Measure Guideline: 

Condensing Boilers—

Optimizing Efficiency and 

Response Time During 

Setback Operation

L. Arena

Consortium for Advanced Residential Buildings

February 2014
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iii

Measure Guideline: Condensing Boilers—Optimizing Efficiency and 

Response Time During Setback Operation

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:

Lois B. 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-0-40342-03

February 2014
iv

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v

Contents

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

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

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

Abstract..................................................................................................................................................... viii

Acknowledgments ................................................................................................................................... viii

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

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

2 Decision-Making Criteria ..................................................................................................................... 4

2.1 Cost and Performance ..........................................................................................................4

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

2.2.1 Oversized Heat Sources...........................................................................................5

2.2.2 Undersized Baseboard .............................................................................................5

2.2.3 Effects of Setback on Indoor Relative Humidity and Building Moisture................5

2.2.4 Outdoor Sensor Placement.......................................................................................5

2.2.5 Specialty Components .............................................................................................5

3 System Interactions ............................................................................................................................. 7

3.1 Setback and the Need To Oversize ......................................................................................7

3.2 Greater Heat Capacity Equals Longer Recovery Periods....................................................8

3.3 Increasing Heat Loss and Decreasing Baseboard Effectiveness........................................13

3.4 Milder Outdoor Conditions and Decreased Recovery Time..............................................13

3.5 Using Added Baseboard Capacity To Recover Without Boost.........................................13

4 Measure Implementation ................................................................................................................... 15

4.1 Determine If Setback Is Appropriate for the Home...........................................................15

4.1.1 Evaluate Climate Zone...........................................................................................15

4.1.2 Who Shouldn’t Use Nighttime Setback.................................................................16

4.2 Design and Install Procedures............................................................................................16

4.2.1 Size the Boiler for Setback Operation ...................................................................16

4.2.2 Size the Baseboard for Setback Operation With a Boost Control .........................18

4.2.3 Size the Baseboard for Setback Operation Without a Boost Control ....................19

4.2.4 Determine the Set Points for the Outdoor Reset Curve .........................................20

4.2.5 Determine the Settings for the Boost Control........................................................21

5 Verification Procedures and Tests ................................................................................................... 24

5.1 Proper Functioning of the Outdoor Reset ..........................................................................24

5.2 Evaluate Boost Control......................................................................................................25

5.3 Additional Resources.........................................................................................................26

References................................................................................................................................................. 27

Appendix: Prescriptive Measure Checklist ............................................................................................ 29
vi

List of Figures

Figure 1. Condensing boiler with an indirect domestic hot water system and outdoor reset control1

Figure 2. Linear boiler reset curve ............................................................................................................ 2

Figure 3. Combustion efficiency versus return water temperature....................................................... 2

Figure 4. Recovery time from thermostat setback and time to fall to the setback temperature for a 

1,200-ft2 super-insulated home in climate zone 6 at varying setback intervals and exterior 

temperatures....................................................................................................................................... 10

Figure 5. Recovery time from thermostat setback and time to fall to the setback temperature for a 

1,200-ft2 2009 IECC compliant home in climate zone 6 at varying setback intervals and exterior 

temperatures....................................................................................................................................... 11

Figure 6. Recovery time from thermostat setback and time to fall to the setback temperature for 

an uninsulated, 1200-ft2 home in climate zone 6 at varying setback intervals and exterior 

temperatures....................................................................................................................................... 12

Figure 7. Recovery times for an 8°F setback for five different size homes built to the 2009 IECC in 

climate zone 6. Heat input is assumed to be twice the design load. ............................................ 13

Figure 8. Comparison of recovery times for climate zones 4 through 8 for similar size homes built 

to the 2009 IECC ................................................................................................................................. 16

Figure 9. Reset curve for a condensing water heater if the supply temperature under design 

conditions is 140°F............................................................................................................................. 21

Figure 10. Flow diagram illustrating the effects of a boost control on the boiler supply temperature23

Unless otherwise noted, all figures were created by CARB.

List of Tables

Table 1. Effective Heat Capacity of a 1200-ft2 Home Built to 2009 IECCa in Climate Zone 6............... 7

Table 2. Recovery Time for an 8°F Setback for Three Homes Insulated to Different Levels With 

100% Oversized Boilers ....................................................................................................................... 8

Table 3. Generic Baseboard Capacities/Linear Foot for Different Flow Rates and Element 

Diameters ............................................................................................................................................ 18

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

Definitions

ACCA Air Conditioning Contractors of America

AHRI Air-Conditioning, Heating, and Refrigeration Institute

CARB Consortium for Advanced Residential Buildings

gpm Gallons per minute

IECC International Energy Conservation Code

kBtu/h Thousands of British thermal units per hour

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
viii

Abstract

Conventional wisdom surrounding space heating has told us a couple of things consistently for 

several years now: sizing the mechanical systems to the heating loads and setting the thermostat 

back at night for a period of at least 8 hours will result in energy savings. The problem is these 

two recommendations oppose each other. A system that is properly sized to the heating load will 

not have the extra capacity necessary to recover from a thermostat setback, especially at design 

conditions. The implication of this is that, for setback to be successfully implemented, the 

heating system must be oversized.

This issue is exacerbated further when an outdoor reset control is used with a condensing boiler,

because not only is the system matched to the load at design, the outdoor reset control matches 

the output to the load under varying outdoor temperatures. Under these circumstances, the home 

may never recover from setback. Special controls to bypass the outdoor reset sensor are then 

needed.

Properly designing a hydronic system for setback operation can be accomplished but depends on 

several factors. The savings to be achieved and the response times possible depend on the 

climate, the heat capacity of the home (mass storage), the level of setback desired, and the 

controls available. Determining the appropriateness of setback for a particular project is the first 

step in the process. This is followed by proper sizing of the boiler and baseboard to ensure the 

needed capacity can be met. Finally, control settings must be chosen that result in the most 

efficient and responsive performance.

This guide is intended to provide step-by-step instructions for heating contractors and hydronic 

designers for selecting the proper control settings to maximize system performance and improve 

response time when using a thermostat setback. It is applicable to both new construction and 

retrofit applications.

