Condensing boilers, while commonly installed for heating hot water applications, often fail to deliver their inherent efficiency advantage because of deficiencies in the system design, start-up, and operation. Condensing or “high efficiency” boilers are constructed to allow them to operate with lower water temperatures than traditional boilers.  This allows condensing boilers to extract more of the useful (latent) energy from the fuel source (typically natural gas or propane) than traditional, non-condensing boilers can use. For condensing boilers to realize their potential for increased efficiency, they must be applied and operated in a manner that allows their combustion gases to condense. Designers and facility operators interested in achieving the efficiency potential of their condensing boilers need to have an understanding of how to design and implement the appropriate system arrangement and control parameters for their boilers.

In this article we will describe how to properly apply a heating hot water reset schedule to maximize the benefits of a condensing boiler. This article provides example calculations for a finned tube (i.e. traditional perimeter heating) system, yet the concepts may be applied to VAV terminal unit heating as well.


One of the byproducts of fuel combustion is water vapor. In non-condensing systems, all of this water vapor is retained in the flue gas and exhausted from the building. This is necessary because when the exhaust vapor condenses the result is a slightly acidic liquid that will quickly eat away materials like cast iron used to construct non-condensing boilers. That water vapor, however, represents an untapped source of energy that can be used to heat your building.  If we go back to high school physics, we remember that it takes 1 Btu of energy to raise 1 lb. of water by 1°F. 

We may also recall that when water is at 212°F (at sea level), we can boil it or turn it into vapor. This phase change (from liquid water to gas) requires 970 Btus per pound of water. Said another way,  making water vapor is 970 times more energy intensive than simply warming the same pound of water 1°F. By nature, the reverse is also true. Lowering the temperature of 1 lb. of water by 1°F releases 1 Btu of energy, but condensing 1 lb. of water vapor releases 970 Btus of energy.

By allowing the return water temperature entering a boiler to go below roughly 130°F, we allow vapor to begin condensing out of the flue gas. This isn’t an all or nothing deal, however. The vapor in the flue gas only starts to condense at about 130°F return water temperature. You get very little increase in efficiency beyond non-condensing operation at this temperature. At slightly above a 70°F return water temperature is when you hit the 98% to 99% efficiency numbers you may see advertised.  This is important to know since a 70°F return temperature is not practical.  This increase in boiler efficiency is roughly linear with a corresponding drop in return water temperature. Knowing this is the key to getting the most out of your condensing boiler. 

We cannot avoid creating water vapor when fuel combustion occurs. We can, however, take advantage of that latent heat by con-densing as much vapor as possible and absorbing that 970 Btu per pound back into our central heating system rather than exhausting that energy out of the building. We’ve already paid for the Btus, so why not use them?


When comparing boilers to use in a building, it is important to compare their efficiencies on an apples-to-apples basis. There are three ratings used for boiler efficiencies and they cannot be compared to one another. When selecting boilers to compare, be sure you are using the same rating method for all the boilers. Table 1 shows a comparison of the standard boiler rating methods.


Condensing boiler manufacturers provide factory default hot water supply temperature (HWST) reset control settings that rely on an outdoor air temperature (OAT) sensor being present. If the outdoor air temperature sensor is not present, improperly installed or simply not connected to the boiler, the boiler will default to a fixed supply water temperature that is typically between 176°F and 194°F — resulting in return water temperatures that are well out of condensing range.  

The boiler manufacturer sets these defaults because they want to ensure their product delivers heat, so they select a default tem-perature range that will satisfy a large majority of demands on the heating system if the OAT sensor is inoperative. However, when operating under the default settings, the capital investment in a condensing boiler does not provide the expected return on investment.

Assuming an OAT sensor is present and properly installed, Figure 1 shows the factory default, out-of-the-box, hot water supply temperature reset schedule as a function of OAT for five different, small-scale condensing boilers (up to about 2.0 MMBTUH) from several well known manufacturers.1 

Taking boiler no. 1 as an example, it will start to condense when the OAT is above roughly 56°F. In Burlington, VT, (a cold climate, ASHRAE 6A), that results in about 2,2902 hours of condensing operation or 33% of the heating hours (below 65 degrees OAT). That leaves 67% of the heating hours when the boiler is not condensing. The other manufacturers have more aggressive curves to varying degrees (boiler 2 condenses roughly 80% of the heating hours in Burlington), but there is still an opportunity with all of the boilers to increase the hours in which these boilers operate in condensing mode.

