Measure Guideline: 

Condensing Boilers— 

Control Strategies for Optimizing 

Performance and Comfort in 

Residential Applications

L. Arena

Consortium for Advanced Residential Buildings

May 2013
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iii

Measure Guideline: Condensing Boilers—Control Strategies for 

Optimizing Performance and Comfort in Residential Applications

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 

61 Washington Street 

Norwalk, CT 06854 

NREL Technical Monitor: Cheryn Metzger 

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

May 2013 
iv

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v 

Contents

Contents....................................................................................................................................................... v

List of Figures ............................................................................................................................................ vi

List of Tables .............................................................................................................................................. vi

Definitions .................................................................................................................................................. vii

Executive Summary................................................................................................................................. viii

Progression Summary............................................................................................................................... ix

ix

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

2 Decision-Making Criteria ..................................................................................................................... 3

2.1 Cost and Performance ..........................................................................................................3

2.2 Risk Identification ................................................................................................................4

2.2.1 Oversized Systems ...................................................................................................4

2.2.2 Undersized Baseboard .............................................................................................4

2.2.3 Outdoor Sensor Placement .......................................................................................5

2.2.4 Specialty Components .............................................................................................5

2.2.5 Occupant Expectations .............................................................................................5

3 Boiler Controls and System Interactions........................................................................................... 6

3.1 Controls for Safety and Durability .......................................................................................6

3.2 Controls for Improved Efficiency ........................................................................................8

3.2.1 Heat Dumping ..........................................................................................................8

3.2.2 Burner Modulation ...................................................................................................9

3.2.3 Input Limits ..............................................................................................................9

3.2.4 Outdoor Reset ..........................................................................................................9

3.3 Control Strategies for Enhanced Occupant Comfort .........................................................11

3.3.1 Boost Controls .......................................................................................................11

3.3.2 Eliminate or Minimize Setback .............................................................................12

3.3.3 Raise Warm Weather Shutoff ................................................................................13

3.3.4 Outdoor Reset and Comfort ...................................................................................13

4 Measure Implementation ................................................................................................................... 14

4.1 Climate-Specific Factors ....................................................................................................14

4.2 Install Procedure ................................................................................................................14

4.2.1 Evaluate the Size of the Boiler ...............................................................................14

4.2.2 Evaluate Baseboard Capacity ................................................................................15

4.2.3 Determine the Set Points for the Outdoor Reset Curve .........................................16

4.2.4 Properly Locate the Outdoor Sensor ......................................................................19

5 Verification Procedures and Tests ................................................................................................... 20

5.1 Ensure the Outdoor Reset Functions Properly ...................................................................20

5.2 Evaluate Boost Control ......................................................................................................21

5.3 Additional Resources .........................................................................................................21

References................................................................................................................................................. 22

Appendix A: Bin Temperature Analysis ................................................................................................. 23

Appendix B: Prescriptive Measure Checklist......................................................................................... 27
vi 

List of Figures 

Figure 1. Condensing boiler with an indirect DHW system and outdoor reset control....................... 1

Figure 2. Flow diagram illustrating boiler operation when high limit or supply set point 

is reached.............................................................................................................................................. 7

Figure 3. Linear boiler reset curve .......................................................................................................... 10

Figure 4. Flow diagram illustrating the effects of a boost control on the boiler supply 

temperature......................................................................................................................................... 12

Figure 5. Wall hung condensing boiler................................................................................................... 14

Figure 6. Flow diagram illustrating iterative process in determining the best setting for Ts,max 

for the reset curve .............................................................................................................................. 17

Figure 7. Predicted boiler supply temperature at 30°F outdoor temperature..................................... 20

Figure 8. Process for evaluating outdoor reset settings using a bin temperature analysis............. 23

Figure 9. Boiler reset curve...................................................................................................................... 25

Unless otherwise indicated, all figures were created by CARB.

List of Tables

Table 1. Economic Analysis of Decreasing the Boiler Supply Temperature, Ts,max ............................. 4

Table 2. Predicted Frequency of Condensing for Various Supply Temperatures.............................. 10

Table 3. Comparison of Baseboard Output per Linear Foot for Single Pipe Versus Double Pipe 

Models ................................................................................................................................................. 16

Table 4 Default Output Ratings of Typical Finned Tube Baseboard ................................................... 18

Table 5. Bin Analysis Results for a Project in Ithaca, New York.......................................................... 24

Unless otherwise indicated, all tables were created by CARB.
vii 

Definitions

ACCA Air Conditioning Contractors of America

AHRI Air-Conditioning, Heating, and Refrigeration Institute

DHW Domestic hot water

gpm Gallons per minute

kBtu/h Thousands of British thermal units per hour

SWA Steven Winter Associates, Inc.

Tout,max Maximum outdoor temperature on the boiler reset curve

Tout,min Minimum outdoor temperature on the boiler reset curve, 

equivalent to the design heating temperature

Ts,max Maximum supply temperature that will be delivered at Tout,min

Ts,min Minimum supply temperature that will be delivered at Tout,max

UA Rate of heat loss, Btu/h °F

ΔT Temperature differential

 
viii

Executive Summary

The combination of a gas-fired condensing boiler with baseboard convectors and an indirect 

water heater has become a common option for high efficiency residential space heating in cold 

climates. Many condensing boilers with rated efficiencies in the low to mid 90% efficient range 

are available; however, if the control systems are not properly configured, these heaters may

perform no better than their noncondensing counterparts. 

Previous research suggests that these types of systems are typically not designed and installed to 

achieve maximum efficiency (Arena 2010). It was found that there is a significant lack of 

information for contractors on how to configure the control systems to optimize overall 

efficiency. For example, there is little advice on selecting the best settings for the boiler reset 

curve or how to measure and set flow rates in the system to ensure that the return temperatures 

are low enough to promote condensing. 

It has also been observed that recovery from setback can be extremely slow and, at times, not 

achieved. Recovery can be affected by the outdoor reset control, the differential setting on the 

boiler, and oversizing of the boiler. 

This guide is intended for designers and installers of hydronic heating systems interested in 

maximizing the overall system efficiency of condensing boilers when coupled with baseboard 

convectors. It is applicable to new and retrofit projects. 

Acknowledgments

Steven Winter Associates, Inc. (SWA) acknowledges the U.S. Department of Energy Building 

America program and its funding and support for the development of this technical report as well 

as research that informed it. SWA would like to thank John Siegenthaler for his invaluable 

consulting services throughout this project. SWA would also like to thank all the manufacturers 

and product suppliers who donated their time, expertise, and equipment: Dave Davis from Heat 

Transfer Products, Dave Scearce from Peerless, Mark Handzel with Bell & Gossett, Bob 

Reimund from Grundfos, and Parker Wheat with Emerson Swan. A special thanks to Scott 

Reynolds with Ithaca Neighborhood Housing Services for his continued support and the use of 

his homes for this research.
ix

Progression Summary

ss

Proceed with caution if:

• The house has or will have a noncondensing boiler

• The occupants are complaining about response time or 

desire to use setback. 

! !

Set Up Controls 

To Optimize 

Boiler 

Performance

Reasons for advanced controls. Besides enhancing efficiency, specialized boiler controls 

can improve occupant comfort and help to reduce wear on the boiler. The following steps 

are required to ensure these controls result in efficient operation and provide optimal 

comfort.

Size the Boiler

Step One: For advanced controls such as outdoor reset to perform as intended, 

the boiler must be properly sized for the loads. Perform load calculations in 

accordance with the Air Conditioning Contractors of America Manual J, and size 

the boiler according to Manual S for optimum performance and efficiency.

