that had three way valves and booster pumps 

that led to mixing of return and supply water. In addition to the control valve issues at the coil level, there were 

certain loops in parallel with unbalanced loads that were remixed before returning to the boilers. The effect of 

the bypass piping at each AHU (from three-way valves) and remixing is shown for a single week in February in 

building 25 (Figure 18). The solid lines are the supply water temperature and return water temperature from the 

boiler plant. The dashed lines are the supply water temperature and return water temperature from the EPA 

loop. The return water temperature from the EPA loop reached temperatures that would have allowed 

condensing operation, yet it was remixed with return water from some of the other loops (which have 

significant bypass at the AHUs, though not at the VAV boxes) and the final return water temperature seen by the 

boilers returned into the 150°-170°F range, disallowing any condensing operation during this time period. 
Condensing Boiler Evaluation – Retrofit and New Construction Page 35

Figure 18 – Trend data for building 25 (2/7/12-2/14/12) 

VI. Summary Findings and Conclusions

A. Overall Technology Assessment at Demonstration Facility

Condensing boilers offer the potential for reduction in natural gas usage through improved operating 

efficiencies. The steady-state efficiency increase can be anywhere from 6% to 18% over a standard natural gas 

fired boiler. This could offer Region 8 significant savings in many of its buildings. The combination of utility data 

and BAS trend data offered quality information on both the operation and the realized savings of the condensing 

boiler installations at the DFC. 

The utility data analysis showed that all of the installations were realizing significant savings. These savings 

ranged from 16% to 41%. This analysis included weather normalization to compare pre- and post-retrofit gas 

usage on equal grounds. 

The economic analysis demonstrated that all five of the projects had positive net present value (using a 25-year 

lifetime) for the case where a boiler replacement was necessary. Boiler replacements were necessary for four of 

the five installations. These four buildings all had existing boilers that were more than 20 years old. ASHRAE 

maintains a database on equipment lifetimes and the replacement age for boilers in that database was 18-25 

years (depending on exact boiler type).

xvii Only Building 810 made the replacement only for improved efficiency. 

One of the criteria set out in the objectives section was achieving condensing mode operation for more than 

20% of the year. Analysis of the BAS trend data showed that only two of the five boiler plants achieved this goal 

during the monitored period. The higher return water temperatures seen by the boilers led to calculated annual 

efficiencies ranging from 87% to 93% depending on firing rate. When the boiler staging control sequence was 

taken into account (to determine approximate firing rate), it was estimated that those efficiencies were in the 

88% to 90% range depending on the building. These efficiencies could be improved if the return water 
Condensing Boiler Evaluation – Retrofit and New Construction Page 36

temperature to the boilers was brought down further in to the condensing zone (less than 130°F) for a larger 

portion of the year. Increased hours in condensing mode could be achieved in a number of different ways. Some 

methods require little upfront investment. A more comprehensive approach is the piping retrofit discussed in 

subsection VI-B, Best Practices, below. 

Low cost/No cost: 

• A more aggressive OAT reset to reduce the supply water temperature and therefore the return water 

temperature (good approach, unless occupant complaints are registered due to reduced heating 

delivery). 

• Another control option that can save more energy than an OAT reset is a load based reset that resets 

supply water temperature based on heating coil valve position. For example if all of the valves are not 

100% open the supply water temperature would be reduced until one valve opens to 100% open. This is 

more complicated control logic, yet is also more precise control.

• Adjust the balancing valves in the bypasses on the three-way valves to limit the amount of water that 

bypasses the coil while still maintaining adequate valve authority (if VFDs are installed, closing bypass 

valves should be investigated, assuming sufficient head delivered by the pumps). 

Retrofit:

• Piping retrofits to address mixing of supply and return water streams. 

• Coil retrofit to higher ∆T coils that will return water below condensing temperature. 

A summary of the results and findings from the current study is presented here:

• Two out of five of the condensing boilers operated in condensing mode for more than 20% of their 

operating hours.

• All of the boilers that were replaced at the end of their life (4 of 5 of the boiler plants) achieved lower 

life-cycle costs than a conventional boiler plant.

• A systematic review of valving, hot water piping, and heating coils in each building can increase the 

efficiency of the condensing boiler plant by reducing return water temperatures.

• Low cost improvements include a lower outdoor air cutoff above which the boilers are shut off and 

checking to ensure that the cutoffs in place are working appropriately. A more aggressive outdoor air 

reset schedule should reduce the temperature of the return water and improve the boiler plant 

performance.