In Figure 1, the horizontal lines indicate a constant HWST for all outdoor air temperatures below the minimum outdoor air temperature setpoint (OATMIN).


Determining the right reset schedule is the key to getting the most efficiency out of your condensing boiler. Using the right reset schedule ensures your boiler will condense for as many hours of the year as possible, thereby maximizing energy savings.

Determining the right reset schedule is reasonably straightforward for building heating, especially where the building envelope component performance is known.3 To do this, we must understand two key concepts. The first is that for most finned-tube or panel-style radiation the heat output is directly proportional4 to the average heating water temperature (AWT). The second concept is that for a fixed indoor air temperature, building heating loads are linearly proportional to the outside air temperature. 

The building heating load is comprised of conduction and air infiltration, and can be expressed as follows:

Q =


       (1.08×CFM×?T)_infiltration where:

Q  = Envelope heating load [Btuh]

U = Overall heat transfer coefficient [Btuh-ft2-°?F]

A  = Envelope area [ft2]

?T = Difference between space temperature and outdoor air temperature [?°F]

CFM = Infiltration airflow [cubic feet per minute]

Or, put in terms of air changes per hour (ACH) of infiltration air:

Q=(U×A×?T)_conduction+ (1.08×ACH×(1/60)×V×?T)_infiltration


ACH = Air changes per hour

V = Volume of space [ft3]



All values to the right of “?T “can be considered constant for a given space. What we have is therefore a direct relationship between ?T and envelope load (Q).

Equation 1:

Q= ?T x K

We will show how this simple equation and the linear relationship between OAT and HWST can be used to develop the right HWST reset schedule.


In our building commissioning work, we have the opportunity to review many central heating system designs. Based on our experience, Table 1 shows a typical hot water reset schedule, where OATMIN is the minimum/design outdoor air temperature setpoint, OATMAX is the maximum outdoor air temperature setpoint (maximum temperature at which a heating load is expected), HWSTMAX is the maximum/design hot water supply temperature setpoint, and HWSTMIN is the minimum hot water supply temperature setpoint.

This reset schedule doesn’t allow a boiler to condense until roughly 40 degrees OAT, and it actually doesn’t have a basis in design values for our region. In Burlington, VT, the design heating temperature is -11 degrees. This matters because the reset schedule above uses 180 degree water for all OATs below 0 degrees (the flat lines in Fig 1 above). If 180 degree water works, i.e. maintains building temperature at -11 degrees, then it is hotter than necessary when the OAT is 0 degrees.  

Designers will argue this is a safety factor, but we would suggest that if a safety factor is desired, this is the wrong place to imple-ment it because of the energy cost penalty it will impose over the life of the boiler. A better solution to ensure thermal conditions are maintained during very cold OAT is to oversize your finned tube.  

In order to develop an optimized reset schedule, use the following steps:

1)  Lower OATMIN to the design temperature. This results in more hours of boiler condensing and more energy savings. Applying Step 1 for Burlington, VT, design conditions looks more like Table 3. With the appropriate design OAT, condensing will begin at approximately 36°F OAT (as opposed to 40°F).

2)  Establish the required heat output of the finned tube at design conditions. Our example has a load of 2,100 Btu/h at -11°F OAT and 70 degrees indoor air temperature (IAT). Assuming 4 ft of finned tube radiation in the space, the output requirement would be 525 Btuh/LF. At -11°F, the HWST will be 180° F, assuming a 20 degree delta T, the average water temperature (AWT) at design conditions will be 170°F. Table 3 provides typical finned tube radiation output from a well-known finned tube manufacturer with the data adjusted for a flow rate of 0.5 ft per second5 through the finned tube.

Table 4 shows that at 170 degrees AWT, this finned tube meets our loads at design conditions.