1

Size the Baseboard

Step Two: As with the boiler, it is essential that the heat emitters be properly 

sized for the loads. An undersized delivery system can lead to poor performance 

and comfort problems. Calculate the necessary length to ensure the boiler 

performs efficiently and provides the desired comfort levels. 

2

Determine the 

Control Settings

Step Three: Reset controls are an essential part of a hydronic system. They can 

dramatically increase system efficiency and can result in improved occupant 3 comfort. Determine the control settings that will best accomplish both tasks.

Locate the Reset 

Sensors

Step Four: Reset controls rely on sensors to inform the boiler about existing 

conditions. Locate these sensors away from exhaust or dryer vents, direct 

sunlight, or potential snow buildup, or anything that may result in a false 

temperature reading.

4

Verify Proper 

Operation

Step Five: Before leaving the job, verify the reset control is accurately controlling 

the boiler supply temperature and any boost controls (if installed) are functioning 5 properly.
1 

1 Introduction

The combination of a gas-fired condensing boiler with baseboard convectors has become a 

common energy-efficient solution for space heating in cold climates. When operating properly, 

condensing boilers paired with baseboard convectors and indirect storage tanks provide a highly 

efficient method of providing heat and domestic hot water (DHW) (see Figure 1). While the 

rated efficiencies of these boilers are high, it is imperative to understand that if the control 

systems are not properly configured, these heaters will perform no better than their 

noncondensing counterparts. 

Figure 1. Condensing boiler with an indirect DHW system and outdoor reset control

Condensing boilers achieve high efficiencies

because they can recapture the latent energy from 

the moisture in the combustion gases when those 

gases condense. For condensing to occur, the 

boiler’s heat exchanger surface temperature must be 

below the flue gas dew point. If the return water 

temperature is low enough, it will cool the heat 

exchanger below the dew point and the gases will 

condense. Therefore, strategies that lower the return 

water temperature will improve the boiler’s 

efficiency.

Latent Heat:

Latent heat is the heat that is 

added or removed due to a change 

in phase from a solid to a liquid or 

from a liquid to a vapor. In the case 

of a condensing boiler, it is the heat 

that is removed from the flue gases 

when the moisture changes from a 

vapor to a liquid.
2 

The most reliable way to ensure that the 

return water temperature remains below the 

dew point is to install an outdoor reset 

control that modulates the boiler’s output 

target temperature relative to the actual 

heating loads. When using this type of 

control, the boiler’s supply temperature 

decreases as the outdoor temperature rises, 

thus ensuring lower return water 

temperatures. The desired outcome is the 

matching of space heating needs with the 

heat delivered. 

Because the output closely matches the 

loads, using an outdoor reset control may 

result in comfort problems if the occupants 

desire to use a setback strategy overnight and during unoccupied hours. When the system has to 

raise the temperature in the space several degrees, this is an additional load above the normal 

operating load. Because the system output so closely matches the normal operating load, this 

may be difficult or impossible. It could take many hours for the home to be brought up to 

temperature after a period of setback when an outdoor reset control is used. In some cases, it may 

not be possible. 

A common solution is to incorporate a boost control that will override the outdoor reset if the 

thermostat is not satisfied within a specified time. The system efficiency will decrease slightly, 

but response time and comfort will improve.. 

These controls are widely available and commonly installed; however, previous research shows 

that these types of systems—condensing boilers paired with baseboard convectors—are typically 

not designed and installed to achieve maximum efficiency and performance (Arena 2010). It was 

found that there is a significant lack of information for contractors on how to configure the 

control systems to optimize overall efficiency. For example, there is little advice on selecting the 

best settings for the boiler reset curve or how to measure and set flow rates to ensure the return 

temperatures are low enough to promote condensing. 

This guide is intended to provide general guidance for heating contractors and hydronic 

designers on selecting the proper control settings to maximize system performance while 

maintaining occupant comfort.

Previous Research Findings 

(Arena 2010): 

• Installers typically do not adjust the 

maximum boiler supply temperature; thus, it 

is left at the factory setting of 180°F. 

• Flow rates through the boiler were typically 

3 gpm or higher. 

• Information for contractors on how to 

configure these systems to optimize overall 

efficiency is lacking. 

• Recovery from setback was extremely slow 

in all homes evaluated in the previous 

research effort and was often not achieved.
3 

2 Decision-Making Criteria

For condensing boilers to operate at peak performance, there are many controls involved. The 

basic functions of these controls are to protect the boiler, maximize efficiency, and ensure 

occupant comfort. Unfortunately, these do not always work together. Particular problems arise 

when the boiler is oversized, the baseboards are undersized, or the outdoor reset sensor is

improperly located or configured. 

For example, if the outdoor reset sensor is located near a heat source or in direct sunlight, it will 

convey to the boiler that the outside temperature is warmer than it actually is, resulting in a 

supply temperature to the conditioned space that 

is too low to meet the space heating demand.

The following sections describe the key steps in 

determining the optimal system settings along 

with control strategies for ensuring occupant 

comfort.

2.1 Cost and Performance

Efficiency is maximized primarily through 

outdoor reset controls which measure air 

temperature and, along with preprogrammed 

algorithms at the boiler’s control board, select a 

boiler temperature to target. When configured 

properly, this results in close matching of the 

boiler output to the loads, improved efficiency, 

and enhanced occupant comfort. The warmer 

the outside temperature, the lower the boiler 

supply temperature (and hence the return 

temperature) will be, which increases 

efficiency. 

When selecting the set points for the outdoor 

reset curve, the main goal is to maximize the 

amount of time the boiler return temperature 

will be lower than 130°F (the point at which 

condensing typically begins for a gas boiler). 

The lower the return water, the higher the boiler 

efficiency. However, decreasing the supply 

temperature may increase the length of 

baseboard required to meet the load. The overall 

installed cost will increase, but the limiting 

factor will be the available space to install the baseboard, not necessarily the added cost. This is 

because baseboard is relatively inexpensive and the energy savings associated with the increase 

in efficiency outweighs the added cost. This is not the case with all emitters—radiant floors, 

panel radiators, etc. If space is an issue, start with the length of baseboard that can be installed 

and work backward or change to a higher output model. 

Condensing Fact:

Butcher (2004) concluded that a 

condensing boiler with outdoor reset 

could operate in a condensing range 96% 

of the season for natural gas, 90% for 

propane, and 80% of the season for #2 

fuel oil. 

Condensing Facts (AHRI 2008):

• Nearly 10% of the heat in natural gas 

is in the water vapor in the flue 

products. 

• Every pound of water vapor that 

condenses gives off about 1,000 Btu.

• The efficiency of a natural gas boiler 

cannot surpass 90% without 

recovering the latent heat of the flue 

gases through condensation.

• Condensing boiler annual fuel 

utilization efficiencies are determined 

based on laboratory testing using 

120°F return and 140°F supply water 

temperatures. 

• Natural gas condensing boilers 

commonly have annual fuel utilization 

efficiencies of 90%–95%.
4 

Table 1 summarizes a basic cost benefit analysis for decreasing the maximum boiler supply 

temperature, Ts,max, from 180°F (a standard factory default) to three lower settings. 

Table 1. Economic Analysis of Decreasing the Boiler Supply Temperature, Ts,max 

Ts,max

Cost of Additional 

Baseboarda

Savings Over 

20 years Simple Payback Net PresentValueb

170.0 $75 $166 9.0 $36

160.0 $133 $351 5.8 $102

150.0 $210 $565 7.4 $169 a

Assumes $8/linear foot of baseboard

b

Assumes a discount rate of 6%, and an energy inflation of 2%/year

The annual heating bills for this 1,200-ft2

 home in Ithaca, New York, were predicted to be 

approximately $1,200 ($200/month). Based on the design heating load calculation, baseboard 

lengths nearly doubled from 29 ft at a Ts,max of 180°F to 56 ft at 150°F. The cost for the 

additional baseboard is approximately $210, but the net present value of the savings over the life 

of a 20-year loan is predicted to be $565. The annual space heating efficiency of this particular 

boiler was predicted to increase from 89.6% to 92% just by changing the set point temperature. 