B. Best Practices 

Condensing boilers are an effective technology for reducing the energy consumed for space heating if best 

practices are followed during installation. As noted above, installing a modular condensing boiler plant alone 

does not guarantee savings beyond what near-condensing boilers can achieve, and there are a number of best 

practices related to the design and operation of a condensing boiler plant, the majority of which can be applied 

to retrofit installations.
Condensing Boiler Evaluation – Retrofit and New Construction Page 37

New Construction - High Performance Condensing Boiler Plant Example

The following design example (Figure 19) is intended to illustrate the design considerations and best practices 

for a high-performance condensing boiler heating system. The following schematic depicts a typical heating 

water system. 

Figure 19 – New Construction Condensing Boiler Plant Schematic

The system has the following components:

• A variable speed pump controlled to maintain a pressure differential set point between supply and 

return piping.

• Three condensing boilers, each with an isolation valve. A boiler controller sequences the boilers 

according to a control strategy designed to optimize system performance.

• A recirculation control valve operates at low load to maintain a minimum flow rate through the 

operating boiler.

• Two heating coils and a hot water baseboard unit are depicted. Each is equipped with a two-way control 

valve to control coil heat output to a leaving air or room temperature set point. 
Condensing Boiler Evaluation – Retrofit and New Construction Page 38

The representative design goal for a high performance condensing boiler plant should be similar to:

• Meet the design 100% heating load with a 180 ̊F supply water temperature and a 120 ̊F return water 

temperature. 

• Maintain a 120 ̊F return water temperature through full load range down to 5% load. The supply water 

temperature may be reset downward as loads decrease.

High Performance Condensing Boiler Plant Design Elements

A listing of the high performance elements that should be included in the design are provided and discussed in 

relation to the hypothetical schematic.

Primary-only vs. primary-secondary distribution system

Figure 19 depicts a primary-only piping system; meaning the system has one variable-speed pump that provides 

variable flow rates through both boilers and coils. Traditional systems with multiple boilers often use primarysecondary piping where each boiler has a dedicated constant-speed primary pump and the coils share a 

variable-speed secondary pump. Primary-secondary piping is not recommended for condensing boilers. The 

problem with primary-secondary is, if boiler and load flow rates are not exactly the same, there will be mixing of 

supply and return water. Mixing will result in either elevated return water temperatures to the boilers or 

reduced supply water temperatures to the coils. Designing a high ∆T system requires minimizing mixing of 

supply and return water. A primary-only system almost always matches boiler and load flows and should be 

implemented when designing and installing condensing boiler plants.

Boiler Turn-Down Ratio

Condensing boilers are typically designed with a low burner turn-down ratio to 20% or lower. Some condensing

boilers have a turn-down ratio as low as 5%. If loads drop below the turn-down ratio, the boilers must cycle on 

and off to meet the load, which significantly reduces the efficiency of the boiler. A boiler with a low turn down 

ratio should be selected. The boilers at the DFC have a 5:1 turndown ratio.

Boiler Flow Requirements

Minimum and maximum operating water flow rates (gpm) are usually more important than the burner turndown ratio. For instance, the boilers in the example can turn down to 5% at the gas burner, but the water flow 

through the boiler can only be turned down to 15%. Thus, operation at low load will result in a low ∆T across the 

boiler and throughout the entire system. Some condensing boilers have a minimum flow ratio as high as 40%, 

which isn’t likely to work for most high-performance systems. A boiler with a low minimum flow ratio should be 

selected. The boilers installed at the DFC do not have a minimum flow requirement. 

Boiler Sizing

Accurate equipment sizing is important to the operational performance for any system; operation below 

minimum turndown must be minimized for good energy performance. Low-mass boilers can be installed as 

modules in banks of 2 to 5 boilers. The controls then enable the number of boilers required to meet the load. 

Larger and smaller boilers can be combined into modular systems and controlled to cover the full operating 

range of heating and flow demands. For condensing boiler plants multiple smaller boilers should be designed to 

meet a peak heating load rather than one large boiler.
Condensing Boiler Evaluation – Retrofit and New Construction Page 39

Boiler Sequencing

Boiler sequencing can be complex given multiple boiler sizes, varying burner and minimum flow turn-down, 

boiler performance characteristics, temperature reset strategies, and the desire to optimize efficiency. Boiler 

sequencing strategies include the following:

• Traditional series control in which one boiler is fully loaded before the next boiler is brought on line.

• Parallel control in which boilers are operated in parallel at low loads; condensing boilers are most 

efficient at low loads provided they operate above their minimum turn-down ratios.

For boilers with a minimum flow ratio of 40%, the above sequences may be nearly identical. In general, parallel 

control is the recommended control sequence for condensing boilers.