Acknowledgments

Steven Winter Associates, Inc. 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. Steven Winter Associates Inc. would like to thank John Siegenthaler of 

Appropriate Designs along with 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 home has one or more of the 

following:

• A non-condensing boiler

• Insulation levels significantly higher than the 2009 IECC

• High mass floors and/or walls. 

! !

Designing for 

Operation with 

Thermostat 

Setback

Reasons for Setback. Using thermostat setback with boost controls for recovery can 

result in improved efficiency, reduced cycling, and less energy consumption than systems 

operated in constant temperature mode. The following are the steps required to ensure 

efficient operation and optimal comfort.

Determine If Setback 

Is Right for the 

Project

Step One: Determine if the use of setback is right for the project based on 1 climate, level of mass in the building, insulation levels, and occupant wishes.

Sizing the Boiler

Step Two: Properly size the boiler to meet the design load plus the energy 

required to recover from a thermostat setback. Perform load calculations in 

accordance with ACCA’s Manual J, and determine the extra capacity needed to 

bring the house to the desired temperature after a period of setback. 

2

Sizing the 

Baseboard

Step Three: As with the boiler, it is essential that the heat emitters are properly 

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

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

performance while providing an acceptable recovery time.

3

Determine the 

Outdoor Reset

Settings

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

dramatically increase the efficiency of the system and can result in improved 

occupant comfort. Determine the control settings that will best accomplish both of 

these tasks.

4

Select Boost Control 

Settings

Step Five: Select the proper settings for the boilers boost control. This control 

allows the boiler to bypass the outdoor reset controller if the space is not brought 

to temperature in a reasonable time. Determine the incremental increase in 

temperature and the amount of time between increases.

5

6 Step Six: Before leaving the job, verify that the outdoor reset control and the 

boost controls are performing as intended.

Verify Proper 

Operation
1

1 Introduction

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

become a common energy-efficient solution for space heating in cold climates. A typical system 

configuration can be seen in Figure 1, which includes a modulating condensing boiler,

distribution piping that includes a primary/secondary loop for hydraulic separation, baseboard 

convectors (not shown), pumps, and an indirect tank for providing the domestic hot water. While 

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

are not properly configured, these heaters will perform no better than their non-condensing 

counterparts.

Figure 1. Condensing boiler with an indirect domestic hot water system and outdoor reset control

The reason condensing boilers achieve such high efficiencies is that they have the ability to 

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 

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

exchanger below the dew point and condensation will occur. Therefore, strategies that lower the 

return water temperature will improve the boiler’s efficiency.

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 load. These controls consist of an outdoor temperature sensor and a special 

controller that communicates the current outdoor conditions to the boiler’s standard controls. 

When using this type of control, the boiler’s supply temperature decreases as the outdoor 

temperature rises, thus ensuring lower return water temperatures. For example, based on the 

standard factory reset curve shown in Figure 2, when the outside temperature is 5°F, the boiler 

supply temperature will be 180°F. But when the outside temperature rises to 50°F, the supply 

temperature is reduced to 120°F. The program that tells the boiler what temperature to deliver 

generally comes preset from the factory, but can be altered by the technician to optimize 

performance. A more detailed explanation of outdoor reset controls and how to program them 

can be found in the publication titled Measure Guideline - Condensing Boilers: Control 

Strategies for Optimizing Performance and Comfort in Residential Applications (Arena 2013).

Figure 2. Linear boiler reset curve Figure 3. Combustion efficiency versus return 

water temperature

The desired outcome is the matching of space heating needs with the heat delivered and improved combustion efficiency. The effect of return water temperature on combustion efficiency 

can clearly be seen in the laboratory data displayed in Figure 3. As the return water temperatures 

were decreased, the combustion efficiency of the boiler clearly increased (Butcher 2011).

Additional efficiency gains can be realized if the thermostat setting is reduced at night and 

during unoccupied hours. This strategy is known as “setback” operation and has been shown in 

numerous field studies to save energy. It can also reduce system cycling, which reduces wear on 

the boiler. However, when the system has to raise the temperature in the space several degrees 

(“recover” from setback) in addition to satisfying the normal operating load, it may not have the 

required capacity. This is particularly true of systems that have been properly sized to meet the 

0

20

40

60

80

100

120

140

160

180

200

0 20 40 60 80

Supply Temperature

Outside Temperature

84

86

88

90

92

94

96

80 100 120 140 160

Return Water Temperature (F)

Efficiency (%)

 

 

 

 
3

design load according to industry-recommended guidelines, 

especially if the system operates under the control of an 

outdoor reset. The reset control will only keep the supply 

temperature at the level required to meet the normal load 

based on the current outdoor conditions. Because of this, the 

system will not be able to provide the additional capacity 

needed to satisfy the demand from the setback. This will 

seriously hinder the system’s ability to raise the temperature 

in the space. In fact, recovery from setback in previous 

studies was found to span several hours, and in some 

circumstances was not achieved at all.

Despite these issues, there are occupants who prefer a 

setback strategy to constant temperature operation. Many 

people prefer their bedrooms to be cooler at night for 

comfort reasons. Cost savings from reducing the indoor 

temperature during unoccupied periods are also often 

quoted as a reason for turning the thermostat down. 

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

boilers employ controls that respond to lags in recovery by boosting the supply temperature and 

bypassing the outdoor reset controller. Once the thermostat in the conditioned space is satisfied, 

the system defaults back to the outdoor reset control. While the system efficiency will be reduced 

slightly during the recovery period, response time and comfort will be improved. Available 

options for a boost strategy are: (1) boiler controls that automatically raise the boiler output 

target temperature if the thermostat setting in the conditioned space is not satisfied within a set 

time frame; (2) an indoor sensor that would work in conjunction with the outdoor reset control to 

compensate for lags in response based on the difference between the interior temperature and the 

thermostat set point; or (3) a simple manual override switch. In addition, oversizing the 

baseboard and possibly the boiler may be necessary to meet the additional load during periods of 

setback recovery.

