According to the U.S. Energy Information Administration (EIA), up to 11% of existing households use some form of hot water or steam heat. Boilers produce hot water that can be used to heat homes through several different distribution methods. The hot water can be sent through plastic pipe loops in the floor for radiant floor heat or through a metal radiator mounted along the wall or a baseboard radiator mounted near the floor. Hot water can also be directed from a combustion tank water heater to a coil in an air handler equipped with a fan to blow air across the coil and through supply air ducts to the home. Most combustion boilers are fueled by natural gas. Fuel oil, propane, and wood are other fuel sources used in locations where natural gas is not readily available. The hot water for a boiler may be heated or preheated by a solar thermal water heating system, a ground-source (geothermal) heat pump, or an air-source heat pump. The boiler may heat water in a tank or it may be a tankless (or instantaneous) wall-hung model. Some boilers provide heat for a potable hot water tank in addition to providing hot water to room heaters; this is referred to as indirect water heating. Some newer, very efficient models combine space heating, water heating, and heat recovery ventilation.

For best performance, the heating system should be properly sized to match the heating design load of the home, as described below. If the home is constructed with high levels of insulation and air sealing, a smaller heating system can often be installed. When equipment is oversized, it can “short cycle” or turn on and off repeatedly before the demand is met, which can have negative impacts on energy use, comfort, and equipment durability.

The International Mechanical Code classifies boilers based on vent type - direct, mechanical, or atmospheric. The National Fuel Gas Code puts vented combustion appliances (furnaces and boilers) in four categories based on flue vent pressures, flue gas temperatures (relates to condensing or noncondensing), and vent pipe materials. (See Gas Boilers for a more complete description of these categories.) The lowest efficiency boilers are atmospherically vented Category I boilers, which may or may not be equipped with a small induced draft fan to start the draft but still rely primarily on high temperatures in the vent stack to draw flue gases up and out the chimney. Category I boilers typically have annual fuel utilization efficiencies (AFUE) of under 80%. Higher efficiency Category III boilers are referred to as forced draft or power vented because they are equipped with a forced-draft fan at the start of the combustion chamber to push combustion air through the chamber and out the vent. Because this fan is operating whenever the boiler is firing, Category III boilers have positive pressure in the vent stack. The stack temperature on a Category III combustion appliance is above 140°F.

Category III and Category IV boilers are both forced draft (also referred to as power vented) appliances meaning that they are equipped with a combustion fan that is located before the burner to push air through the combustion chamber and out of the vent. The fan is continually operating when the burner is firing so the vent stack pressure is always positive.

Category IV boilers, like Category III boilers, vent their combustion exhaust gases directly outside through a sealed pipe so they cannot be back drafted. Category III and IV appliances should be installed as sealed-combustion/direct vent appliances, which means their combustion chamber is sealed off from the combustion appliance zone (CAZ), which is the room where the equipment is located, and they draw their combustion air from outside via a second vent pipe or concentric pipes that bring combustion air directly to the combustion chamber from outside. (See Figures 1, 2, and 3.) However, although manufacturers do not recommend it, they are sometimes installed as non-direct-vent appliances (where the exhaust pipe is installed but the pipe for incoming air is not installed so the boiler draws its combustion air from the CAZ). If this approach is taken, ample combustion air must be supplied to the CAZ; see Combustion Furnaces for more on calculating the amount of air needed in the CAZ for non-direct-vented appliances.

Category III boilers have efficiencies of 80% to 90%. For information on Category I and III vented combustion appliances, see Gas-Fired Boilers and Oil-Fired Boilers.

An important difference between Category III and Category IV boilers is that Category IV boilers are condensing boilers that have a second heat exchanger (or sometimes one extra-large heat exchanger), which allows the combustion gases to cool and condense, releasing more heat in the boiler rather than sending it up the flue as water vapor. Because the flue gases are cooler (<140°F), they can be vented through the wall or roof by means of a PVC pipe rather than a chimney. Condensing boilers are the most efficient type of boilers with AFUEs of 90% to 96%.