Heating Coil Selection

Proper heating coil selection is the most important element of designing a high-performance heating water 

system. Traditionally, a ∆T on the water side is used to size heating coils. High-performance systems require coil

selection at a much higher ∆T, 60 ̊F, for this example. Performance of a hot water heating coil is given by the 

following equation:

Q = (mĊ p)min × ε × (EWT − EAT) 

Where

Q = Rate of heat transfer (Btu/hr)

(mCp)min= Minimum fluid capacitance rate (Btu/hr-°F)

ε = Heat exchanger effectiveness

EWT = Entering water temperature (°F)

EAT = Entering air temperature (°F)

Fluid capacitance rate is the product of the mass flow rate and the fluid specific heat for both the air and water 

sides of the coil. For water-side control (control valve), the water-side capacitance rate must be less than the airside capacitance rate at design conditions. The heat exchange rate is a function of the minimum capacitance 

rate and the control valve will vary the minimum capacitance rate to control the leaving air temperature. Coils 

may be selected using any manufacturer’s coil selection software and coils with a 60 °F ∆T are recommended for 

condensing boiler plants.

Hot Water Baseboard Design

Hot water baseboard selection is more difficult than forced air coil selection. Hot water baseboard relies on the 

air temperature exiting the unit to drive the airflow. As a result, the heat output is strongly affected by the 

entering water temperature as the adjacent chart indicates. However, it is possible to achieve a low leaving 

water temperature by adding additional baseboard length. The next chart (Figure 20) indicates the heat output 

along 60 feet of baseboard operating with a flow rate of 0.35 gpm and a supply water temperature of 180 ̊F.
Condensing Boiler Evaluation – Retrofit and New Construction Page 40

Figure 20 - Hot Water Baseboard Temperature

A return water temperature of 120 ̊F is achieved with 30 feet of baseboard length. The average heat output is 

also a function of entering water temperature and design of Hot water baseboard for high-performance 

condensing boiler plants requires a model to calculate heat output based on supply water temperature, gpm, 

pipe diameter, and baseboard length. Hot water baseboard designs should target leaving water temperatures of 

120°F at design conditions.

Control Valve Selection

Control valves can negatively affect the ability of the system to maintain a high ∆T at low loads. Theoretically, 

fluid ∆T will increase at low load; restricting the water-side capacitance (flow) with the control valve increases 

the ∆T. Poor performance is caused by imprecise control at low flow rates, which causes the valve to cycle 

between fully closed and a valve position where stable control is possible. The cycling results in higher return 

water temperatures from the coil. For example, the figure indicates a typical hot water control valve operating 

with a constant supply water temperature; the valve loses its ability for proper control at 40% load where the 

curve gets close to vertical. As can be seen in Figure 21, the performance can be greatly improved with a supply 

water temperature reset strategy based on outdoor air temperature or on response to control valve position. It 

is recommended that all high-performance systems be equipped with modern pressure independent control 

(PIC) valves. PIC valves can provide accurate control down to the range of 0.5% load.
Condensing Boiler Evaluation – Retrofit and New Construction Page 41

Figure 21 – Control Valve Capacity versus Flow Ratexviii

Hot Water Recirculation Control

Most systems are designed for some hot supply water flow into the return piping without going through a 

heating coil. Design piping features may include:

• A recirculation pipe connecting the supply and return piping at the end of the supply piping to ensure 

hot supply water is available to all heating coils and that pumps are not forced to operate with zero 

flow, which could over heat the motor. The pipe may include a balancing valve.

• Similarly, the last control valve at the end of the supply piping may be equipped with a three-way valve 

serving the same purpose as the recirculation pipe described above.

• A recirculation control valve may be included to maintain the required minimum flow through the 

boilers. This valve is controlled to maintain a water flow set point at a flow measuring device in the 

supply piping leaving the boilers, or to maintain a minimum pressure drop across the boilers 

corresponding to the boiler minimum flow. This feature is nearly unavoidable in a primary-only system.

It is recommended that the recirculation valve be installed towards the end of the supply piping to insure hot 

supply water temperatures at the end of the piping. Recirculation can be minimized through selection of boilers 

with low minimum flow rates in combination with multiple boilers sequenced for maintaining minimum boiler 

flow.

System Control

When it comes to optimizing hydronic system controls, the main focus is centered on reducing boiler supply 

water temperature to maximize boiler condensing time and efficiency. Control strategies, including optimal 

boiler sequencing, outdoor air reset, and load reset are commonly employed. 

• Optimal Boiler Sequencing for a variable-flow primary-only distribution system is dominated by the 

minimum boiler water flow (gpm). If the flow turn-down is 20% and the boiler is operating at 15% load, 
Condensing Boiler Evaluation – Retrofit and New Construction Page 42

roughly 25% of the hot water supply must be recirculated to the return water header. This characteristic 

favors a control strategy of loading one boiler fully before bringing the next one on. However, operating 

more boilers at lower load fractions will improve efficiency, if recirculation is avoided. Boiler efficiency is 

more sensitive to supply water temperature than percent load, as illustrated in Figure 22. 