While these controls are widely available and commonly installed, it is apparent from previous

research efforts 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 design the 

system to quickly and efficiently recover from periods of thermostat setback.

This guide is intended to provide step-by-step instructions for heating contractors and hydronic 

designers for selecting the proper control settings to maximize system performance and improve 

response time when using thermostat setback.

Facts About Setback: 

• For an 8-hour nighttime 

setback period, every 1°F 

setback will result in ≈ 

1% savings in energy 

consumption. 

• The colder the weather, 

the greater the energy 

savings from thermostat 

setback. 

• Savings are generally 

greater during periods of 

nighttime setback than 

during periods of daytime 

setback.
4

2 Decision-Making Criteria

In a prior research study, using thermostat setback with boost controls for recovery resulted in 

improved efficiency, reduced cycling, and less energy consumption than systems operated in 

constant temperature mode. In order to ensure efficient responsive operation, however, careful 

design and planning are necessary. Extra capacity is required to overcome the additional load 

resulting from temperature setback, and controls that ensure quick response should be 

incorporated.

The following sections describe the key steps in determining the optimal system settings and 

control strategies for providing satisfactory response times and maximum overall system 

efficiency when thermostat setback is feasible and desired.

2.1 Cost and Performance 

Thermostat setback is accomplished with a standard thermostat, programmable or not, and can 

result in reduced operating time, reduced wear on the boiler, and decreased energy use. Field 

studies have shown that setback operation can result in savings ranging from 5%–17% over 

constant temperature operation (Tariku et al. 2008; Arena 2013).

The basic system setup recommended in this guide includes a modulating condensing boiler (or 

similar heating appliance) coupled with baseboard convectors, both of which have been sized to

provide the necessary capacity to recover from thermostat setback. In some circumstances, this 

may mean selecting a boiler larger than originally intended. With respect to baseboard, since the 

conventional practice is to oversize, there may not be any additional costs involved. If the 

contractor did size the baseboard to the design load, an increase in baseboard will likely be 

needed.

In addition, an outdoor reset control, a programmable setback thermostat, and boost controls to 

bypass the outdoor reset are recommended. Fortunately, both outdoor reset and boost controls 

are becoming standard features with condensing boilers and generally do not result in additional 

installation costs. Increased costs come primarily from the need for increased boiler and 

baseboard capacity.

2.2 Risk Identification 

The risks associated with operating a hydronic heating system with a setback are primarily linked 

to occupant comfort and satisfaction. An outdoor reset controller won’t damage the boiler, but it 

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

problem may result in short periods of reduced efficiency, but again, will not result in damage to 

the equipment. If the baseboard is sized properly, this reduction in efficiency can be minimized.

When considering whether or not to employ these strategies, consideration should be given to the 

boiler’s capacity, the baseboard’s capacity, and how the occupants expect the system to behave. 

Information on how to deal with these risks can be found in Section 4 of this report, in works like

Burdick 2011 and 2012; Butcher 2011, 2009, 2006, and 2004; and in the publication titled,
5

Measure Guideline - Condensing Boilers—Control Strategies 

for Optimizing Performance and Comfort in Residential 

Application (Arena 2013).

2.2.1 Oversized Heat Sources

Conventional wisdom states that heating systems should be 

sized no larger than 110% of the design loads. Under constant 

temperature operation, following this practice should result in 

decreased short cycling, increased life of the equipment, and 

increased efficiency. However, operation of hydronic systems 

with thermostat setback actually requires capacities much higher than 110% of the design load to 

recover. And, as the mass of the home increases, the oversize factor must also increase. For 

example, a home built to the latest energy code may require a system that is 100% oversized to 

recover from a night setback of 8°F, as compared to an uninsulated, old home of the same size 

and location which may only need a system that is 50% oversized. These calculations are 

discussed in more depth in Section 3.

2.2.2 Undersized Baseboard

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

true for baseboard. Undersized baseboard will result in reduced response time, reduced 

efficiency, increased cycling and reduced life of the boiler. Properly sizing the baseboard to 

handle the added load of the recovery period is critical to performance and overall system 

efficiency.

2.2.3 Effects of Setback on Indoor Relative Humidity and Building Moisture

Reducing the interior temperature in the home may increase relative humidity above desired 

levels, thereby increasing the negative effects associated with elevated moisture levels. In 

addition, the temperature of the surfaces in the home will decrease during periods of setback 

increasing the potential for condensation on windows and building envelope components (Tariku 

et al. 2008; CMHC 2009). Before recommending setback strategies, an evaluation of existing 

moisture issues and the expected interior moisture levels should be conducted.

2.2.4 Outdoor Sensor Placement

Where outdoor reset is used, the boiler supply temperature is dependent 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, or too near 

the boiler exhaust, the boiler will be fooled into thinking it is warmer outside than it actually is.

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

can lead to comfort problems and/or drastically increased response times.

2.2.5 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 

Important:

Setback can have a 

sizable impact on peak 

heating loads because 

setback recovery occurs 

during the early morning 

hours on top of the peak 

heat loss. 
6

supply water temperature were 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.
7

3 System Interactions

This section discusses a few typical interactions that need to 

be considered when installing a hydronic system that will be 

controlled with an outdoor reset control and operated with a 

temperature setback.

3.1 Setback and the Need To Oversize

As noted earlier, studies show that thermostat setback results 

in decreased energy use compared to systems operated at 

constant indoor temperatures. However, added capacity is 

needed to recover from that setback as opposed to operation 

under constant temperature. The extra capacity needed 

depends on the heat capacity, or thermal mass, of the building.

The heat storage capacity of the building includes the heat flow into or out of the mass of the 

building, including the furnishings and air in the space. This depends on the rate of heat transfer 

and on the frequency. The static heat capacity can be calculated by multiplying the volume of the 

materials by their specific heat and density, as shown in Table 1. The effective heat capacity is 

usually calculated as 40%–80% of the static heat capacity, assuming typical wood frame 

construction (Kreider and Rabl 1994). The effective heat capacity is generally used for materials 

not directly coupled to the space such as framing members and insulation behind drywall and 

subfloors. 