The simple payback period associated with reducing Ts,max from 180°F to 150°F and doubling the 

baseboard is estimated to be 7.4 years.

If a condensing boiler is being installed, the costs to ensure condensing are generally associated 

with the costs of additional baseboard to compensate for lower supply water temperatures.

Properly configuring the controls should not increase project costs. And, as indicated, the 

additional baseboard is cost effective.

2.2 Risk Identification 

The risks associated with the identified control strategies are primarily linked to occupant 

comfort and satisfaction. An outdoor reset controller will not damage the boiler, but it can 

drastically affect response time if night setback is used. A boost control installed to combat this 

problem may reduce efficiency, but again, will not damage the equipment. 

When considering whether to employ these strategies, consideration should be given to how 

oversized the boiler is compared to the loads, if the baseboard is undersized for a low 

temperature system, and how the occupants expect the system to behave. 

2.2.1 Oversized Systems

Although it may seem counterintuitive, oversizing a boiler can actually decrease response times. 

It also increases short cycling, reduces equipment life, and decreases efficiency (see Section 3.1). 

2.2.2 Undersized Baseboard

While installing an oversized boiler is bad for comfort and efficiency, the exact opposite is true 

for baseboard. Undersized baseboard will have the same negative effects as an oversized 

boiler—reduced response time, reduced efficiency, increased cycling, and reduced boiler life. 
5 

2.2.3 Outdoor Sensor Placement

Where outdoor reset is used, the boiler supply temperature depends on the outdoor reset sensor. 

Therefore, proper placement of this sensor is critical. For example, if placed in a location where 

it is exposed to a heat source such as air from a dryer vent, in direct sunlight, too near the boiler 

exhaust, etc., it indicates to the boiler that the outside temperature is warmer than it actually is.

This will result in lower supply temperatures than are needed to meet the loads and can cause

comfort problems and drastically increase response times. 

2.2.4 Specialty Components

Components that require a minimum supply water temperature, such as toe kick heaters, should 

be specified with the knowledge that boiler temperatures vary depending on the outside 

conditions, and supply temperatures may drop below that required by various components. 

For example, a fan coil unit may have a low limit control to prevent the fan from turning on if 

the supply water temperature drops below 140°F. If the outdoor reset control is set such that the 

boiler would operate under that temperature for a significant portion of the heating season, the 

occupants would be left without heat in that area of the home. 

2.2.5 Occupant Expectations

Occupants typically control temperature with standard thermostats. It is common in heating 

dominated climates to turn the thermostat down to a lower temperature during unoccupied hours 

and overnight. Turning down the thermostat has been estimated to save approximately 1% on 

annual heating bills per each degree if the setback period lasts eight hours.

1 However, a properly 

sized boiler combined with baseboard matched to the design load will not have the capacity to 

recover from setback, especially if that system is designed with an outdoor reset control. The 

reset control will keep the supply temperature at the level required to meet the normal load based 

on the current outdoor conditions, severely hindering the system’s ability to raise the temperature 

in the space. In fact, recovery from setback in previous studies spanned several hours.

If a setback strategy is desired, controls to speed up recovery may sometimes be necessary. Some 

boilers employ controls that respond to lags in response by boosting the output temperature to 

the zones and bypassing the outdoor reset controller. Once the thermostat is satisfied, the system 

would default to the outdoor reset control. Available options for a boost strategy are: 

• Boiler controls that automatically raise the boiler output target temperature if not satisfied 

within a set time frame

• An indoor sensor that works in conjunction with the outdoor reset control to compensate 

for lags in response based on interior temperature 

• A simple manual override switch. 

The baseboard—and possibly the boiler—may need to be oversized to meet that additional load 

induced during periods of setback recovery. 

 

1 http://www.energysavers.gov/your_home/space_heating_cooling/index.cfm/mytopic=12720
6 

3 Boiler Controls and System Interactions 

This section discusses of a few typical boiler controls in depth and explains how they can affect

efficiency and comfort. 

3.1 Controls for Safety and Durability

To maximize condensing, condensing boilers typically have heat exchangers with smaller, 

narrower passages than those of noncondensing boilers. As a result, there is a substantial 

pressure drop through the boiler. This pressure drop, combined with the many small passages 

where water could stagnate, can result in overheating in the heat exchanger and possibly boiling 

which could damage the equipment. These systems thus have special piping configurations, high, 

steady flow rates, and special controls as standard features. 

The boiler is protected primarily with high temperature, low water, high pressure, and low 

pressure cutoffs. The high temperature cut off is the most commonly activated in residential 

applications, especially in homes with low loads. In a system employing an outdoor reset control, 

this high temperature limit may be the boiler’s set point based on the reset curve or the boiler’s 

maximum temperature.

Figure 2 shows that when conditions in the boiler trigger one of the cut offs, the controller will 

shut down the burner. In the case of a higher temperature cutoff, the boiler will not fire again 

until a certain amount of time has passed or until the water temperature falls below a certain set 

point as determined by the “differential” setting. If there is still a call for heat, the pumps will 

continue to operate, but the boiler will not fire. 
7 

Figure 2. Flow diagram illustrating boiler operation when high limit or supply set point is reached

The differential settings, which dictate when the 

boiler will re-fire after it has been shut down by one 

of these controls, are intended to minimize short 

cycling, and hence, wear on the boiler. The 

differential may be based on time or temperature.

For the time setting, the boiler will not fire again 

until a certain amount of time has passed. With 

respect to the temperature setting, the boiler will not 

fire again until the water in the return loop has 

dropped a minimum number of degrees below the 

current set point. Unfortunately, these controls can 

affect response time and occupant comfort.

Thermostat Calls 

for Heat

Pumps Circulate 

Water

Burner is 

Ignited

Warm Water is 

Supplied to 

Space

Supply 

Setpoint 

Reached

Thermostat 

Satisfied

Yes

No

Yes

Burner & Pumps 

Shut Off

Burner Shuts 

Down

Differential 

Met

Pumps 

Continue to 

Run

No

Yes

No

Differential:

This is the difference in either time 

or temperature between boiler 

shutdown and startup. 

Example: A differential of 10°F 

means the boiler will not fire again 

until the temperature (usually the 

return) has dropped 10°F below the 

set point.
8 

For example, assume that the boiler’s supply target 

temperature is 120°F based on the boiler’s curve settings, 

and the differential setting in the boiler’s controller has 

been set to 30°F. Once that target temperature is reached 

(plus an allowable overshoot), the burner will shut down 

and will not fire again until the return temperature has 

decreased to 30°F below the boiler’s set point regardless 

of whether there is still a call for heat. The smaller the 

loads compared to the boiler’s output, the quicker the 

boiler will reach that set point. 

Because the overshoot is not necessarily half the 

differential, this could result in average supply 

temperatures below the desired set point. If the overshoot were only 5°F for the above example, 

the burner would shut off at 125°F and would not turn back on until the return temperature was

90°F. The resulting average water temperature supplied to the space under those conditions 

would only be approximately 108°F when, in fact, the desired temperature to meet the load was 

120°F. Thus, the system may not be able to maintain 

the desired space temperature, or the time needed for 

the system to recover from a setback period may 

increase. 

3.2 Controls for Improved Efficiency

Past research has determined that most condensing (in 

a gas boiler) occurs when the return temperature to the 

boiler is 130°F or lower, and therefore, any control 

technique that reduces the return water temperature—

including lowering the boiler set point—will

significantly improve the efficiency (Butcher 2004; 

Arena 2010).

