Expert Meeting:

Optimized Heating Systems 

Using Condensing Boilers and 

Baseboard Convectors

L. Arena

Steven Winter Associates

January 2013
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Expert Meeting: Optimized Hydronic Heating Systems Using 

Condensing Boilers and Baseboard Convectors

Prepared for: 

The National Renewable Energy Laboratory 

On behalf of the U.S. Department of Energy’s Building America Program

Office of Energy Efficiency and Renewable Energy 

15013 Denver West Parkway 

Golden, CO 80401 

NREL Contract No. DE-AC36-08GO28308 

Prepared by: 

L. Arena 

Steven Winter Associates, Inc.

of the 

Consortium for Advanced Residential Buildings (CARB)

61 Washington Street 

Norwalk, CT 06854 

NREL Technical Monitor: Cheryn Metzger

Prepared under Subcontract No. KNDJ-0-40342-00 

January 2013 
iv

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v

Contents

List of Tables ............................................................................................................................................... v

Definitions................................................................................................................................................... vi

Executive Summary.................................................................................................................................. vii

1 Introduction........................................................................................................................................... 1

2 Meeting Agenda.................................................................................................................................... 3

3 Presentations........................................................................................................................................ 5

4 Discussion............................................................................................................................................. 6

5 Summary ............................................................................................................................................... 7

References................................................................................................................................................... 8

Appendix A: Presentation by Lois Arena, Steven Winter Associates, Inc........................................... 9

Appendix B: Updated Presentation by Xia Fang, National Renewable Energy Laboratory.............. 47

List of Tables

Table 1. Expert Meeting Agenda................................................................................................................ 3

Table 2. Expert Meeting Attendees ........................................................................................................... 4

Unless otherwise noted, all tables were created by CARB.
vi

Definitions

BEopt Building Energy Optimization

BNL Brookhaven National Laboratory

CARB Consortium for Advanced Residential Buildings

DOE U.S. Department of Energy

OAT Outside air temperature

VFD Variable-frequency drive
vii

Executive Summary

On August 11, 2011, a Building America Expert Meeting was held to review and discuss results 

and future plans for research to improve the performance of hydronic heating systems using 

condensing boilers and baseboard convectors. The meeting was held in Denver, Colorado, in 

conjunction with the Building America Residential Energy Efficiency Technical Update 

Meeting. A meeting objective was to create an opportunity for other Building America teams and 

industry experts to offer feedback and specific suggestions for the planned research. 

Lois Arena of Steven Winter Associates summarized the findings from previous Consortium for 

Advanced Residential Buildings (CARB) field monitoring research and presented the plan for 

further research to be conducted next year. CARB has been collaborating with Brookhaven 

National Laboratory (BNL) and industry manufacturing partners to design, install, and monitor 

condensing boiler space and water heating systems under occupied conditions. Previous research

included the monitoring of condensing boilers in six existing homes, targeted bench top testing 

by BNL, and monitoring of CARB-designed hydronic heating systems in three new homes.

Xia Fang of the National Renewable Energy Laboratory presented preliminary energy simulation 

results and identified gaps in modeling capability. Additional modeling work was done based 

upon comments raised during the meeting. An updated presentation of findings is included as an 

appendix to this report. 
1

1 Introduction

With annual fuel utilization efficiencies around 95%, condensing boilers and furnaces promise 

significant energy savings by recovering the latent energy of flue gas water vapor. Condensing 

boilers paired with low-temperature baseboard heating systems are one of the most cost-effective 

methods for heating high performance, and thus low load, homes. Ducted systems (furnaces and 

hydro-air systems) are often subject to reduced efficiencies resulting from duct leakage and fan 

energy requirements, and other hydronic distribution systems, such as radiant-floor systems and 

some specialized cast-aluminum radiation systems, are far more expensive. 

Although condensing boilers promise to deliver substantial improvements over conventional 

boilers, condensing boilers as typically installed (according to common practice and 

manufacturer recommendations) do not achieve consistent flue gas condensation. To ensure 

condensation, the return temperature to the boiler must be below the flue gas saturation 

temperature, which is generally 54°C (130°F) for natural gas equipment (Butcher 2006). 

For Building America to achieve its goals, it is important that applications of condensing boiler 

technology reliably achieve the intended efficiency and energy savings. In response to this need, 

CARB teamed with the New York State Energy Research and Development Authority, Ithaca 

Neighborhood Housing Services, and Brookhaven National Laboratory to evaluate the 

performance of condensing boilers using baseboard convector delivery systems. Specific 

research objectives included: (1) evaluating and defining the optimal operating parameters of the 

condensing-boiler/hot-water baseboard combination; (2) evaluating and defining the optimal 

operating parameters of the condensing boiler/indirect domestic hot water combination; (3) 

testing those findings in real-world residential settings; and (4) documenting those parameters in 

a technically accurate, installer friendly manner.