3) Determine the K constant for the space. In order to determine the load factor we use Equation 1 shown previously:

Q= ?T x K

For our example at design conditions we determine ?T as follows:

[?T= 70°F]_IAT-([-11°F ]_OAT )= 81 degrees

Solving for K:


We can now use the building K value6 of 25.93 to find the load at any given OAT or the OAT that corresponds to a given load.

4) Determine setpoints under the warmest outdoor air conditions when heating may be required. We do this by lowering the minimum hot water supply temperature.  Most condensing boiler manufacturers have a minimum return water temperature of 80 to 90 degrees. It is at this point that the boiler will reach peak efficiency. A typical design uses a 20 degree ?T from supply to return. Using a 20 degree ?T and a 90 degrees minimum return water temperature gives us the lowest allowable HWSTMIN:

HWSTMIN= 90 degrees+20 degrees = 110 degrees 

Returning to the load factor equation, we find the heating load at 60 degrees OAT

Q = (70 [degree]_IAT-60 [degree]_OAT ) x 25.93=259.3 Btu/hr

With 4 ft of finned tube, the necessary output at 100 degree AWT is 64.8 Btuh/LF (259.3 / 4). Referring back to Table 4 we see the finned tube more than meets this load with its output of 142 Btu/H/LF.

5) Determine the lowest allowable OATMAX. Doing this increases the hours of condensing. At an AWT of 100 degrees we know the finned tube will produce 142 Btu/H/LF. Since we have 4 ft of it, it will produce 568 Btu/H.  Use the estimated K value to solve for the ?T and corresponding OATMAX.


[[OAT]_MAX= 70 °F]_IAT-[22°F]_?T= 48°F


We’ve just demonstrated how to take a boilerplate HW reset schedule so that condensing will occur at 18 degrees OAT and above instead of only above 40 degrees. Doing so increased the hours of condensing from roughly 3,400 (56% of heating hours) to over 5,1007 (86% of heating hours). All of that efficiency gain has come with no capital investment since the finned tube and boiler were already selected for the project —  we merely took full advantage of the boiler’s ability to condense.

To maximize the benefit of a condensing boiler, the design must include increased finned tube capacity so that the boiler can deliver lower temperature water at design conditions; since this approach is applied in new construction, a better building envelope can reduce the heat load and minimize the need for more radiation.

The reset schedule in Table 5 takes advantage of condensing operation for 86% of the hours below an OAT of 65 degrees without compromising the 70 degree indoor air temperature requirement, but to maximize the boiler’s efficiency a supply temperature of no higher than 140 degrees is used, enabling the boiler to condense for essentially all hours of operation.

The finned tube radiation used in the example above will not provide enough heat for a space with a 2,100 Btuh design load under this reset schedule. In new construction or major renovations the finned tube can be selected to satisfy the design condition with a HWST of 140°F with minimal impact on physical size. The two different finned tubes are compared in Table 7 below (all other parameters not shown are the same).


In existing buildings, heating systems are very often oversized such that there is an opportunity to lower the HWS temperatures. It is much easier to determine the right HWS temperatures using an informed trial and error process rather than design calculations. Any improvements made to the reset schedule will result in an immediate payback because there is no additional capital cost.

First, take a look at the hot water reset schedule. Does it allow condensing? If the HWSTMIN set point is below 140°F the boiler will likely condense in some applications.  If the minimum HWSTMIN set point is above 140°F you’ve definitely got an opportunity waiting to be tapped. Start by lowering the lowest HWST to 120°F or even 110°F. Making this adjustment won’t affect the building in cold weather, but will yield savings during milder weather. If the adjustment results in some cold complaints from occupants during mild weather, the operator can incrementally raise the HWSTMIN set point in response. If complaints are only occurring early in the day, the operator can adjust the night setback schedule to account for the reduction in finned tube output at lower water temperatures.