Figure 22 – Boiler Efficiency Curve

• Outside Air Temperature (OAT) Reset is common boiler control strategy that reduces boiler return 

water temperature in response to OAT. Traditionally OAT reset is focused on reducing control valve 

cycling at low flow, but OAT reset also will reduce boiler supply water temperature and, thus, increase 

condensing boiler efficiency. For each installation the site is encouraged to implement the most 

aggressive outside air reset that they can for the given installation. A previous GSA Green Proving 

Grounds report on condensing boilers entitled ‘Condensing Boiler Assessment: Peachtree Summit 

Federal Building Atlanta, Georgia’ has a good example of an aggressive outside air reset schedule that 

could be applied to future installations and would have resulted in more hours in condensing mode for 

this installation.Error! Bookmark not defined.

• Load Reset is like OAT reset, except boiler supply water temperature is reset in response to zone 

temperatures or control valve position. With the feedback from the loads, load reset is more effective 

than OAT reset and is the recommended control sequence for condensing boiler plants.

Retrofit - Condensing Boiler Plants

The design and system integration options are limited when installing condensing boiler plants as a retrofit. The 

major obstacle is addressing the design ∆T of the existing hot water coils, which are not cost effective to replace 

unless the entire HVAC system is being upgraded through a facility modernization. Of the best practices listed 

above, the following apply to retrofit projects:

• Primary-only vs. primary-secondary distribution system

• Boiler Turn-Down Ratio
Condensing Boiler Evaluation – Retrofit and New Construction Page 43

• Boiler Flow Requirements

• Boiler Sizing

• Boiler Sequencing

• Control Valve Selection

• Hot Water Recirculation Control

• System Control 

Each of these best practices should be implemented for condensing boiler plants. The boiler plants installed at 

the DFC were installed correctly as modular systems with the proper controls to ensure that the boilers 

maintained a low firing rate throughout most of the year. The installation of an outdoor air reset reduced the 

supply hot water temperature on warmer days, which, in turn, reduced the return water temperature to allow 

the boilers to operate in condensing mode more often. The installation of a condensate treatment system in the 

boilers at DFC reduces the environmental impact of the acidic condensate water. 

Improvement could be made to the existing system to further improve the efficiency of the system, such as the 

implementation of a load based reset, more aggressive outdoor air reset, installation of pressure independent 

control valves, removal of three-way control valves and the installation of variable speed drives on booster 

pumps. This will reduce mixing and lower the return water temperature. The economics of addressing these 

issues was analyzed for building 25. A total of 28 AHU’s serve the office space in building 25. The AHU’s are 

currently set up with a small hot water booster pump and a two way control valve with a bypass. The booster 

pumps are rated at 1⁄2, 3⁄4 and 1 hp depending on the size of the AHU. Figure 23 shows the piping arrangement for 

the control valve and bypass leg. The supply water into the heating coil is modulated with the two-way control 

valve and the excess water that is diverted through the by-pass pipe. The supply water that by-passes the coil is 

mixed with return water.

Figure 23 – Two Way Control Valve with Bypass

Image Credit: Jesse Dean (NREL)
Condensing Boiler Evaluation – Retrofit and New Construction Page 44

Figure 24 – Schematic of Two Way Control Valve with Bypass

One potential solution to eliminating the mixing problem would be to close the balancing valve on the by-pass 

leg and turn off the booster pumps. If the booster pumps need to operate to overcome the static pressure 

within the unit, then the site can explore the option of installing a variable frequency drive on each pump and 

modulating the booster pump flow rate based on hot water valve position. The installed costs were estimated 

assuming the average booster pump size was 3⁄4 hp. The associated costs would consist of installing a small VFD 

and a new control point. The cost per control point was estimated as $1,000 per point and the installed costs 

were estimated with RSMeans and were 50% higher than the values provided in RSMeans Electrical Cost Data 

2011.