Table 1. Effective Heat Capacity of a 1200-ft2 Home Built to 2009 IECCa in Climate Zone 6

Area

(ft2)

Thickness

(ft)

Volume

(ft3)

Specific 

Heat

(Btu/lb∙°F)

Density

(lb/ft3

)

Heat 

Capacity

(Btu/°F)

Drywall, Ceiling 1,276 0.04 53.2 0.26 78.0 1,074

Drywall, Walls 1,689 0.04 70.4 0.26 78.0 1,422

Plywood Floors 1,276 0.06 79.8 0.29 34.0 786

Furniture same as floors 786

Air 1,276 8.00 10,208.0 0.24 0.08 184

Studs 805 0.458 369.0 0.45 31.0 3,088

Cellulose 3,960 0.458 1,814.8 0.33 2.20 791

Total Effective Heat Capacityb 8,131 a International Energy Conservation Code

b Only 60% of the heat capacity of the studs and insulation is included in the total, hence the term effective heat capacity.

The example in Table 1 applies to a 1,200-ft2 single family home located in climate zone 6 built 

to the 2009 IECC. The extra capacity required to bring this home back to temperature from a 5°F 

setback in 1 hour would be 5°F × 8,131 Btu/°F = 40,655 Btu/h. If a 2-hour recovery period were 

acceptable, 20,328 Btu/h additional capacity would be needed. Considering the design load for 

Heat Capacity (Kreider 

and Rabl 1994):

• The heat capacity of 

the building is the 

energy required to 

raise it 1°F. 

• The effective heat 

capacity of walls, floors 

and ceilings is roughly 

40%–80% of the static 

heat capacity,

assuming typical 

construction. 
8

the home is approximately 15,000 Btu/h, the capacity of the system would have to be doubled to 

recover from a 5°F setback in 2 hours under design conditions.

The negative effects associated with oversizing are actually minimized when combined with 

setback operation. This is due to the fact that the temperature difference between the emitters and 

the space is much greater than when operated under constant temperature control. During the 

recovery period, more heat is emitted to the space, resulting in lower return water temperatures, 

longer system runtimes, and reduced cycling. Simply operating the system for fewer hours per 

day (as is the case for setback operation) results in reduced cycling. Combining this with boost 

controls also reduces cycling by further decreasing the system runtime.

3.2 Greater Heat Capacity Equals Longer Recovery Periods

Table 2 lists the effective heat capacities for three different homes—a super insulated home with 

double-stud walls, a code level home insulated with cellulose, and an old, uninsulated home.

Each home is assumed to be equipped with a heat source that is twice the design load. The 

recovery times show that similar size buildings with lower heat capacities warm up or cool down 

faster under similar climatic conditions.

Table 2. Recovery Time for an 8°F Setback for Three Homes 

Insulated to Different Levels With 100% Oversized Boilers

Efficiency Level Heat Capacity

(Btu/°F)

Design Load

(Btu/h)

Boiler Size

(Btu/h)

Recovery Time

(hours)

Super Insulated 10685 8,540 17,080 6.9

2009 IECC 8131 15,441 30,882 3.4

Old, Uninsulated 6600 44,030 88,060 1.0

Figure 4 through Figure 6 show the recovery time versus the time it takes to fall to the setback 

temperature for the three homes listed in Table 2. The charts are read as follows:

1. Find the appropriate outdoor temperature on the left side of the plot in the body of the 

graph.

2. Follow the line to the right and up until it intersects with the setback desired.

3. Draw a line vertically to the bottom axis and read the number of hours expected to 

recover from setback given that outdoor temperature, setback, and an input equal to twice 

the design load.

4. Draw a line horizontally to the left to determine the number of hours that it will take for 

the house to fall to the setback chosen.

The following example illustrates the use of these charts.
9

Figure 4 shows a home that is super-insulated. Note that the outdoor temperature goes to only

20°F and the maximum setback shown is 6°F. This is due to the fact that the home loses heat 

much more slowly and would not see an 8°F drop in temperature in less than 8 hours, even at –

10°F outside. At 20°F outside, the temperature would drop only 3°F. While these low heat loss 

conditions are desirable, note that to recover from a 3°F setback at 20°F outside it would take 4 

hours with a heat input twice that of the design load. This indicates that setback for a superinsulated home should be avoided. The added costs of tripling or quadrupling the heating system 

Example

(A) How long will it take for a home built to code to recover from a setback of 6°F if the outdoor 

temperature is 20°F? (B) How long will it take for the home to drop those 6°F?

Answer Part A:

From Figure 4 (see below), find 20°F along the left side of the plot.

Follow the 20°F line up and to the right until it intersects with the 6°F setback temperature.

Drop straight down until the line intersects with the horizontal axis and read the number.

The answer to part A is approximately 2.25 hours to recover from a 6°F setback.

Answer Part B:

Draw a line horizontally to the left until it intersects with the vertical axis and read the answer. It would 

take just over 5 hours for the home to drop 6°F.
10

output capacity to decrease the recovery time would far outweigh the savings for a home with 

such a low heat load. It could be argued that the smallest boilers available would satisfy even 4 

times the load of the super-insulated house without any additional expense to go to the next size, 

however, the baseboard would also have to be quadrupled. Based on how small the heating loads 

are, the potential savings from operating in setback mode would be miniscule compared to the 

extra cost of the baseboard.

In contrast, Figure 6 represents a very old, uninsulated home with plaster walls. With a heat input 

rate twice that of the design load, this home recovers from an 8°F setback in less than 1.5 hours 

at –10°F outside.