Once that adjustment has been effectively implemented and the building operator is comfortable with the heating system’s operation, take a look at the OATMIN set point.  Does it represent a design condition or is it higher than necessary? Consulting an ASHRAE reference8 is the preferred approach for determining the low OAT setpoint, but we all know how cold it gets. If the lowest OAT on your reset schedule is 0°F and you’re in Minneapolis, MN, there is wiggle room because the OAT design condition is -16°F; the OATMIN setpoint can be lowered to -16°F with confidence assuming the building’s heating system already operates effectively at that temperature. If you’re in Rochester, NY, where the OAT design condition is 1°F, the OATMIN in the default reset schedule is likely as low as it can be made.

There is still one more step to be taken. Observe the boiler plant operation on the coldest days. If the boilers run consistently, without much downtime between cycles, there are minimal additional opportunities; the boilers are sized very close to the building load. If the boilers consistently cycle off for periods exceeding roughly 15 minutes there is likely an opportunity for the last step — lower HWSTMAX.  Implement this adjustment in small increments and over a reasonable period of time e.g. lower the HWSTMAX 5 to 10 degrees, then observe the building over a two to three day period.  Any reduction in the HWSTMAX will yield a few more hours of condensing operation because it lowers the OAT at which the boiler will condense (see Table 8 and Table 9).


We often see heating system designs that incorporate an indirect domestic hot water system, one where the heating boiler is used to make domestic hot water via a heat exchanger. On the surface this seems like a good idea because you eliminate a fuel burning appliance (the domestic water heater).  

The problem is that to do this, the heating boiler must operate at an elevated temperature, usually between 160 and 180 degrees, to make the 120 degree or 140 degree domestic hot water. This forces the boiler out of condensing mode and compromises its efficiency. In addition, in most commercial buildings, the heating load is significantly larger than the domestic hot water load so the large heating boilers are required to run all summer long to provide domestic hot water, which results in substantially reduced efficiency due to start up and purge losses, even in low-mass boilers.  For buildings with a large DHW load a dedicated condensing DHW boiler can be used. It should be noted that in low-load (very well insulated) residential buildings, we find that DHW loads actually eclipse the heating loads.


In order for condensing boilers to realize their full potential for increased efficiency, they must be designed and operated to condense. This is accomplished by using and optimizing a hot water supply temperature reset schedule. Designing/selecting the right terminal heating can allow condensing boilers to condense for all hours of the heating season. However, significant low and no cost opportunities to increase the condensing hours and efficiency of condensing boilers can be achieved through the right reset schedule. Many boiler manufacturers offer more than one pre-set, reset curve, while others allow manipulation of the reset setpoints directly.

In either case, with a little planning and some understanding of how the boiler reset schedules work, you can increase the seasonal efficiency of your boiler with little or no added first cost. Who can say no to that offer? 


Matthew Napolitan and Brent Weigel are engineers at Cx Associates LLC in Burlington, VT. 

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1. Manufacturers’ reset schedules are typically similar for larger boilers as well.  Larger boilers are more often controlled by central building management systems which tend to make their control a bit more transparent when compared to factory only controls.

 2. Assumes no heating past 65°F OAT.

 3. As in a new or recently constructed building. In new construction it is recommended that the construction of the building envelope be verified via commissioning.  In situ testing of key assemblies before the building is complete should be undertaken to ensure the design thermal and air leakage values are achieved.

4 . Very minor deviations from perfect proportionality will be found in manufacturers’ literature.  The deviations are small enough that they do not change the approach of this article.

5. I=B=R ratings use 3.0 ft per second or 7.6 GPM in a 1-in tube.  Using 0.5 ft per second will reduce the pressure drop from more than 4 ft/100 ft of pipe to less than 0.2 ft/100 ft and reduces the flow rate from 7.6 GPM to 1.3 GPM while only reducing the heat output by 7%.  This significantly reduces the pumping energy and the savings will persist for the life of the system.

6. This approach can be used for the entire building where the building enclosure is nearly uniform.  A unique K value should be calculated for any space with a significantly different enclosure, as in a conference room with floor to ceiling glass in a building that is otherwise punched windows.

7. For Burlington, VT, based on TMY3 data for all hours below the 65 degree bin.

8. ASHRAE Standard 90.1 contains recommended design outdoor air conditions.