The initial phase of the project involved monitoring boiler performance in six existing homes in 

Ithaca, NY, beginning in late January 2009 and concluding in August 2009. In the next phase, 

information gained from the first phase was used to design and size systems for three new homes 

also located in Ithaca. Boiler performance in the new homes was tested and monitored to further 

define the best design parameters to ensure maximum boiler efficiencies.

Research findings were presented to industry leaders at a Building America Expert Meeting in 

October 2010, and although there was much agreement and acceptance of the issues presented, 

no agreement as to the best solutions was reached and further research was encouraged. Thus, 

the objectives of the planned research are as follows:

• Evaluate and define the optimal combinations of components and plumbing configuration 

to ensure highly efficient, reliable condensing boiler performance.

• Test those findings in real-world residential settings.

• Gain recognition and acceptance from the industry for the new designs.

• Document those parameters in a technically accurate, installer friendly manner.

• Disseminate the results to the larger residential building industry. 
2

During the expert meeting held on August 11, 2011, the details of the test plan were reviewed 

and discussed. A goal of the meeting was to determine whether the planned research was 

appropriate and comprehensive.
3

2 Meeting Agenda

The meeting was held at the Renaissance Hotel in Denver on August 11, the third day of the 

Building America Residential Energy Efficiency Technical Update Meeting. The meeting 

agenda and attendees are presented in Tables 1 and 2, respectively. 

Table 1. Expert Meeting Agenda

Time Item Presenter/Facilitator

9:00–9:05 Welcome and Meeting Overview Bill Zoeller, CARB

9:05–10:00

Previous Research: monitoring and evaluations of 6 

homes (Phase I and Phase II)

Lois B. Arena, CARB

Benchtop research conducted by Dr. Thomas 

Butcher, Brookhaven National Laboratory

Conclusions and open questions based on Phase I and 

Phase II results

Emerging Technologies (VFD pumps, Versa-Hydra, 

etc.)

Phase III: Proposed designs, collaboration with w/ 

industry partners. 3 new homes with 3 different 

systems.

10:00–10:30

Analysis of condensing boiler modeling in homes 

using BEopt/DOE-2 vs. BEopt/EnergyPlus

Xia Fang, NREL Addressing Issues with OAT reset with respect to 

modeling

Performance curve implementation issues in 

EnergyPlus vs. DOE-2

10:30–10:55 Feedback, questions, suggestions from 

teams/attendees All

10:55–11:00 Closing Remarks and Next Steps Bill Zoeller, CARB

Notes: VFD, variable-frequency drive; BEopt, Building Energy Optimization; OAT, outside air temperature; DOE, 

U.S. Department of Energy
4

Table 2. Expert Meeting Attendees

Name Company E-Mail Address

Charles Adams A.O. Smith Corporate Technology 

Center, Milwaukee, WI cadams@aosmith.com

Lois Arena Steven Winter Associates, Inc., 

Norwalk, CT larena@swinter.com

David Bohac Center for Energy and 

Environment, Minneapolis, MN dbohac@mncee.org

Larry Brand Gas Technology Institute, 

Des Plaines, IL larry.brand@gastechnology.org

Kim DeVoe City of Fort Collins, CO kdevoe@fcgov.com

Cheryn Engebrecht National Renewable Energy 

Laboratory, Golden, CO Cheryn.engebrecht@nrel.gov

Xia Fang National Renewable Energy 

Laboratory, Golden, CO xia.fang@nrel.gov

Dianne Griffiths Steven Winter Associates, Inc. dgriffiths@swinter.com

Philip Kerrigan Building Science Corporation phil@buildingscience.com

Bill Zoeller Steven Winter Associates, Inc. wzoeller@swinter.com
5

3 Presentations

Lois Arena’s presentation (Appendix A) reviewed the earlier research findings and detailed the 

specific designs to be installed and monitored in the next phase of research. She had given a 

more detailed presentation of the research findings the previous day as part of the Technical 

Update Meeting. 

Xia Fang’s presentation reviewed her initial effort to model the performance of condensing 

boilers and control strategies using EnergyPlus. A work-around using performance curves was 

utilized for the current version of EnergyPlus. Xia noted that more data should be gathered to 

validate the performance map used. Preliminary modeling results did not demonstrate substantial 

energy savings, but the system capacities were not optimized for each alternative modeled. 
6

4 Discussion

The modeling results were presented as preliminary and several suggestions were made. The

performance specifications for the modeled home, specifically the infiltration assumption, were 

questioned, and it was recommended that two house models would be appropriate—a new, 

tightly constructed home and an existing home. Utilities need to understand the impacts of 

different parameters to justify their programs. This additional modeling work was done after the 

meeting. Those findings are presented in the updated presentation located in Appendix B. 