Figure 4. Recovery time from thermostat setback and time to fall to the 

setback temperature for a 1,200-ft2 super-insulated home in

climate zone 6 at varying setback intervals and exterior temperatures
11

Figure 5. Recovery time from thermostat setback and time to fall to the 

setback temperature for a 1,200-ft2 2009 IECC compliant home in

climate zone 6 at varying setback intervals and exterior temperatures
12

Figure 6. Recovery time from thermostat setback and time to fall to the 

setback temperature for an uninsulated, 1200-ft2 home in 

climate zone 6 at varying setback intervals and exterior temperatures

Although response times for the super-insulated home look very long under design conditions 

and an 8°F setback, only a fraction of the heating season will see those conditions. As 

temperatures increase outside, response times will be faster if using a boost control. In addition, 

during warmer conditions, the home will not fall to the full setback temperature which will also 

decrease the recovery time. While design conditions should be evaluated, decisions to use 

setback versus constant temperature control would be best evaluated at average low temperatures 

for the project location, not necessarily design conditions.

Another interaction that should be noted is the effect of house size on recovery rate. As can be 

seen in Figure 7, recovery time actually increases as house size increases. All five homes 

evaluated were located in the same climate zone and have the same configuration and insulation 

levels. The reason the recovery time increases is because the increase in heat load is smaller than 

the increase in the heat capacity as the house increases in size.
13

Figure 7. Recovery times for an 8°F setback for five different size homes built to the 

2009 IECC in climate zone 6. Heat input is assumed to be twice the design load.

3.3 Increasing Heat Loss and Decreasing Baseboard Effectiveness

Assuming a constant outdoor temperature, response time will be affected by the fact that the heat 

loss from the conditioned space will increase as the indoor temperature increases, but the output 

of the baseboard will actually decrease. This is because the temperature difference between the 

baseboard and the room air has decreased. This is mostly a concern if the added capacity in the 

system is coming only from additional length of baseboard and not from increased supply 

temperatures that would result from using a boost control. Fortunately, most baseboard ratings 

are based on 65°F room air. Therefore, for room air temperatures under that baseline, the output 

should be higher than rated. This will offset the lower-than-rated output once the house 

temperature rises above 65°F.

3.4 Milder Outdoor Conditions and Decreased Recovery Time

As temperatures increase outside, temperatures in the home may not fall to the setback 

temperature resulting in quicker recovery times (if using a boost control). Considering that the 

design heating condition occurs for only 1% of the heating season, the evaluation of recovery 

time should be conducted for a more common operating condition. A recommendation would be 

to look at the average low temperature for each of the three coldest months, and use that 

temperature to determine the extra capacity needed.

3.5 Using Added Baseboard Capacity To Recover Without Boost 

Another option for recovering from setback is to oversize the baseboard and supply warmer 

water so that the heat input to the space under design conditions is greater than the load. For 

example, if the design load were 15,000 Btu/h, install enough baseboard to supply 30,000 Btu/h 

under design conditions. For maximum efficiency, assume a supply temperature at design that 

will result in condensing operation. Typically systems are designed so that there is a 20°F 

0.0

1.0

2.0

3.0

4.0

5.0

6.0

1000 2000 3000 4000 5000

Hours

House Size [ft2]

Recovery Time by House Size
14

difference between the supply and return to the 

boiler at design temperatures. Under these 

conditions, the maximum boiler supply temperature 

should be set to 150°F on the outdoor reset curve, 

and the baseboard should be sized to provide 30,000 

Btu/h at an average of 140°F water.

The downside to this approach as opposed to using a 

boost control is particular to using outdoor reset. As 

noted earlier, as the outdoor temperature increases 

the boiler supply temperature will decrease. 

Unfortunately, the baseboard output at lower 

temperatures is not proportional to the decreased 

load. Baseboard is less efficient at lower 

temperatures. Recovery times will actually increase 

as the outdoor temperatures increase and the boiler 

supply temperature decreases. Therefore, while 

increased baseboard is recommended under design 

conditions for the most efficient steady-state 

operation when the house is already at the desired 

temperature, it should be used in conjunction with a 

boost control to more efficiently and quickly recover from setback.

Caution:

When relying solely on added 

baseboard length to recover from 

setback (without the aid of a boost 

control), baseboard effectiveness 

will be reduced as:

• The temperature in the home 

increases but the baseboard 

temperature remains constant;

i.e., outdoor temperature is 

constant and boiler has reached 

programed supply temperature.

• The outdoor reset control reduces 

the boiler supply temperature as 

the exterior temperature 

increases. 

Both of these conditions result in 

extended recovery times. 

Baseboard should be sized 

accordingly.
15

4 Measure Implementation

4.1 Determine If Setback Is Appropriate for the Home

4.1.1 Evaluate Climate Zone

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

can result in savings in all zones. Actual savings will be greatest in colder climates; however, 

savings in milder climates are a larger percentage of the overall load (Tariku et al. 2008).

Recovery times from setback will vary from climate to climate, with size of the home, and with 

differences in the levels of mass in relationship to the load. The recovery times for a similar size 

home in climate zones 4 through 6 are displayed in Figure 8. All homes were modeled with 2009 

IECC insulation levels. The increase in recovery time from climate zone 4 to climate zone 5 is 

due to the increase in lumber in the walls and the increase in insulation levels. Homes in zone 4 

were assumed to be constructed with 2 × 4 walls, while homes in zones 5 through 8 are assumed 

to have 2 × 6 walls. The thermal mass in the homes in zones 5 through 8 are fairly similar, but 

the heat loads are quite different. This results in shorter recovery times as the climate gets colder.

Essentially, there is more heating capacity available per square foot of mass because the heating 

system is larger the colder the climate, but the heat capacity of the building changes very little.

No climate zones need be excluded from setback operation.

Scope of Work

A. Determine if the project is right for thermostat setback operation. 

B. Size the boiler to provide the capacity needed for recovery from setback. 

C. Size the baseboard to provide the capacity at low supply temperatures and to recover from 

setback.

D. Determine the proper settings for the outdoor reset control

E. Determine the proper settings for the boost control.

 
16

Figure 8. Comparison of recovery times for climate zones 4 through 8 for 

similar size homes built to the 2009 IECC

4.1.2 Who Shouldn’t Use Nighttime Setback

For any home with a hydronic heating system, if the answer to any of the following questions is 

“yes,” the project is probably not right for nighttime setback and should be maintained under 

constant temperature operation.