Low flue vent temperatures allow the water vapor (a by-product of combustion) to condense before or as it enters the vent pipe. This condensate water is highly acidic (pH of 2) and could be dirty if the fuel is dirty or if the boiler needs servicing. If the vent pipe exits through a side wall rather than through the ceiling, the horizontal portion of the pipe should be installed at a slight upward angle so that any condensate water drains back to a condensate drain line rather than going out the vent. The condensate drain line should be directed to a sewer or septic system. However check local codes to see if pretreatment of the condensate is required. Some jurisdictions require that the condensate be passed through a neutralizer containing calcium carbonate or a similar material to raise its pH before disposal. Such pretreatment is recommended whether required by code or not.

The design of the piping distribution system can reduce energy usage and improve comfort because they allow for zoning. In general, distribution systems such as parallel or primary-secondary piping arrangements perform better than the simpler series piping where all heat emitters are connected to a single pipe loop. For more information on distribution, see Gas-Fired Boilers.

Boiler Controls

While older boilers are either on or off, newer boilers with multi-stage or modulating burners have adjustable output to better match heating loads. This reduces the number of on-off cycles (and cycling losses) and allows the boiler to operate for longer hours at lower firing rates, which improves efficiency. Non-modulating boilers have efficiencies of 85% to 90%. Boilers that operate in modulation mode rather than just on-off can improve average boiler efficiency by up to 8%. Higher-efficiency models are also equipped with electronic controllers that can increase equipment life, improve boiler efficiency, and enhance comfort, by adjusting boiler water temperature, creating time-delay relays, performing automatic post-purge, preventing warm-weather boiler operation, controlling the position of mixing valves, and controlling pump speeds. These controls can increase the efficiency of noncondensing boilers by 10% or more and reduce idle losses to 0.3%. Condensing gas boilers that are fully modulating and have advanced controls can achieve efficiencies ranging from 92% to 96%.

There are many settings that can be adjusted on a modern boiler to improve the efficiency and comfort performance of the equipment, these adjustments may provide better performance than the default factory settings.

An outdoor reset control, which matches the system output to the actual outdoor temperature conditions, will improve comfort for owners of both condensing and non-condensing equipment by preventing extreme spikes in indoor temperature when the outdoor temperatures are warmer than design conditions. If you install an outdoor reset, recommend that homeowners do not use a night-time temperature setback strategy unless special controls have been installed that can override the reset control. Locate the outdoor sensor where it will not be exposed to a heat source such as direct sunlight or a dryer exhaust vent.

When installing an outdoor reset control with a noncondensing boiler, choose settings so that the return temperature to the boiler is no lower than 140°F to prevent condensing. However, when selecting the outdoor reset curve set points for a condensing boiler, choose settings so that the temperature of the water returning to the boiler is below 130°F. This ensures that the return temperatures are low enough to promote condensing, which will greatly increase the energy efficiency of the system (see Arena 2012 for more details). To ensure the return temperature is below 130°F, the supply temperature will likely have to be reduced to below the factory setting. Make sure the heat emitters used (baseboards, radiators, etc.) are properly sized based on the average temperature in the distribution loop. If undersized, they won’t release enough heat to the space and the water will return to the boiler at too high a temperature, preventing condensing. Radiant floor systems are typically set up to run at lower temperatures when installed, so they do not require additional adjustment to the boiler’s supply temperature. 

If you are specifying toe kick heaters in homes that have condensing boilers with outdoor reset controls, make sure the toe kick model specified is capable of operating at low temperatures. Many of the toe kick heaters currently available will not operate below a supply temperature of 140°F. A properly designed and configured condensing hydronic system will have return temperatures below 130°F most of the year, leaving the occupants without heat in rooms with toe kick heaters.

In highly insulated, energy-efficient homes with correctly sized equipment, nighttime setback can cause comfort issues and customer complaints. A boiler that is correctly sized to meet the home’s design heating load will not have enough capacity to recover from the setback in a reasonable amount of time, especially if the system is designed with an outdoor reset control. Outdoor reset controls match the boiler’s supply temperature to the heating load based on the current outdoor conditions, severely hindering the system’s ability to raise the temperature in the space. If the boiler was set up with an outdoor reset control and no capability to override it, advise homeowners not to set their thermostat temperature back during nighttime hours. This is also recommended if the home is highly energy efficient and the boiler was sized to meet the design heating load.

If you know that the homeowner will employ a setback strategy or if you would like to provide that capability, you can install controls to speed up temperature recovery such as 1) a boost control that automatically raises the boiler output target temperature if heating demand is not satisfied within a set number of minutes, 2) an indoor sensor that works with the outdoor reset control to compensate for lags in response based on interior temperature, or 3) a simple manual override switch. Oversizing the heat emitters and possibly the boiler may be necessary to meet the additional load induced during periods of setback recovery.