There was also a concern that the cost of the OAT reset control applied for the cost-effectiveness 

analysis was too high. The preliminary conclusions on cost effectiveness have been removed 

from the updated presentation. 

Zone valve reliability was discussed with the group. The builder partnering with CARB for this 

research prefers to use circulators for each heating zone as opposed to zone valves. He believes 

that zone valves fail every few years and the cost to replace them outweighs the savings of 

installing high efficiency systems. The group was asked their opinion on this issue. None of the 

participants had experience or direct knowledge with the reliability of zone valves versus

circulators.

Larry Brand noted that the Gas Technology Institute has tested pumps in its laboratory and 

agreed that the trades believe in the “bigger pump is better” philosophy.

Installing condensing boilers in a retrofit application was discussed. It was agreed that, while 

baseboard convectors are probably adequate in existing homes because they are traditionally 

grossly oversized, existing high mass radiators may not be a feasible heat emitter when 

combined with a condensing boiler in a retrofit application. 

Charles Adams noted that AO Smith has acquired Lochinvar, and they might be interested in 

partnering with us during the next phase. It was mentioned that Lochinvar boilers have a built-in 

primary loop, which is one of the design elements that CARB is trying to eliminate because it 

contributes to higher return water temperatures and lower installed efficiencies. The designs 

currently under consideration eliminate primary/secondary loop configurations all together.

The application of hydrocoil systems was briefly discussed and, while of interest, it was 

acknowledged that they are not within the scope of this research. 
7

5 Summary

During this expert meeting, the details of the test plan to be implemented by CARB during the 

2011/2012 winter were reviewed and discussed. Meeting attendees were asked if the test plan 

was appropriate and comprehensive and if there were other design configurations that should be 

considered. Attendees supported the test plan and no alternative design suggestions were made 

at, or subsequent to, the meeting. Meeting attendees were invited to provide input to the test plan 

and proposed designs at any time.
8

References

Butcher, T. (2006). “Condensing Boilers and Baseboard Hydronic Systems.” ASHRAE 

Transactions (112:part 1). 
9

Appendix A: Presentation by Lois Arena, 

Steven Winter Associates, Inc.
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Appendix B: Updated Presentation by Xia Fang, National 

Renewable Energy Laboratory
NREL is 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.

CONDENSING BOILER MODELING IN 

BA HOMES

Condensing Boiler 

Expert Meeting

Xia Fang

Rev 1: 11-18-2011

Innovation for Our Energy Future
 Innovation for Our Energy Future

Residential Condensing Boilers

2

•Thermal efficiency 93%~98%

•Fully Modulating 5:1 Turn Down 2
 Innovation for Our Energy Future

Condensing Performance

3
4

DOE2:

Thermal Efficiency is a function of 

return water temperature, default 

efficiency is rated at 80 F HWRT.

EnergyPlus:

EnergyPlus v6.0 uses condensing 

efficiency as a function of supply 

water temp instead of return water 

temp.

New release of E+ v7 coming in Dec. 

Custom EMS program on 

performance curve as current work 

round.

80%

85%

90%

95%

100%

105%

75 100 125 150 175

PLR 1

PLR 0.783

PLR 0.625

PLR 0.466

PLR 0.254

HWRT (°F)

Thermal Efficiency

84%

86%

88%

90%

92%

94%

96%

98%

100%

95 120 145 170 195

PLR 1

PLR 0.783

PLR 0.625

PLR 0.466

PLR 0.254

HWST (°F)

Thermal Efficiency
 5 Innovation for Our Energy Future

86

88

90

92

94

96

80 100 120 140 160 180

PLR 1

0.783

0.625

0.466

0.254

%

HWRT (°F)

Created a regression fit of 

Boiler thermal efficiency as biquadratic

function of part load ratio and return 

water temperature.

TE = 87% @160 F HWRT

Condensing Performance

85% TE 95% TE 85% TE 95% TE

DOE2 EPLUS

DOE2 models loads and system 

separately: interactions caused by 

OAT reset cannot be properly 

captured.
 Innovation for Our Energy Future

Modeled House Syracuse, NY (EnergyPlus)

6

Efficient House

Tightest 1.9ACH50

2x6 24” OC stud, R21 batts

with 1” foam - R24.2

Vented Attic R60 cellulose 

blown in

Low-E Low-SHGC window

CFL lighting, EnergyStar

Appliance

R10 Whole Slab Insulation

Hydronic Heating, SEER 16 AC

Gas Premium Water Heater 

EF0.67

Typical House

Leaky 14.5ACH50

2x6 24” OC stud, R21 batts

with 1” foam - R24.2

Vented Attic R11 cellulose 

blown in

Dbl-pane Std window

40% Fluorescent Lighting, Std 

Appliance

2ft R5 Perimeter

Hydronic Heating, SEER 16 AC

Gas Standard Water Heater 

EF0.59

Building America House Simulation Protocol assumptions on operating conditions.