1. Does the home have a boiler that cannot modulate its output?

2. Is the home insulated to levels significantly better than the 2009 IECC?

3. Is the heat emitter radiant flooring or radiant panels?

4. Are the walls or floor of the conditioned space constructed with concrete? If yes, is that 

concrete uninsulated to the interior?

5. Do the occupants expect/desire a recovery time shorter than 1 hour?

4.2 Design and Install Procedures

The following steps are necessary for proper design and installation of a hydronic system 

intended to be operated with a nighttime thermostat setback. An example is provided for each 

step and is based on a 2000-ft2 home located in climate zone 6 built in compliance with the 2009 

IECC. The home is assumed to be two stories of conditioned space over an unconditioned 

basement.

4.2.1 Size the Boiler for Setback Operation

The very first step in a good design is proper sizing of the mechanical equipment. A right-sized 

heating system provides the desired occupant comfort and runs efficiently. Even though 

oversizing is necessary for recovery from setback, the design load must be determined first. Then 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Zone 4 Zone 5 Zone 6 Zone 7 & 8

Hours

Recovery Time by Climate Zone Burner Modulation:

Burners in newer natural gas and 

propane boilers typically have 

the ability to supply a fraction of 

the total capacity by modulating 

the burner.

This feature is often referred to 

as the turndown ratio. A 

modulation rate of 20% equates 

to a 5:1 turndown ratio. 
17

the proper capacity needed to recover from setback can be determined and the boiler can be 

selected based on those two pieces of information.

In order to properly size the boiler, accurate heating loads must be calculated. These calculations 

should be performed in compliance with the Air Conditioning Contractors of America’s (ACCA) 

standard protocols as outlined in the most recent version of Manual J: Residential Load 

Calculation (ACCA 2009a).

For constant temperature operation, it is ideal if the highest modulation rate of the boiler matches

the design load of the home, leaving plenty of room for the unit to reduce its firing rate for the 

loads during non-peak times. However, for setback operation, the boiler should be approximately 

twice the design load for a code compliant home to about 1.5 times the heating design load for an 

old inefficient home.

Example 1:

The buyers of new spec home in Ithaca, New York would like to install a condensing boiler with 

baseboard convectors. They desire to use a nighttime setback in their new home because they are 

accustomed to sleeping in cooler temperatures. The home will be 2000 ft2 and will be built in 

compliance with the 2009 IECC. The design load on the house was calculated using Manual J and is 

approximately 25,000 Btu/h. What size boiler should the contractor install?

Answer:

Ideally, the contractor should install a boiler with that has a capacity equal to twice that of the design 

load:

2 × 25,000 = 50,000 Btu/h

The boiler should also have a modulation rate of at least 5:1 (typically the best ratio available in 

residential boilers) resulting in a low output of between 10,000 and 15,000 Btu/h.

Note: If this were a very old inefficient home, the contractor would only have to oversize the boiler by 

50% or 1.5 × 25,000 = 37,500 Btu/h

Additional Resources:

• ACCA Manual J V.8: Residential Load Calculation (ACCA 2009a)

• ACCA Manual S V.3: Residential Equipment Sizing (ACCA 2009b)

• Heating and Cooling of Buildings (Kreider and Rabl 1994)

• Condensing Boilers: Control Strategies for Optimizing Performance and Comfort in Residential 

Applications (Arena 2013)
18

4.2.2 Size the Baseboard for Setback 

Operation With a Boost Control

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 result in reduced condensing 

which in turn results in lower system 

efficiencies. Additionally, many boilers 

compare the temperature difference between 

the supply and return water. 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.

Manufacturers of baseboards publish output 

capacities for their equipment at varying 

temperatures. Table 3 lists some generic 

baseboard capacities for different flow rates 

and element diameters assuming only one pipe passes through the baseboard. To use the chart, 

determine the average baseboard temperature at design conditions, the flow rate and the size of 

the element to be installed. For example, if the boiler supply temperature at design is 150°F, 

assume a 20°F difference between the supply and the return, which would result in an average 

water temperature of 140°F in the baseboard. Assuming the size of the element will be 3⁄4 in. and 

a 1 gpm flow rate through the system (unless 4 gpm or higher can be verified), the baseboard 

output from Table 3 would be 290 Btu/h/linear foot.

Table 3. Generic Baseboard Capacities/Linear Foot for 

Different Flow Rates and Element Diameters

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

Determining Baseboard Capacity in an 

Existing Home:

1. Measure the length of the baseboard 

(length with fins).

2. Determine the manufacturer.

3. If manufacturer is unknown, compare 

the dimensions of the casing, fins,

and pipe diameter to those of several 

manufacturers. Pick the one that is 

closest.

4. Determine the boiler supply 

temperature at design and subtract 

10°F (assuming the system was 

designed for a 20°F temperature 

differential) 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.

8. If setback operation is desired and 

the boiler does not have a boost 

control, double the length of the 

baseboard if possible or increase the 

supply temperature to a setting that 

will result in double the output 

capacity of the baseboard.
19

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 of the boiler. This may be the only option in situations where the smallest boilers are too 

large for the design load or there are several zones, all of which have 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 was 30 kBtu/h, installing enough baseboard to regulate the system would require installing 

103 linear feet of baseboard (30,000 Btu/h ÷ 290 Btu/h∙ft). For those homes where space is 

limited, high output baseboards or buffer tanks should be considered to counter the negative 

effects of severely oversized equipment.

4.2.3 Size the Baseboard for Setback Operation Without a Boost Control

As noted earlier, recovery from setback can be met with the use of a boost control by oversizing 

the baseboard and selecting a boiler supply water temperature that results in heat input to the 

space which is greater than the load. For maximum efficiency, assume a supply temperature at 

design that will result in condensing operation. But, because the baseboard is less efficient at 

lower temperatures, recovery times will actually increase as the outdoor temperatures increase 

and the boiler supply temperature decreases.