Table 14 – Building 25 Variable Speed Booster Pump Cost Estimate

The energy savings were estimated using a sensitivity analysis that reduced the return water temperature in 

increments of 5%, 10%, and 20%. The current efficiency at the annual average operational firing rate of 50% is 

88.2%. The new boiler efficiency, if the return water temperature is reduced by 5%, 10%, and 20% is 89.5%, 

90.8%, and 93.2% respectively. The annual cost savings were estimated to range from $2,826 to $8,386 per year, 

which includes the cost savings from the increased boiler efficiency and electricity savings from the variable 

speed booster pumps. The simple payback ranged from 11 to 33 years. This modification has the potential to be 

cost effective. For this particular building the installed costs would not have been reduced if this was 

implemented when the boiler plant was installed since the modifications need to be made at the AHU’s 
Condensing Boiler Evaluation – Retrofit and New Construction Page 45

themselves. Additional analysis would be required to determine the specific percent reduction in return water 

temperature and the project would need to bid out to determine the exact installed costs. The specific reduction 

in return water temperature could be estimated with an energy modeling program. If some of the booster 

pumps can be eliminated, the savings have the potential to be even greater at reduced initial costs.

For the facilities that do not have CV booster pumps and have more traditional three-way control valves and CV

primary pumps, the energy savings from the variable flow primary pump will typically pay for itself with 

electrical energy savings alone, and the reduced boiler energy use would only further reduce the total energy 

savings and reduce the payback period. The hot water piping system for building 54 is currently set up as a CV 

system. The three-way control valves on the 27 AHU’s should be replaced with two way control valves, as this 

would significantly reduce pumping energy use and boiler energy use. Building 810 has limited potential to 

reduce the return water temperatures due to the high entering water temperatures required by the unit heaters 

in the warehouse and the baseboard radiators. Although the energy savings will not be as significant in buildings 

810 and 45, this measure should be evaluated to determine the cost effectiveness for each facility. Building 

710A is an ideal candidate for control adjustments to reduce the return water temperature. All of the heating is 

controlled by VAV boxes equipped with two-way valves. Experimenting with the outdoor air reset to lower the 

supply water temperature could result in significantly colder return water temperatures. Although no hot water 

bypass of the VAV systems was pointed out by the staff, it is likely that one is installed to maintain minimum 

flow through the boilers. Adjusting the control of the valve in the bypass to maintain only the minimum flow 

though the boilers will minimize mixing of supply water and return water and could further reduce return water 

temperatures. These low cost items should pay for themselves quickly in building 710A. 

Condensing Boiler Design Tool

There are many variables to consider in designing a high performance system, too many for the human mind to 

consider simultaneously. Therefore, Group14 Engineering Inc. developed a Condensing Boiler Design Tool

(CBDT), to predict system performance over the full range of part-load operating conditions. Design information 

is entered on AHU heating coils, terminal unit reheat coils and baseboard radiators within the facility. Boiler and 

control valve characteristics, control options and set points are all entered into the tool to determine the 

impacts of the total system design on the operational efficiency of the boiler plant. The CBDT determines the 

boiler supply water temperature from the system and uses conditional formatting to identify problem areas, 

such as high return water temperature, control valve cycling, and coil bypass operation. The user can address 

each performance issue individually at each component to optimize performance. In the future, the CBDT will 

calculate the seasonal efficiency of the system and determine annual operating costs. The excel based design 

tool is available upon request from Sue Reilly at Group14 Engineers (sreilly@group14eng.com) and can help 

inform both design decisions and energy model input parameters. 

C. Barriers and Enablers to Adoption

A condensate treatment system, shown in Figure 25, may be required to treat the acidic condensate before it 

enters the drain. All of the boiler installations at the DFC included condensate treatment. The increased cost for 

installation of this piece of equipment, as well as the stainless steel exhaust flue, increases the installation costs 

significantly, compared to a standard boiler. 
Condensing Boiler Evaluation – Retrofit and New Construction Page 46

Figure 25 – Example condensate neutralization system

Image Credit: Cleaver Brooks

In addition to the boiler system costs, there are potential hydronic system upgrades that need to be monetized 

and accounted for. As shown above, the correct integration of the condensing boiler technology with the rest of 

the HVAC system is critical to achieving maximum return on investment. The complexity involved in the retrofit 

of the hot water system can be a barrier to entry. Development of a process for each system assessment is 

recommended to ensure consistency in the decision making. 

D. Market Potential within the GSA Portfolio

Condensing boilers should first be considered for retrofit in buildings that already return water temperatures 

below 130o

F throughout much of the year. The building automation system in many building can provide this 

information. Typical candidates include buildings with low temperature heating systems, such as in-floor radiant 

heating, and buildings with hot water loops that can easily be retrofitted to avoid coil bypasses and three way 

valves. In buildings where it is impossible or difficult to avoid return water temperatures above 130o

F, high 

efficiency near-condensing boilers, with thermal efficiencies above 84%, may be more cost effective. It is 

important to note that basic steps, like a more aggressive outdoor air temperature reset and balancing the 

three-way valves, should be taken before a conclusion can be made regarding the return water temperature. 

Comprehensive retrofits, including replacement of three-way valves with two-way valves and installation of 

VFDs on the pumps should be considered as well. 