Thermostat Setpoint: heating setpoint 71 F w/setback 65 F 11PM – 6AM.
 Innovation for Our Energy Future

Control Strategies to Boost Performance

7

• Baseline 0: Boiler 80% TE

• Baseline 1: Boiler 85% TE

• ECM-1: Condensing up to ~95% Efficient

• ECM-2: ECM-1 + Outside air Temperature Reset

• ECM-3: ECM-2 + 160F HWST Baseboard Doubling Area

• ECM-4: ECM-3 + OAT Reset

• ECM-5: ECM-4+VFD Pumping

Claims: “For every 4°F the boiler water temperature is reduced, 

there is a 1% energy savings. Thus, if the boiler was run based 

on a fixed set point of 180°F vs. running the boiler using outdoor 

reset at 120°F will provide a minimum saving of 15%.”....

OAT (F) HWST (F)

5 180

68 95

Note: 95F is higher than the 

monitored houses which drops 

down to 80F.
 Innovation for Our Energy Future

Convective Baseboards in EnergyPlus

8

 At the same BB surface area, or heat transfer effectiveness ε: when boiler supply 

temp to BB (Twater,inlet) , BB outlet air temp (Tair,outlet) , and BB leaving water temp 

(Twater,outlet). . This modeling capability in E+ confirms:

• When designing condensing boiler HWST: in order to maintain BB outlet air 

temp, BB length / surface area needs to be properly increased for lower HWST.

• When operating: convective BB heat transfer effectiveness will change as a 

result of changing Twater,inlet, resulting in changing Tair,outlet. This effect can be 

properly captured in EnergyPlus.

Convective BB: air is pulled in and out based on natural convection effect. 
 Innovation for Our Energy Future

Efficient House Modeled Recovery– Jan 24th.

120

130

140

150

160

170

180

0

10

20

30

40

50

60

70

80

1/23/07 9:36

1/23/07 14:241/23/07 19:12

1/24/07 0:001/24/07 4:481/24/07 9:36

1/24/07 14:241/24/07 19:12

1/25/07 0:001/25/07 4:48

Outdoor Dry Bulb [F] LIVING:Zone Temp[F] Heating Setpoint [F]

LIVING:Zone Cooling Setpoint [F] BOILER:Boiler Water Inlet Temp [F] BOILER:Boiler Water Outlet Temp [F]

Deg F
 Innovation for Our Energy Future

Typical House Modeled Recovery– Jan 24th.

120

130

140

150

160

170

180

0

10

20

30

40

50

60

70

80

1/23/07 9:36

1/23/07 14:241/23/07 19:12

1/24/07 0:001/24/07 4:481/24/07 9:36

1/24/07 14:241/24/07 19:12

1/25/07 0:001/25/07 4:48

Outdoor Dry Bulb [F] LIVING:Zone Temp[F]

Heating Setpoint [F] LIVING:Zone Cooling Setpoint [F]

BOILER:Boiler Water Inlet Temp [F] BOILER:Boiler Water Outlet Temp [F]

Deg F
 Innovation for Our Energy Future

Efficient House Source Energy Consumption

11

Point 1: Gas 80% AFUE boiler

Point 2: Gas 85% AFUE boiler

Point 3: Gas 95% AFUE boiler, 180F HWST

Point 4: Gas 95% AFUE boiler, OAT reset on HWST control

Condensing Frequency 34%
 Innovation for Our Energy Future

Typical House Source Energy Consumption

12

Point 1: Gas 80% AFUE boiler

Point 2: Gas 85% AFUE boiler

Point 3: Gas 95% AFUE boiler, 180F HWST

Point 4: Gas 95% AFUE boiler, OAT reset on HWST control

Condensing Frequency 48%
 Innovation for Our Energy Future

Summary

13

1. OAT reset increases condensing efficiency when boiler is left at the 

factory setting of 180 F. 

2. OAT reset control on condensing boilers overrides space 

thermostat reset control, so the boiler does not know to fully fire 

to catch up the space heating setpoint. This system and load 

interaction is only captured in EnergyPlus.

3. The problem gets worse in efficient houses where space heating 

loads are less responsive to OA condition. It’s less of a problem in 

typical houses. 

4. To increase condensing frequency without sacrificing thermal 

comfort, optimization can be further explored among system 

options of thermostat reset, OAT reset, VFD pumping etc.

5. Condensing boiler savings ~10% source energy.

6. Future work: zoning for 2 story bldg; space heating and domestic 

hot water heating combined systems.
DOE/GO-102013-3482 ▪ January 2013

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