Example 2:

For the home in Example 1, determine the length of baseboard needed to provide the capacity needed 

to recover from setback (50 kBtu/h). The maximum boiler temperature is 180°F, and the maximum 

supply temperature on the outdoor reset curve is programmed at 150°F to ensure 130°F return water 

temperatures at design conditions. Assume a flow rate of 1 gpm and 3⁄4-in. elements. The occupants 

wish to set back their thermostat 8°F at night. Size the baseboard assuming the boiler has a boost 

control and can supply water up to 180°F. 

Answer:

If the supply temperature is 180°F and the system has been designed for a 20°F temperature 

difference at design conditions, the average baseboard temperature would be 170°F. Therefore, the 

capacity at 170°F should be used to size the baseboard. Determine the length at 170°F average 

temperature at the design load of 50 kBtu/h:

50,000 Btu/h ÷ 480 Btu/h∙ft = 104 ft

To make sure this length can meet the design load once the home is up to temperature, determine the 

length of baseboard needed when the system is running at the temperature dictated by the outdoor 

reset curve—150°F. Assume a 20°F temperature difference which would result in an average 

baseboard temperature of 140°F at the design load of 25kBtu/h:

25,000 Btu/h ÷ 290 Btu/h∙ft = 86 ft

To meet the additional load from setback, 22 additional feet of baseboard would have to be installed. 

At $10/ft, this would result in an additional cost of $220. If the savings due to the setback were 1% for 

every 1°F, the homeowners would save approximately 8% annually. Assuming $1200/yr for heating, 

the savings would be ≈ $96/year and would have a simple payback of 2.3 years.
20

4.2.4 Determine the Set Points for the Outdoor Reset Curve

Determining the optimal settings for the reset curve is an iterative process. While the goal is to 

create conditions where the return water temperature is always below the dew point for the 

particular fuel being used, this could result in baseboard lengths that are too long for the space 

available. Therefore, several supply temperatures may need to be evaluated. This process is 

outlined in detail in the publication titled Measure Guideline - Condensing Boilers: Control 

Strategies for Optimizing Performance and Comfort in Residential Applications (Arena 2013).

Ultimately, the goal is to maximize the time the boiler is in condensing operation. Systems 

should be configured to condense at design conditions. This can be accomplished by supplying 

temperatures lower than the boiler’s maximum supply temperature. Then, when recovery from 

setback is needed, the boiler can boost to higher temperatures to meet the added load. For 

modulating, condensing boilers, design the system to meet the design heating load such that the 

return temperature does not exceed 130°F. For loops designed for a 20°F temperature difference 

between the supply and return, this would equate to a 150°F boiler supply temperature and an 

average loop temperature of 140°F.

If using a condensing water heater, design the system to meet the design heating load such that 

the tank temperature does not exceed 130°F. Because the heat exchanger is submerged in the 

tank, the tank temperature will have a much greater effect on condensing than the return water 

temperature. If maintaining a temperature below 130°F, the supply temperature to the zones

would be no more than 125°F assuming that there is a 5°F temperature loss in the heat 

exchanger. Figure 9 shows the reset curve for a condensing water heater where the supply 

temperature under design conditions was set to 140°F. Under these conditions, the tank must be 

kept at 145°F in order to supply 140°F water to the space heating zones. This means that the tank 

will be kept above the condensing temperature of 130°F until the outdoor temperature rises 

above 15°F.

Example 3:

For the home in Example 2, size the baseboard assuming there is no boost and the maximum boiler 

supply will be 150°F to promote condensing. Use the baseboard capacities list in Table 3.

Answer:

Check the length required at 140°F average baseboard temperature at the design load of 50 kBtu/h:

50,000 Btu/h ÷ 290 Btu/h∙ft = 173 ft

Compared to constant temperature operation, this option results in an additional $870 of baseboard 

and a simple payback of 9 years as opposed to $220 additional for setback operation coupled with a 

boost as determined in Example 2.
21

Figure 9. Reset curve for a condensing water heater if the supply temperature 

under design conditions is 140°F

If designed for a 20°F temperature difference between the supply and return under design 

conditions, the average loop temperature would be 115°F, and the baseboard should be sized 

accordingly. Capping the maximum supply temperature to 125°F doesn’t only encourage 

condensing, it also reduces standby losses because the entire tank is kept at a lower temperature.

Running the tank at a temperature higher than 130°F will result in efficiencies that are only in the 

mid to upper 80% range. Incorporating a boost with these systems would allow warmer supply 

temperatures for a short period of time, but would not require keeping the tank at higher 

temperatures once the space reaches the desired temperature. This would reduce the amount of 

baseboard needed. It is not recommended that condensing tanks are used for constant 

temperature operation unless that temperature is below 130°F.

4.2.5 Determine the Settings for the Boost Control

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 in homes when outdoor 

reset controls are used without boost, since these controls are specifically trying to very closely 

match the output of the system 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 space heating is accomplished, the 

system defaults back 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

80

90

100

110

120

130

140

150

-30 -10 10 30 50 70 90

Supply Water Temperature (°F)

Outdoor Temperature (°F)

Space Heating Supply Temperature

Tank Temperature
22

degrees F is added to the target temperature of the boiler. Both of these values are adjustable 

based on selections in the installer menu.

Figure 10 illustrates how such a boost control would operate. “Lag” is the number of minutes (x) 

that would pass before the boiler’s supply temperature was increased, and “offset” refers to the 

number of degrees (d) by which the supply temperature would be raised. This process would 

continue until either the thermostat was satisfied or until the supply temperature reached the high 

limit setting of the boiler.

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 would be

increased to 128°F. If the central heating thermostat continues to call for another 20 minutes, the 

temperature would jump again to 146°F, and so on. The actual settings chosen can be adjusted 

based on response and occupant preference. 

To determine the best setting for the boost control, the following should be considered:

1. The capacity of the baseboard at different temperatures. Some baseboard almost doubles 

in capacity with a 20°–30°F increase in supply temperature.

2. The heat capacity of the home. See Figure 4 through Figure 6 to determine the response 

time for the desired setback and building efficiency.

These two pieces of information along with the occupants’ desires will determine the necessary 

increase in supply temperature and the time lag between increases.

Note that the temperature will not exceed the maximum supply temperature dictated by the 

boiler’s safety setting.