To this end, there are several common controls used 

with condensing boilers that help improve system

efficiency. A few of the most common and effective 

include heat dumping, burner modulation, maximum 

input limits, and reset controls. 

3.2.1 Heat Dumping

Heat dumping is a strategy whereby excess boiler heat 

is diverted to the DHW tank after a space heat demand 

is satisfied. Generally, the process continues until the 

return temperature to the boiler is lower than a 

specified minimum differential or until a time limit has 

been exceeded. Butcher (2011) showed that this 

process can greatly improve overall system efficiency.

Example of differential operation:

You receive a call from occupant Jones 

that their house isn’t reaching the 

desired temperature after periods of 

setback. You visit the site and determine 

the following:

• The outdoor rest control calls for 

120°F supply water temperatures at 

40°F outside. 

• The load on this home is 15kBtu/h at 

40°F outside. 

• The boiler’s lowest modulation rate 

is 30 kBtu/h.

• The boiler has been set up with a 

30°F differential.

Conclusion: Because the system is so 

oversized, the supply water temperature 

quickly reaches its set point. Allowing for 

a small overshoot, the burner shuts off 

at 125°F and, because of the differential 

setting, does not turn back on until the 

return temperature is 90°F (120°–30°F). 

The resulting average water temperature 

supplied to the space is only 108°F. 

Overshoot:

The allowable range over 

the current set point or high 

temperature limit. 

Example: A boiler with an 

overshoot of 5°F for the high 

limit of 180°F would allow 

the supply water 

temperature to reach 185°F 

before shutting down.
9 

3.2.2 Burner Modulation

As noted earlier, excessive cycling of a boiler will 

result in efficiency losses, reduced durability, and 

occupant comfort issues. As homes have become more 

efficient, the difference between the smallest boilers on 

the market and the design loads in residential buildings 

has increased, aggravating the problem. In response to 

this, manufactures developed modulating burners for 

natural gas and propane boilers with the ability to 

supply a fraction of the total capacity. 

Burner modulation in smaller residential boilers has been available for more than 10 years.

Modulation rates can be as low as 20% of the rated capacity of the boiler and are generally 

controlled by adjusting the rate at which air and gas are supplied to the burner (Siegenthaler 

2012). 

The level to which the boiler modulates can be controlled in more than one way. In some cases 

the output is varied based on the difference between the supply and return temperatures. It can 

also be controlled by sensing the supply temperature and increasing or decreasing the firing rate 

to maintain a certain set point. The ability to lower the input results in less cycling under part 

load conditions and better control of space temperatures.

3.2.3 Input Limits

In a similar vein, some boiler manufacturers have started offering controls that can limit the 

boiler’s maximum input. This can be especially useful for either space heat or DHW operation if 

one load is significantly less than the other. Because the boiler briefly modulates to a high firing 

rate on startup, this limit reduces cycling in situations where the maximum firing rate is 

significantly higher than the demand.

3.2.4 Outdoor Reset 

Rest controls have by far the greatest impact on system 

efficiency. Because loads decrease as outdoor temperatures 

increase, lower supply water temperatures can be used to 

satisfy the demand. With an outdoor reset control, the boiler 

attempts to deliver a supply temperature that varies according 

to outdoor temperature.

Figure 3 provides an example of a linear reset curve for a 

common boiler. Its settings are:

• Maximum boiler supply temperature (Ts,max) is 180°F 

at an outside temperature (Tout,min) of 5°F (design temperature)

• Minimum supply temperature (Ts,min) of 95°F at an 

outside temperature (Tout,max) of 68°F.

Outdoor reset control:

This allows the designer to 

match the boiler’s supply 

temperatures to the load. 

As outdoor temperatures rise, 

the boiler supply temperatures 

are lowered resulting in lower 

return water temperatures, 

increased condensing, and 

increased efficiency.

Modulation rate:

This is often referred to as the 

turndown ratio. A modulation 

rate of 20% equates to a 5:1 

turndown ratio. 
10 

Figure 3. Linear boiler reset curve

Ts,max is the maximum boiler supply temperature that will be supplied to the conditioned space 

when the outdoor temperature equals the design temperature, Tout,min. Ts,min is the minimum boiler 

supply temperature that will be delivered at the maximum outdoor operating temperature, 

Tout,max. The maximum outdoor operating temperature is commonly set between 68°F and 72°F. 

The line on the graph shows that the colder the outside temperature, the higher the boiler outlet 

target temperature. Because boilers operate more efficiently at lower return water temperatures, 

strategies that significantly reduce temperature have the most impact on condensing. Therefore, 

keeping the upper limit as low as possible will lower return temperatures and increase system 

efficiency (see Table 2). 

Table 2. Predicted Frequency of Condensing for Various Supply Temperatures

Frequency of Condensing at Different Ts,max 

(°F) 

(1, 2, and 3 gpm)

Ts,min 150 160 170 180

95 99% 91% 87% 90% 80% 77% 79% 68% 64% 66% 57% 53%

105 99% 87% 83% 86% 72% 67% 71% 58% 54% 56% 47% 44%

110 99% 84% 79% 82% 66% 60% 62% 50% 45% 48% 41% 39%

115 98% 80% 73% 72% 56% 50% 48% 42% 35% 40% 34% 32%

120 97% 70% 60% 66% 45% 40% 43% 34% 24% 32% 25% 23%

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40 50 60 70 80

Supply Temperature [F]

Outside Temperature [F]

Boiler Reset Curve

Ts,max

Ts,min

Tout,min Tout,max
11 

Table is applicable for Ithaca, New York, and 

assumes an outdoor reset control and a 20°F 

temperature differential (ΔT) between the supply 

and return under design conditions. 

Ts,max in Table 2 refers to the high limit on the boiler 

reset curve (see Figure 3). The higher the supply 

temperature, the less often the boiler will condense. 

It should also be noted that raising the lower limit on 

the reset curve, Ts,min, significantly reduces

condensing frequency. This setting corresponds to 

the minimum boiler supply temperature that will be 

delivered to the zones at the maximum outdoor 

temperature. For example, in Figure 3, the minimum 

boiler temperature, Ts,min, is set to 95°F when the 

outdoor temperature is 68°F, Tout,max. If that 

minimum supply temperature is increased to 140°F 

at an outside temperature of 68°F, condensing will 

be minimal. This is because the differential between 

the supply and return temperatures decreases as the 

loads decrease and will be only a few degrees (not 

20°F as under design conditions) by the time the 

outdoor temperature reaches 68°F. Therefore, raising Ts,min can drastically limit the number of 

occurrences when the return temperature will actually fall below 130°F.

3.3 Control Strategies for Enhanced Occupant Comfort

3.3.1 Boost Controls

Occupants typically control comfort with standard thermostats. In many heating-dominated 

climates, occupants often reduce or set back the temperature settings on their thermostats during 

unoccupied hours and overnight. As mechanical systems are more closely matched to the loads, 

especially in higher efficiency homes, systems are less able to respond to large setbacks because 

excess capacity was not installed. Recovering from a setback of 8°–10°F can take several hours

when outdoor reset controls are used alone, because these controls are specifically trying to very 

closely match the system output to the load. 

Some boilers employ controls that respond to lags by boosting the output temperature to the 

zones and bypassing the outdoor reset controller. Once a space is heated, the system defaults to 

the outdoor reset control. Boost controls set to override the outdoor reset control work as 

follows: if a heating demand is not satisfied within x minutes, an offset of d °F is added to the 

target temperature of the boiler. Both values are adjustable based on selections in the installer 

menu. 