If the boiler is oversized compared to the design load, oversizing the heat emitters 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, each of which has very small loads compared to the boiler’s capacity. In these cases, oversizing the emitters will reduce cycling, improve response time, and increase efficiency. Note that many manufacturers set a maximum temperature difference between the boiler’s supply and return to protect the heat exchanger. Oversizing the heat emitter will result in an increase in the delta T, so make sure that you do not oversize to the point that the manufacturer’s limit is exceeded. If installing a non-condensing boiler, make sure that increasing the emitter does not result in return water temperatures below 140°F.

For both condensing and noncondensing boilers, a warm weather shutoff turns off the boiler when the temperature setting is exceeded by the outdoor temperature. Boilers commonly come from the factory with the shutoff set between 68°F and 72°F. In locations with large day-night temperature swings or in spring and fall in homes that use a setback, if the shutoff is set too low, warm midmorning outside temperatures could prevent the heat from coming on even if it is still cold inside. Make sure the warm weather shutoff setting is no lower than the desired indoor winter temperature. For example, if 70°F is the normal setting, the warm weather shutoff should be no lower than 70°F.

Make sure your system includes an automatic post purge control, which keeps the system pump on for several minutes after the boiler stops firing to disperse the heat still residing in the mass of the boiler.

Some boiler manufacturers have started offering controls that can limit the boiler’s maximum input. This can be especially useful if the boiler is used for both space heating and domestic hot water and one load is significantly less than the other. This limit reduces cycling in situations where the boiler’s maximum firing rate is significantly higher than the demand, for example when the water heater is calling for heat but the space heater is not.

Heat dumping is a strategy that diverts excess boiler heat to the domestic hot water (DHW) tank after the space heating demand is satisfied. Studies have shown this technique can greatly improve overall system efficiency (Butcher 2011).

See the Building America report Condensing Boilers – Control Strategies for Optimizing Performance for additional guidance on setting boiler controls.

How to Select and Install a Boiler

1. Choose the highest performing boiler that project funding will allow to meet the design heating load of the project. If you are participating in an energy-efficiency program, select a boiler that complies with the requirements for your climate zone, as described in the Compliance tab.
2. Install in accordance with relevant standards including ACCA Standard 5: HVAC Quality Installation Specification and the ACCA’s Technician's Guide for Quality Installations and ACCA Standard 9: HVAC Quality Installation Verification Protocols.
3. Design an efficient distribution system that allows for zoning.
4. Properly size the boiler by first calculating the heating load of the home. Calculate heat load as described in the ASHRAE Fundamentals Handbook. Many software products are also available that can guide you through the calculations, and several boiler manufacturers include sizing guidelines or software on their web sites. If the design load is equal to or lower than the selected boiler’s lowest output rating, consider alternative low-load heating equipment options that better match the design load of the home.
5. Install the boiler as a direct-vent installation where combustion air is piped directly to the boiler combustion chamber from outside. If the boiler must use the CAZ for combustion air, verify that required combustion air is provided in the CAZ and perform a combustion safety test after installation. See methods for calculating and providing combustion air in the guide Combustion Furnaces.
6. Select appropriate vent piping in accordance with the National Fuel Gas Code (see the Compliance tab).
7. Set the equipment control settings to optimize system efficiency as described above and in Arena 2012.
8. If your boiler heats a hydro coil for forced air heating, see Sizing Heating/Cooling System Distribution.
9. After the boiler is installed and before the initial filling, fill the system with water plus a cleaning solution. Allow this to circulate for several hours to remove grease, oil, and chemicals from solder and flux. Drain then fill with clean water. If the city water is corrosive, include an initial treatment. If properly installed, the boiler should operate indefinitely without needing additional water or cleaning.
10. For condensing boilers, ensure that condensate drains properly to the sewer or directly outdoors. Because the condensate is highly acidic, follow local code requirements regarding pretreatment of condensate before disposing to the sewer. Protect the condensate line from freezing. Provide a secondary (emergency) drain pan constructed of durable material.
11. Verify correct boiler operation by testing the outdoor reset control, and evaluating the boost control if one is installed.