To conduct an assessment of the cost effectiveness of the condensing boilers at the DFC, and evaluate the larger 

market potential for the GSA portfolio, market research was conducted on the installed costs of condensing 

boilers. This market research covered numerous sources including: RSMeans, the Consortium for Energy 

Efficiency, NREL’s National Residential Efficiency Measures Database, and the actual installed costs from the 

DFC. Information available for the initial costs of commercial, condensing boilers is limited in general. The results 

from the market research are listed here and an “average” installed cost is used to evaluate the condensing 

boiler technology. 

RSMeans costs were calculated by taking the average cost of three small boilers listed in RSMeans. These boilers 

had an average output of 167 MBH and an average Annual Fuel Utilization Efficiency of 83%.

xix This average cost 

was $50.56/MBH. The Consortium for Energy Efficiency’s market analysis estimated the initial cost of 

condensing boilers to be approximately 3 times that of a conventional boiler.

iii At that time, the market for 
Condensing Boiler Evaluation – Retrofit and New Construction Page 47

condensing boilers was only around 2% of total boiler sales, according to the Consortium. NREL’s National 

Residential Efficiency Measures Database estimates the average cost of a condensing boiler retrofit to be 

$55/MBH. The three larger condensing boiler plants at the DFC, with a total output of greater than 1,000 MBH, 

averaged only $41.36/MBH. Discussions with two local boiler distributors (including the installer for the DFC 

projects) placed the installed cost of condensing boilers at approximately 175% to 200% the cost of a standard 

boiler. The cost of a near-condensing boiler was estimated to be roughly 10% less than a condensing boiler. 

Near-condensing boilers require the same stainless steel flue that condensing boilers require, which increases 

the installation cost to almost that of a condensing boiler.

The estimate from NREL’s database and the three large DFC installation costs were averaged to calculate the 

expected initial costs for condensing boiler installations used in this analysis. For simple payback and net present 

value calculations with the actual DFC installed costs, see the Utility Bill Analysis subsection located in the 

Results section (pg. 24). The standard boiler installed costs shown in Table 15 were gathered from RSMeans 

(including removal costs of the existing systems), and near condensing costs were calculated as 10% less than 

the condensing boiler cost (as per the distributor’s estimate).

Table 15 – Installed cost estimates for the three boiler types

Boiler Type Rated 

Efficiency Installed Cost ($/MBH)

Standard 80% 26.21

Near-Condensing 86% 43.46

Condensing 86-98%1 48.18

1

Depending on return water temperature and part load operation.

The costs established here were compared with the cost estimates documented by Pacific Northwest National 

Laboratory (PNNL) in a report assessing condensing boiler performance at a GSA site in Atlanta, Georgiaxx. The 

costs established by PNNL are shown in Table 16. 

Table 16 – Initial cost estimates established by PNNL report

Boiler Type Rated 

Efficiency Installed Cost ($/MBH)

Standard 80% 35.40

Near-Condensing 84% 38.50

Condensing 95% 42.60

It can be noted that that the costs for the condensing boilers are very similar between the PNNL report and the 

current report (12% difference between estimates). The near condensing costs are also similar, especially 

considering that the higher cost shown in Table 15 is for a slightly higher efficiency model (which incurs more of 

the high cost stainless steel). 

Table 17 shows the economic evaluation of replacing the existing boilers with condensing boilers for each of the 

five sites at the DFC. The initial costs used for the condensing boiler systems were based on the normalized cost 

of $48.18/MBH, established in this report. The cost of the standard replacement boilers were estimated using 

the 86% efficient boiler costs from Table 15. The standard replacement costs were used in calculating the 
Condensing Boiler Evaluation – Retrofit and New Construction Page 48

incremental cost when a replacement is necessary, therefore it was necessary to use a system cost that would 

reflect the typical replacement option. It was assumed that GSA would replace older boilers with at least a nearcondensing system in order to pursue energy reduction on-site and stay compliant with current federal 

requirements. When calculating the incremental energy cost savings of the condensing boiler over the nearcondensing replacement option, the calculated energy cost savings were multiplied by the ratio of the rated 

efficiency improvement (95−86

95−80 or 60% of the savings achieved by the condensing boilers).