Figure 4 illustrates how such a boost control would operate. “Lag” is the number of minutes (x) 

that pass before the boiler’s supply temperature is increased, and “offset” refers to the number of 

degrees (d) by which the supply temperature is raised. This process continues until either the 

thermostat is satisfied or the supply temperature reaches the boiler’s high limit setting.

Caution: Lowering Ts,max:

When designing systems with 

modulating, condensing boilers, 

sufficient lengths of baseboard 

must be installed so maximum 

water temperatures can be kept 

as low as possible and still meet 

the loads.

Caution: Lowering Ts,min: 

Be careful when specifying toe 

kick heaters that they are capable 

of operating at low temperatures. 

Many common units will not 

operate below supply 

temperatures of 140°F. A Ts,min set 

to this temperature would 

essentially eliminate any chance 

for condensing.
12 

Figure 4. Flow diagram illustrating the effects of a boost control on the boiler supply temperature

Assuming that x = 20 minutes and d = 18°F, if the outdoor reset algorithm calculates a target of 

110°F and the central heating call exceeds 20 

minutes, the target supply temperature is

increased to 128°F. If the central heating 

thermostat continues to call for another 20 

minutes, the temperature increases again to 

146°F, and so on. It is important to realize that, 

although this control expedites the response, it 

may also decrease system efficiency because it 

increases the supply temperatures.

3.3.2 Eliminate or Minimize Setback

Another option is to eliminate setback altogether. Although significant savings are associated 

with setback strategies in modern efficient homes, the savings will be much lower than those in a 

very inefficient home. If the occupants are comfortable with a slow response time, the setback 

will save energy. If they desire a boost control, there may be no savings from setback. 

If Time = Time + x

(lag time, x, is 

determined by 

installer)

t = t + d

(offset, d, is 

determined by 

installer)

Thermostat Calls 

for Heat

Pumps Circulate

Water

Burner is 

Ignited 

Warm Water is 

Supplied to 

Space

Thermostat 

Satisfied

t = Ts (boiler setpoint)

Time = 0 (start up)

Yes

Burner & Pumps 

Shut Off

No

Caution: Boost Controls:

Care must be used when implementing 

this function in multi-zones systems as the 

boiler may boost to the highest 

temperature if calls from several zones 

overlap.
13 

3.3.3 Raise Warm Weather Shutoff

Another control issue that could affect occupant comfort is the setting for the warm weather 

shutoff. This setting disables the boiler’s heating operation when it is exceeded by the outdoor 

temperature, and commonly comes from the factory set between 68°F and 72°F. For occupants 

who maintain a constant temperature in their homes, this setting should be no lower than that 

desired temperature. So, if 70°F is their normal setting, the warm weather shutoff should be no 

lower than 70°F.

For occupants using setback, this setting may cause comfort issues in the swing seasons of spring 

and fall when the nights are still cool, but the daytime temperatures rise above the warm weather 

shutoff. This can be a particular problem in climates that experience large daily temperature

swings. In a home employing setback, the interior temperature may have fallen enough during 

that period that the occupants want to heat the space even though the outdoor temperature is 

warm. Unfortunately, the boiler will not fire. Increasing this limit in the boiler’s controller will 

help eliminate this problem. If this is a problem, this shutoff should be increased 2°–4°F above 

the desired interior temperature. For instance, if the occupants desire to keep the space at 70°F, 

the warm weather shutoff should be increased to 72°–

74°F. 

3.3.4 Outdoor Reset and Comfort

In addition to improving system efficiency, the 

outdoor reset control can improve comfort. When 

using a reset control, the boiler’s supply set point 

temperature is closely matched to the load conditions. 

This prevents overshooting the setting on the 

thermostat, a common problem in homes with high 

mass systems, such as older homes with radiators or 

homes with radiant slab floors. Because these types 

of systems have so much mass, there is a lag between 

when the boiler shuts off because the thermostat is 

satisfied, and when the delivery system has finished 

dissipating heat. This can cause the thermostat setting 

to overshoot several degrees, which causes large 

temperature swings. Outdoor reset control can 

minimize this effect by supplying cooler 

temperatures when the loads are smaller.

Caution: Outdoor Reset

If used with a noncondensing

boiler, the lower limit on the 

outdoor reset curve should be set 

to a supply temperature that will 

prevent condensing. 

The corresponding return 

temperatures should be higher 

than the dew point temperature 

for the space heating fuel type.

As a rule of thumb, dew points for 

various fuels are:

Natural Gas: 130°F

Oil: 115°F

Propane: 125°F

Additional Resources:

• ACCA Manual J V.8: Residential Load Calculation

• ACCA Manual S V.3: Residential Equipment Sizing. 

• Burdick (2011). Strategy Guideline: Accurate Heating and Cooling Load Calculations: 

http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/hvac_load_calc.

pdf

• Burdick (2012). Strategy Guideline: HVAC Equipment Sizing 

http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/strategy_guide_

hvac sizing.pdf
14 

4 Measure Implementation

4.1 Climate-Specific Factors

This guide is applicable to all climate zones where space heating is required.

4.2 Install Procedure

4.2.1 Evaluate the Size of the Boiler

The first step in a good design is to properly size the 

mechanical equipment. A right-sized heating system 

provides the desired occupant comfort and runs 

efficiently. Oversized heating equipment has negative 

impacts on energy use, comfort, and equipment 

durability because the system will short cycle, which

means that a heating or cooling device turns off before 

the demand is met. As with most mechanical 

equipment, peak operational efficiency and 

effectiveness are reached when the unit runs for long, 

steady periods to address the loads (Burdick 2011). 

To properly size the boiler, accurate heating loads 

must be calculated. These calculations should be 

performed in compliance with the ACCA’s standard 

protocols as outlined in the most recent version of 

Manual J: Residential Load Calculation. Manual J 

calculations are based on: 

• The efficiency levels of the building envelope, 

windows, and doors 

• The square footage of all the surfaces 

• The infiltration rate

• The location of the project. 

Procedures for properly selecting equipment based on the results of those calculations can be 

found in the ACCA’s Manual S: Residential Equipment Selection.

Scope of Work

A. Determine appropriate limits for the outdoor reset controller. 

B. Locate outdoor reset sensor away from heat sources and out of direct sunlight. 

C. Review manufacturers’ differential control settings and adjust as necessary. 

D. Discuss operation with homeowner to determine if setback is desired.

E. Evaluate response time and determine the need for any additional controls.

Figure 5. Wall hung condensing boiler
15 

Ideally, the highest modulation rate of the boiler would match the design load of the home, 

leaving plenty of room for the unit to reduce its firing rate for the loads during nonpeak times. 

This may not be possible in many situations, however, because many new houses and apartments 

have loads that are significantly lower than the lowest modulation rate on the boiler. The smallest 

boilers available to date have an input capacity of about 50 kBtu/h. With a turndown ratio of 5:1, 

the smallest output capacities are generally 10–15 kBtu/h. And, as the energy efficiency of 

residential buildings is increased under code or beyond code programs, the heating loads can 

easily fall below these modulation rates, especially during nonpeak heating periods and when the 

home has been separated into numerous small zones. 

If the peak heating load is close to or less than the lowest modulation rate on the boiler, adding 

thermal mass to the system, such as a buffer tank, will help reduce short cycling and improve 

comfort and response time. Alternatively, a high efficiency domestic water heater can also be 

used to provide space heating. Whatever the solution, the goal is to reduce cycling to increase 

efficiency, occupant comfort, and equipment longevity. 

4.2.2 Evaluate Baseboard Capacity

Installing sufficient capacity to transfer the heat 

to the conditioned space is imperative. An 

undersized distribution system will lead to all 

the same troubles as an oversized boiler—slow 

response, increased cycling, and reduced 

efficiency. 