Additional Resources

• Modern Hydronic Heating for Residential and Light Commercial Buildings, 3rd edition; John 

Siegenthaler, P.E. (Siegenthaler 2012)

• I=B=R Guide—Residential Hydronic Heating: Installation and Design, AHRI
23

Figure 10. Flow diagram illustrating the effects of a boost control on the boiler supply temperature

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-zone systems as the boiler may boost 

to the highest temperature if calls from several zones overlap.
24

5 Verification Procedures and Tests

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

achieved. The inspections outlined below should take only a few minutes. A field checklist has 

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

5.1 Proper Functioning of the Outdoor Reset

Verifying that the boiler is operating as intended is not very difficult unless it is an extremely hot 

day when testing is desired. Under those conditions it may not be possible to get the boiler to 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 that 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.

Caution: Ensuring Success

Items to verify:

1.The boiler is properly sized for recovery from setback.

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.The maximum supply temperature on the boiler reset curve is set to promote condensing at 

design conditions.

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.
25

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. Don’t 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 yet, 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.

Example 4:

Determine the boiler supply temperature for a system with a reset curve as shown in the figure if it 

were 30°F outside.

Answer:

Follow a straight line up from 30°F on the horizontal axis until you intersect with the reset curve. Then 

follow that point over 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. 

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
26

5.3 Additional Resources

There are many different control and design strategies for optimizing hydronic system efficiency 

and occupant comfort. Those mentioned in this report are just a small subset. Following are a 

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

• I=B=R Guide: Residential Hydronic Heating Installation & Design (AHRI 2009) 

• Modern Hydronic Heating, John Siegenthaler, PE (Siegenthaler 2012)

• ACCA Manual S, Residential Equipment Selection (ACCA 2009b)

• ACCA Manual B, Balancing and Testing Air and Hydronic Systems (ACCA 2009c).

Verifying Setup

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

• Verify the return temperature on non-condensing systems is not below the critical threshold.
27

References

ACCA. (2009a). Manual J, Residential Load Calculations. Third Edition, V. 1.00. Arlington, 

VA: Air Conditioning Contractors of America.

ACCA. (2009b). Manual S, Residential Equipment Sizing. Third Edition, V. 1.00. Arlington, 

VA: Air Conditioning Contractors of America.

ACCA. (2009c). Manual B, Balancing and Testing Air and Hydronic Systems. Third Edition, 

V.1.00. Arlington, VA: Air Conditioning Contractors of America

AHRI. (2009). “I=B=R Guide; Residential Hydronic Heating.” Arlington, VA: The Hydronics 

Institute Division of the American Heating and Refrigeration Institute.

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

National Energy Technology Laboratory, Morgantown, WV; US Department of Energy, 

Building Technologies Program.

Arena, L. (2013). Measure Guideline: Condensing Boilers - Control Strategies for Optimizing 

Performance and Comfort in Residential Applications. Golden, CO: National Renewable Energy 

Laboratory. NREL/SR-5500-57826. 

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

National Energy Technology Laboratory, Morgantown, WV; US Department of Energy, 

Building Technologies Program.

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

Technology Laboratory, Morgantown, WV; US Department of Energy, Building Technologies 

Program.

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

December 2011, p36-41.

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; US Department of Energy, Office of Buildings Technology.

Butcher, T.; (2006). “Condensing boilers and baseboard hydronic systems.”, ASHRAE 

Transactions, V. 112, part 1, 2006. 

Butcher, T; (2009). “Optimal Design and Operating Parameters of the Condensing Boiler/Hot 

Water- Baseboard Combination: Task Report.” NYSERDA Agreement, No. 10927. Albany, NY; 

NYSERDA.

CMHC. (2005). “Effects of Thermostat Setting on Energy Consumption.” Canadian Center for 

Housing Technology, Technical Series 05-100.
28

Kreider J. F., Rabl A. (1994). Heating and Cooling of Buildings (Design for Efficiency).

McGrawHill, USA.

Nelson, Lorne W. and J. Ward MacArthur (1978) “Energy Savings through Thermostat 

Setbacks”, ASHRAE Transactions, Volume 83, AL-78-1 (1): 319-333.

Plourde, A. (2003). “Programmable Thermostats as Means of Generating Energy Savings: Some 

Pro’s and Con’s.” Canadian Building Energy End Use Data and Analysis Center; CBEEDAC 

2003–RP-01.

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

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

Tariku, F.; Kumaran, M.; Fazio, P.; (2008). “Thermostat Setback Effect in Whole Building 

Performance.” National Research Council Canada; NRCC-50859.
29

Appendix: Prescriptive Measure Checklist

Boiler Setup for Setback Operation: 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. Determine if Setback Operation is 

Right for the Project

Note: If the answer to any of the questions in this section is 

“yes”, setback operation may not be appropriate for the job.

a. Does the home have a boiler that 

cannot modulate its output? Yes No

A boiler that cannot modulate should not be used for setback 

because it will be significantly oversized leading to excessive 

short cycling.

b. Is the home insulated to levels 

significantly better than the 2009 

IECC?

Yes No Homes with insulation levels much above current code 

generally contain higher levels of mass and respond slowly.

c. Is the heat emitter radiant flooring 

or radiant panels? Yes No These types of systems contain a lot of mass and are meant to operate at very low temperatures.

d. Are the walls or floor of the 

conditioned space constructed with 

concrete? If yes, is that 

concrete uninsulated to the 

interior?

Yes No High mass homes respond very slowly and will not recover 

from setback in an acceptable amount of time.

e. Do the occupants expect/desire a 

recovery time under one hour? Yes No Even the best designed systems are unlikely to recover in under 1 hour.

2. 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 add 

a buffer tank to the system

3. 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 

4. 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
30

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

5. 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

6. Verifying performance

MP a. Outdoor temperature °F

CV b. Predicted supply setpoint 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

7. Evaluating the Boost Control (if 

applicable)

CV a. Predicted supply setpoint 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.
buildingamerica.gov

DOE/GO-102014-4369 ▪ February 2014

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