Table 17 – Calculated simple payback and net present value for the five sites for condensing boilers 

(using normalized initial costs)

Replacement Necessary Replacement Optional

Building

Initial Cost 

Replacement 

Boiler 

Initial Cost 

Condensing 

Boiler 

Incremental 

Energy Cost 

Savings 

Simple 

Payback 

Period 

(Years) 

Net 

Present 

Value

Energy 

Cost 

Savings 

Simple 

Payback 

Period 

(Years) 

Net Present 

Value

25 $629,084 $697,406 $15,326 4.46 $254,452 $25,544 27.30 -$159,449

45 $183,836 $203,801 $6,361 3.14 $114,001 $10,602 19.22 $19,477

54 $524,236 $581,171 $9,161 6.21 $136,004 $15,269 38.06 -$259,606

710A $122,557 $135,868 $2,344 5.68 $36,046 $3,906 34.78 -$53,607

810 $629,084 $697,406 $9,821 6.96 $138,504 $16,368 42.61 -$352,695

Net present value was calculated according to NIST Handbook 135xxi with an assumed boiler life of 25 years, a 

discount rate of 3% and natural gas as a fuel for a commercial building located in Colorado. The life expectancy 

of 25 years is the estimate provided by the Federal Energy Management Program’s Energy Efficient Product 

Procurement guide.

xxii The replacement necessary category considers the difference in costs of a standard 

versus a condensing boiler, while the replacement optional category considers only the cost of a new, 

condensing boiler. 

E. Recommendations for Installation and Commissioning

One important factor in achieving an economic condensing boiler retrofit is to size the boiler plant so that it 

operates efficiently without purchasing more heating capacity than is necessary. In the data analyzed for 

building 710A for the period beginning February 22 and ending April 1, the actual heat output of the plant is only 

around 10% of the maximum output that the boiler plant is capable of producing. For every boiler plant 

replacement, a heating load calculation should be conducted by an engineer to ensure that a plant is installed 

that meets the load, while limiting the size of the plant to a minimum to keep installed costs at a minimum.

Relying on the size of the previous plant is not a sizing method, given changes that have occurred to the 

buildings, as well as having to rely on previous sizing methods that may or may not be accurate. 

A potential boiler plant design that warrants further investigation by GSA is the use of several condensing boilers 

that serve as the primary boilers, coupled with an existing conventional boiler that serves as a backup. The 

condensing boilers could be sized to serve the heating load approximately 75% of the year, and the conventional 

boiler will only be used as backup during the coldest weather. This set up would keep initial costs at a minimum, 

while providing the potential to operate in condensing mode for most of the year. This strategy, coupled with an 

aggressive outdoor air reset during moderate weather, could allow the condensing boilers to operate in 

condensing mode throughout much of the year without significant investments in new valves, piping, and coils. 

Of course, the return water temperatures are building-specific and must be treated as such. Conducting an 
Condensing Boiler Evaluation – Retrofit and New Construction Page 49

independent analysis of the heating system of each building that receives a boiler retrofit is the surest way to 

achieve condensing operation as often as possible while meeting GSA’s stated objective of seeking the lowest 

life cycle cost system. 

As stated throughout the report a systems approach to condensing boiler installations will help to achieve the 

high operating efficiencies that condensing boilers can provide. Taking from the lessons learned in this study and 

following the best practices outlined in this report will assist in achieving the highest level of savings for the 

capital investment.
Condensing Boiler Evaluation – Retrofit and New Construction Page 50

Appendices

A. Utility Regression Analysis 

Utility regressions, often described as inverse modeling, were performed on the natural gas use within the 

buildings before and after the condensing boiler plants were installed. The regressions analyze natural gas usage 

per day vs. heating degree days per day to estimate the relationship between outdoor temperature and gas 

usage. The analysis attempts to normalize natural gas consumption over different weather conditions so that 

natural gas usage can be compared before and after the new boiler plants were installed over a year with 

average weather conditions. 

y = 2.2609x + 20.3

R2 = 0.9709

y = 2.5631x + 4.2233

R2 = 0.9848

0

20

40

60

80

100

120

140

0 10 20 30 40

Gas Usage [Dekatherms/day]

Heating Degree-Days per Day [°F]

Pre Retrofit

After Retrofit

Linear (After Retrofit)

Figure 26 – Building 25 Utility Regression Analysis
Condensing Boiler Evaluation – Retrofit and New Construction Page 51

y = 0.8796x - 0.8979

R2 = 0.9837

y = 0.6471x - 0.2585

R2 = 0.9499

-5

0

5

10

15

20

25

30

35

40

0 10 20 30 40 50

Gas Usage [Dekatherms/day]

Heating Degree-Days per Day [°F]

Before Retrofit

After Retrofit

Linear (Before

Retrofit)

Linear (After

Retrofit)

Figure 27 – Building 45 Utility Regression Analysis

y = 2.07x - 0.3897

R2 = 0.9667

y = 1.7474x - 3.2007

R2 = 0.9355

-10

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50

Gas Usage [Dekatherms/day]

Heating Degree-Days per Day [°F]

Before Retrofit

After Retrofit

Linear (Before

Retrofit)