If the baseboard is unable to deliver enough 

energy to the space, the return water 

temperature to the boiler will be higher than 

desired. This can reduce condensing, which 

lowers system efficiency. Many boilers also 

compare the temperature difference between the 

supply and return. If the difference is too small, 

the burner shuts off. If the baseboards are 

undersized, the system will short cycle (even if 

the boiler is properly sized), because it simply 

cannot release enough heat to the space. 

Baseboard manufacturers publish output 

capacity for their equipment at varying 

temperatures.

Table 3 lists the outputs for two different baseboards: a single pipe design and a double pipe with 

twice the fin area as the single pipe, which significantly increases output capacity. If in the above 

example, the baseboard installed was a single pipe model intended to handle the design load at 

an average water temperature of 140°F, the output capacity would be 410 Btu/h/ft of baseboard. 

If the design load on the building were 20 kBtu/h, a minimum of 49 ft of baseboard would be 

required to meet that load.

Determining Baseboard Capacity in 

an Existing Home:

1.Measure the length of the baseboard 

(length with fins). 

2.Determine the manufacturer. 

3.If the manufacturer is unknown, 

compare the dimensions of the casing, 

fins, and pipe diameter to those of 

several manufacturers. Select the 

closest.

4.Determine the boiler supply 

temperature at design and subtract 

10°F (assuming the system was 

designed for a 20°F ΔT) to get the 

average water temperature.

5.Look up the output listed in the 

manufacturer’s tables. 

6.Multiply the length by the capacity.

7. Verify that the baseboard capacity is 

greater than the peak heating load.
16 

Table 3. Comparison of Baseboard Output per Linear Foot for 

Single Pipe Versus Double Pipe Models

Average Water Temperature

(°F)

90 100 110 120 130 140 150 160 170 180

Btu/h Output

Single Pipe – – 200 270 340 410 490 570 650 730

Double Pipe 130 205 290 385 460 546 637 718 813 911

Increase in Output (%) – – 45 43 35 33 30 26 25 25

Evaluating the baseboard’s ability to meet the design 

load is only one part of the equation. If the boiler is 

oversized compared to the design load, oversizing 

the baseboard will help reduce short cycling. This 

may be the only option in situations where the 

smallest boiler is too large for the design load or 

there are several zones, each of which has very small 

loads compared to the boiler’s capacity. In these 

cases, oversizing the baseboard will reduce cycling, 

improve response time, and increase efficiency. In 

the case above, if the lowest modulating rate on the 

boiler is 30 kBtu/h, installing enough baseboard to 

regulate the system may require doubling it. Instead 

of 49 ft, 98 ft could be installed. While this may 

seem excessive, at approximately $8/ft, it would only 

add $400 to the system cost and would enhance performance. For homes with limited space, high 

output baseboards or buffer tanks should be considered to counter the negative effects of 

severely oversized equipment. 

4.2.3 Determine the Set Points for the Outdoor Reset Curve

Determining the optimal settings for the reset 

curve is an iterative process. Although the goal 

is to create conditions where the return water 

temperature is always below the dew point for 

the fuel being used, this could result in 

baseboards that are too long for the space 

available. Therefore, several supply 

temperatures may need to be evaluated. Figure 

6 illustrates the general steps involved in 

determining the best settings, and provides 

some simple recommendations for reaching a 

satisfactory decision. The goal is to maximize 

the time the boiler is in condensing operation. 

Caution: Oversizing the 

Baseboard

NOTE: Many manufacturers set a 

maximum ΔT between the boiler’s 

supply and return to protect the 

heat exchanger. Oversizing the 

heat emitter increases the ΔT. 

Although oversizing the emitters 

will improve system efficiency and 

response time, it is important to 

ensure that the manufacturer’s limit 

is not exceeded.

Additional Resources:

Siegenthaler (2012). Modern Hydronic 

Heating for Residential and Light 

Commercial Buildings, 3rd edition.

AHRI (2008). I=B=R Guide – Residential 

Hydronic Heating: Installation and 

Design. 
17 

Figure 6. Flow diagram illustrating iterative process in determining 

the best setting for Ts,max for the reset curve

Baseboard 

Length 

OK?

Determine Baseboard 

Length 

Determine Ts,max & ΔT 

Desired 

Yes

Start

Finish

If baseboard length 

is too long, increase 

Ts,max

Calculate Average 

Baseboard Temperate 

Tave

Locate Baseboard 

Output at Tave on Spec 

Sheet

Start with Treturn <= 

dewpoint for the fuel Tave = (Ts,max + Treturn) ÷ 2

Divide the design 

heating load for 

each room by 

baseboard output

Based on 1 gpm 

flow rate.

Outdoor Reset Control:

It is important that the upper limit of the reset curve be set lower than the high cutoff limit so it does not 

interfere with operation. If the upper limit on the reset curve, Ts,max, is set higher than the high limit shutoff, the 

boiler will never supply water at Ts,max. 

If the system is designed to deliver Ts,max under design conditions, it would be incapable of keeping up with 

those loads.
18 

The process outlined in Figure 6 is a guide for determining the optimal length of baseboard when 

paired with a condensing boiler and an outdoor reset control. The higher the Ts,max, the lower the 

system efficiency. If a more precise prediction of annual performance is desired, a “bin 

temperature” analysis can be performed. This type of analysis uses the temperature profile for 

the location, the building’s rate of heat loss, and the boiler’s reset curve to analyze what 

Example

The owners of an existing home in Ithaca, New York, would like to replace their old boiler with a 

condensing boiler that is controlled with an outdoor reset sensor. The home is heated with natural gas 

and has 80 ft of baseboard convectors. The baseboard is a 1⁄2-in. single pipe model. The flow rate 

through the system is estimated at 2.5 gpm with a 20°F ΔT under design conditions. The contractor 

determined that the design heating load is 40 kBtu/h. 

A) What should the contractor enter for Ts,max on the reset curve given the current configuration? B) 

How much additional baseboard would have to be installed for the return temperature to be 130°F 

under design conditions?

Solution A:

1. Determine the output capacity per foot of baseboard needed to meet the design load 

40,000 Btu ÷ 80 ft = 500 Btu/h/ft 

2. Consult Table 4 to determine what Tave will result in an output of 500 Btu/h/ft

for 1⁄2-in. element at 1 gpm, an average baseboard temperature of 180°F would 

provide 550 Btu/h/ft.

3. If designed for a 20°F ΔT, the installer would have to enter a Ts,max of 190°F.

180°F + 1⁄2 × 20° ΔT = 190°F

Btu/h/ft Based on Average Temperature Listed

Flow 140°F 150°F 160°F 170°F 180°F 190°F 200°F 210°F 220°F

3⁄4-in. Element 1 gpm 290 350 420 480 550 620 680 750 820

4 gpm 310 370 440 510 580 660 720 790 870

1⁄2-in. Element 1 gpm 310 370 430 490 550 610 680 740 800

4 gpm 330 390 450 520 580 640 720 780 850

Reproduced from AHRI’s I=B=R Residential Hydronic Heating: Installation & Design Guide.

Solution B: 

At a Ts,max of 190°F and a 20°F temperature difference, the boiler’s return temperature would only be 

170°F under design conditions and would not condense. Adding more baseboard would allow the 

installer to decrease the Ts,max while meeting the design load. The following steps can be followed to 

determine how much additional baseboard is needed.

1. Because the home is heated with natural gas, start with the assumption that the return water 

temperature should be no higher than 130°F to ensure condensing throughout the year. If a 

20°F ΔT is desired between the supply and return, this gives a Ts,max of 150°F and a Tave of the 

baseboard of 140°F.

2. From the default table, the output for an average temperature of 140°F, assuming 1 gpm flow 

rate would be 310 Btu/h/ft.

3. The total length needed to meet the 40 kBtu/h design load would be:

40,000 Btu/h ÷ 310 Btu/h/ft = 129 ft

129 ft – 80 ft = 49 additional ft of baseboard would be needed.

Table 4 Default Output Ratings of Typical Finned Tube Baseboard
19 

percentage of the year the boiler is predicted to operate in condensing mode. This procedure is 

explained in detail in Appendix A. 

4.2.4 Properly Locate the Outdoor Sensor 

The outdoor sensor for the reset control must be carefully located. Avoid locating the sensor near 

or in the path of any heat sources, including: 

• Direct sunlight

• Dryer vents 

• Boiler flue

• Bathroom or kitchen exhausts. 

Also avoid installing it at a height that would allow it to be covered by snow. Installing the 

sensor improperly will provide incorrect climate data to the boiler’s controller, resulting in 

supply temperatures that are lower than required for the outdoor conditions. This will result in 

comfort issues for the occupants.
20 

5 Verification Procedures and Tests

Before leaving the site, confirm that the desired operation of the controls has actually been 

achieved. The inspections outlined in this section should take only a few minutes. A field 

checklist has been included in Appendix B to assist in the verification and testing process.

5.1 Ensure the Outdoor Reset Functions Properly

Verifying that the boiler is operating as intended is not very difficult, unless testing is done on an 

extremely hot day. Under those conditions the boiler may not fire if the thermostat cannot be set 

above the ambient conditions. Under these circumstances, it may be necessary to postpone 

commissioning until the weather turns colder. 

To verify the outdoor reset control is set up as intended, increase the temperature setting on the 

thermostat so the boiler fires. Be sure to choose a temperature several degrees above the indoor 

condition so the boiler runs for a few minutes. On the boiler’s control screen, note the maximum 

supply temperature after it stops rising or at the time the burner shuts down (which may happen 

quickly if oversized for the load). Compare this temperature to the temperature from the boiler 

curve, which coincides with the current outdoor temperature. The predicted and actual 

temperatures should be very close.

 

Figure 7. Predicted boiler supply temperature at 30°F outdoor temperature

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40 50 60 70 80

Supply Temperature [F]

Outside Temperature [F]

Boiler Reset Curve

Example: Determine the boiler supply temperature for a system with a reset curve as shown in 

Figure 7 if the outside temperature were 30°F).

Solution: Follow a straight line up from 30°F on the horizontal axis until you intersect with the reset 

curve. Then follow that point to the left until you intersect with the vertical axis. The corresponding 

boiler supply temperature at 30°F outdoor temperature would be approximately 145°F.
21 

At this time, you should also verify one more time 

that the outdoor sensor was properly located. 

5.2 Evaluate Boost Control 

If a boost control is installed, allow the system to 

run long enough for it to kick in. Setting the 

thermostat several degrees above the indoor 

temperature will ensure that the boiler will run long 

enough. Do not assume that the system is shutting 

off if the burner shuts down. The boiler may just 

have met the current set point. If the pumps continue 

to run, the thermostat has not been satisfied, and the 

controller is keeping track of the time. 

After the programmed time has passed, the boiler 

should increase the temperature of the supply water.

Give the boiler a couple of minutes to increase the 

supply temperature and then compare that 

temperature to the set point noted from the previous 

test outlined under Section 5.1. It should be higher 

than the original set point temperature by the 

differential entered into the controller. If the boiler 

does not supply a higher temperature to the zones, 

the boost function is not operating properly. 

5.3 Additional Resources

Many control and design strategies can be used to optimize hydronic system efficiency and 

occupant comfort. Those mentioned in this report are just a small subset. The following 

resources may be consulted for different control strategies and additional information: 

• AHRI (2008). I=B=R Guide: Residential Hydronic Heating Installation & Design

• Siegenthaler (2012). Modern Hydronic Heating 

• ACCA Manual S, Residential Equipment Selection

• ACCA Manual B, Balancing and Testing Air and Hydronic Systems 

Verifying Setup:

• Don’t forget to reset any settings that you bypassed for testing.

• Verify the return temperature on noncondensing systems is not below the critical threshold.

Caution: Ensuring Success

Items to Verify:

1. The boiler is not oversized for the 

home.

2. Enough baseboard has been 

installed to deliver the capacity 

needed and ensure a minimum of a 

20°F differential between the supply 

and return temperatures under 

design conditions.

3. Ts,max on the boiler reset curve is 

below the boiler’s high limit setting.

4.Warm weather shutoff is high 

enough to prevent no-heat 

situations during the swing 

seasons.

5.The outdoor reset sensor has been 

placed away from any exhaust 

vents, including kitchen, bath, dryer, 

and mechanical system vents and 

will not be in direct sunlight during 

any portion of the day.
22 

References 

Air Conditioning Contractors of America (ACCA). 2009. “Manual J, Residential Load 

Calculations”. Third Edition, V. 1.00. ACCA, Arlington, VAAHRI; (2009). “I=B=R Guide; 

Residential Hydronic Heating.” Arlington, VA; The Hydronics Institute Division of AHRI. 

Air Conditioning Contractors of America (ACCA). 2009. “Manual S, Residential Equipment 

Sizing”. Third Edition, V. 1.00. ACCA, Arlington, VAAHRI; (2009). “I=B=R Guide; 

Residential Hydronic Heating.” Arlington, VA; The Hydronics Institute Division of AHRI. 

Air Conditioning Contractors of America (ACCA). 2009. “Manual D, Residential Duct 

Systems”. Third Edition, V. 1.00. ACCA, Arlington, VAAHRI; (2009). “I=B=R Guide; 

Residential Hydronic Heating.” Arlington, VA; The Hydronics Institute Division of AHRI. 

AHRI. (2008). I=B=R Ratings for Boilers, Baseboard Radiation, Finned Tube (Commercial) 

Radiation, and Indirect-Fired Water Heaters. The Hydronics Institute Division of AHRI.

Arena, L. (2010). “In-Field Performance of Condensing Boilers in Cold Climate Region.” 

National Energy Technology Laboratory, Morgantown, WV; U.S. Department of Energy, 

Building Technologies Program.

Burdick, A. (2011). “Strategy Guideline: Accurate Heating and Cooling Load Calculations.” 

National Energy Technology Laboratory, Morgantown, WV; U.S. Department of Energy, 

Building Technologies Program.

Burdick, A. (2012). “Strategy Guideline: HVAC Equipment Sizing.” National Energy 

Technology Laboratory, Morgantown, WV; U.S. Department of Energy, Building Technologies 

Program. 

Butcher, T. (2011). “Performance of Combination Hydronic Systems.” ASHRAE Journal, 

December 2011, pp. 36–41. 

Butcher, T. (2004). “Hydronic Baseboard Thermal Distribution System with Outdoor Reset 

Control to Enable the Use of a Condensing Boiler.” Brookhaven National Laboratory; Uptown, 

NY; U.S. Department of Energy, Office of Buildings Technology.

Siegenthaler, J. (2012). “Modern Hydronic Heating for Residential and Light Commercial 

Buildings, 3rd Edition.” Mohawk Valley Community College, Utica, NY. 
23 

Appendix A: Bin Temperature Analysis

A process for determining the best settings for the outdoor reset control was presented in Section 

4.2.3. The following procedure takes that analysis a step further and helps the designer determine 

the frequency of condensing given the reset curve settings, the building’s rate of heat loss, the 

climate, and the baseboard output. The basic steps are outlined in Figure 8. 