Figure 28 – Building 54 Utility Regression Analysis
Condensing Boiler Evaluation – Retrofit and New Construction Page 52

y = 0.2296x - 0.2684

R2 = 0.8553

y = 0.1428x - 0.1316

R2 = 0.9434

-2

0

2

4

6

8

10

12

0 10 20 30 40 50

Gas Usage [Dekatherms/day]

Heating Degree-Days per Day [°F]

Before Retrofit

After Retrofit

Linear (Before

Retrofit)

Linear (After

Retrofit)

Figure 29 – Building 710A Utility Regression Analysis

y = 3.2373x + 2.1968

R2 = 0.9674

y = 2.7035x + 0.9749

R2 = 0.9591

0

20

40

60

80

100

120

0 10 20 30 40

Gas Usage [Dekatherms/day]

Heating Degree-Days per Day [°F]

Before Retrofit

After Retrofit

Linear (Before

Retrofit)

Linear (After

Retrofit)

Figure 30 – Building 810 Utility Regression Analysis
Condensing Boiler Evaluation – Retrofit and New Construction Page 53

B. Efficiency Analysis with Metered Data

To calculate the annual efficiency of the boilers, an hourly estimate of the return water temperature is needed 

for each building’s boiler plant throughout the year. To estimate the return water temperature throughout the 

year, a linear regression was performed with the metered data. These regressions are shown below. Hourly TMY

weather data was used to estimate the return water temperature throughout the year. The boilers are modeled 

as being off if the outside temperature is above 72°F, due to the outdoor air reset controls. The return water 

temperature was used to calculate boiler efficiency at each firing rate by performing a linear interpolation of the 

efficiency of each boiler at the different return water temperatures. Interpolating was necessary, because the 

boiler manufacturer only provides efficiencies at several return water temperatures.

y = -0.786x + 188.89

R2 = 0.8538

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40 50 60 70 80 90 100

Hot Water Return Temperature (F)

Outfoor Temperature (F)

Figure 31 – Building 25 Hot Water Return Temperature
Condensing Boiler Evaluation – Retrofit and New Construction Page 54

y = -0.9208x + 192.52

R2 = 0.8476

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40 50 60 70 80 90 100

hot Water Retrun Temperature (F)

Outdoor Temperature (F)

Figure 32 – Building 45 Hot Water Return Temperature

y = -1.0252x + 196.89

R2 = 0.8903

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40 50 60 70 80 90 100

Hot Water Return Temperature (F)

Outdoor Temperature (F)

Figure 33 – Building 54 Hot Water Return Temperature
Condensing Boiler Evaluation – Retrofit and New Construction Page 55

y = -0.7515x + 172.75

R2 = 0.5798

0

20

40

60

80

100

120

140

160

180

0 10 20 30 40 50 60 70 80 90 100

Hot Water Return Temperature (F)

Outdoor Temperature (F)

Figure 34 – Building 710A Hot Water Return Temperature

y = -1.3266x + 199.95

R2 = 0.8345

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40 50 60 70 80 90

Hot Water Return Temperature (F)

Outdoor Temperature (F)

Figure 35 – Building 810 Hot Water Return Temperature
Condensing Boiler Evaluation – Retrofit and New Construction Page 56

C. Calculation of Average Firing Rate in Building 25 

The average firing rate in building 25 was estimated by comparing the outdoor air temperature with the heat 

produced by the boiler plant over a TMY in Denver. Boiler heat output can be calculated from the metered data, 

using the formula: Q̇ = ṁ Cp∆T = 500.4 × GPM × (SWT − RWT). The heat output calculated from the 

metered data was correlated to the outdoor air temperature (this was done for both occupied and unoccupied 

hours). The resulting linear regression was applied to hourly data throughout a TMY data set. Once a load was 

estimated for every hour, the staging control logic was implemented to predict the firing rate for each boiler at 

every hour. This allowed for a prediction of annual firing rates for the boilers in building 25.

D. Glossary 

∆ T – Temperature Differential

AHU – Air Handling Unit

BAS – Building Automation System

BTU – British Thermal Unit

CV – Constant Volume

GPM – Gallons per Minute

HVAC – Heating, Ventilation, and Air Conditioning

LCC – Life Cycle Cost

MBH – 1,000 BTU per hour

OAT – Outdoor Air Temperature

O&M – Operations and Maintenance

RWT – Return Water Temperature

SWT – Supply Water Temperature

TMY – Typical Meteorological Year

VAV – Variable Air Volume

VFD – Variable Frequency Drive

 

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xix 2011 RS Means Green Building Cost Data. 1st Annual Edition. Construction Publishers and Consultants. Reed Construction 

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