Figure 8. Process for evaluating outdoor reset settings using a bin temperature analysis

This process (called a bin temperature analysis) yields a table similar to Table 5. The data in the 

first two columns of Table 5 are called bin temperature data. This term describes the number of 

hours per year that a location experiences temperatures in a certain range. The range is generally 

five degrees (for example, 61°–65°F). In the following example, the data state that Ithaca, New 

Acquire Bin 

Temperature 

Data

Generate 

Equation for 

Reset Curve

Baseboard 

Length 

OK?

Yes

No

Estimate 

Frequency of 

Condensing

Determine 

Baseboard 

Length 

Determine Ts,max

& ΔT Desired

Estimate 

Satisfactory

?

No Yes

Start

Finish

If baseboard length 

is too long, increase 

Ts,max

If increased condensing 

operation is desired, 

decrease Ts,max. or increase 

ΔT by decreasing flow rate 

or increasing baseboard
24 

York, experiences temperatures of 61°–65°F for 738 hours each year. These values are averages 

over many years of data and can be acquired for many locations across the country. 

Table 5. Bin Analysis Results for a Project in Ithaca, New York

Bin 

Mean

(°F)

Hours

Supply 

Temp

(°F)

Load 

(Btu/h) 

Return 

Temp

(°F)

ΔT

(°F) 

% of 

Heating 

Season

62 738 103.2 1066 101.1 2.1 11.1

57 715 110.0 1732 106.5 3.5 10.7

52 691 116.7 2398 111.9 4.8 10.4

47 643 123.5 3064 117.4 6.1 9.7

42 686 130.2 3730 122.8 7.5 10.3

37 776 137.0 4396 128.2 8.8 11.7

32 767 143.7 5062 133.6 10.1 11.5

27 507 150.5 5728 139.0 11.5 7.6

22 381 157.2 6395 144.4 12.8 5.7

17 297 164.0 7061 149.8 14.1 4.5

12 209 170.7 7727 155.3 15.5 3.1

7 119 177.5 8393 160.7 16.8 1.8

2 68 180.0 9059 161.9 18.1 1.0

–3 28 180.0 9725 160.5 19.5 0.4

–8 20 180.0 10391 159.2 20.8 0.3

–13 8 180.0 11057 157.9 22.1 0.1

–18 2 180.0 11723 156.5 23.5 0.0

Step 1. Acquire bin data 

The first step in the process is to acquire bin temperature data which are available from a variety 

of sources. ASHRAE and the U.S. Air Force are two such sources. These data can also be 

acquired from several energy analysis software packages.

Step 2. Calculate the supply temperature based on the reset curve

The second step is to generate the equation for the reset curve so the supply temperature for each 

bin can be calculated. For this example, consider Figure 9, which shows a maximum boiler 

temperature of 180°F at 5°F outside and a minimum boiler supply temperature of 95°F at 68°F 

outside. 
25 

Figure 9. Boiler reset curve

The boiler supply temperature (Ts) is calculated using the boiler curve and the mean temperature 

for each bin (Tbin). 

Ts = slope of the line × Tbin + Constant 

Ts = (Ts,min-Ts,max)/(Tout,max-Tout,min) × Tbin + Constant

The only unknown, if we use design conditions to start, is C. To solve for C, substitute in the 

design conditions for Ts and Tbin: 180°F supply at 5°F 

outside. 

This yields 

180 = (95–180)/(68–5) × 5 + C 

C = 180 – (–85/63) × 5 = 186.91 

The general equation (for this reset curve) to input into the 

Supply Temp column is then: 

Ts = –1.0345 × Tbin + 186.91 

 

Step 3. Calculate the load for each bin 

For this analysis, the building’s UA, or its rate of heat loss, 

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40 50 60 70 80

Supply Temperature [F]

Outside Temperature [F]

Boiler Reset Curve

A Building’s UA

For this analysis, the building’s 

UA, or its rate of heat loss, can 

be calculated using the design 

load from the sizing calculation.

The calculation is:

Building UA = Design Load ÷ 

(Tin – Tout)

Tin = assumed thermostat set

point

Tout = design temperature for 

your location 
26 

can be calculated using the design load from the sizing calculation.

UA = Design Load ÷ (Tin – Tout) 

Tin = assumed thermostat set point 

Tout = design temperature for your location 

Use the UA from this calculation to calculate the load for each bin. The load for each bin 

temperature can be calculated by multiplying the UA by the difference between the indoor (Tin) 

and outdoor temperatures (Tbin). 

Load = UA × Tin–Tbin 

Step 4. Calculate the return temperature

Finally, assuming the flow rate through the zones is 1 gpm, the return temperature (Tr) is

calculated as follows:

Tr = Ts – (Load/(gpm × 500)) 

Step 5. Determine % of the heating season system will condense 

The % Heating Season column is simply the number of hours for that bin divided by the total 

number of hours in the table. Using this column and the Return Temp column, the frequency of 

condensing can be analyzed for Ts,max. Simply identify the return temperatures at or below 130°F 

and add up the % of the heating season for which that condition applies. 

For example, in Table 5, the predicted frequency of condensing for these settings is 

approximately 64% of the heating season. This means that the boiler will likely be in condensing 

mode 64% of the heating season. It is NOT the predicted efficiency of the boiler. 
27 

Appendix B: Prescriptive Measure Checklist

Boiler Control Setup: Field Checklist

Tool Box Items

Digital Thermometer Compass Tape Measure Boiler Control Manual

Where to Locate Information

BC – Boiler Controller MS - Manufacturer Specifications CV - Calculated Value MP – Measured Parameter

System Parameters Comments/Suggestions

1. Verifying Proper Sizing of the Boiler

CV a. Design Load kBtu/h Calculated according to ACCA Manual J

MS b. Max output capacity of boiler kBtu/h

MS c. Min output capacity of boiler kBtu/h If min output capacity > the design load, choose a smaller boiler or 

one that can modulate below the design load or add a buffer tank to 

the system

2. Assessing Baseboard Capacity

CV a. Average baseboard water 

temperature under design conditions

°F Assume a 20°F temperature difference between the supply and the 

return for each zone

MS b. Baseboard output capacity at 

average water temperature

kBtu/h/ft 

MP c. Total length ft

CV d. Total capacity (output x length) kBtu/h If baseboard capacity < design load, add more baseboard or choose a 

higher design supply water temperature 

3. Entering Control Settings

MS a. High limit °F

BC b. Ts,max °F Ts,max must be lower than high limit

BC c. Tout,max °F

BC d. Ts,min °F

BC e. Tout,min °F

BC f. Differential °F

BC g. Warm weather shutoff °F This setting should be 2°F to 4°F higher than desired temperature for

occupied periods

h. Thermostat setting (s) °F to °F If no setback, enter only one temperature

4. Sensor Placement

MP a. Side of house (North, South, East

West)

MP b. Height off ground ft Verify that sensor is installed above the typical snow line

MP c. Heat sources nearby Yes No If yes, move sensor

5. Verifying performance

MP a. Outdoor temperature °F

CV b. Predicted supply set point from 

boiler curve °F

BC c. Actual supply temperature being 

delivered

°F Predicted supply should be no more than a couple of degrees different 

than the actual 

MP d. Time for space to increase 1°F min If response is less than 5°F per hour, recommend reducing setback or 

installing a boost
28 

6. Evaluating the Boost Control (if 

applicable)

CV a. Predicted supply set point from 

boiler curve °F

BC b. Lag time allowed min

BC c. Intended temperature offset °F If the supply temperature did not increase after the lag time passed 

and the thermostat is still calling for heat, recheck the settings in the 

boiler’s controller and consult the manufacturer’s installation guide.
DOE/GO-102013-3895 ▪ May 2013

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