Boilers & Condensing Boilers



Heat production & delivery


Biomass heat production (wood- and pellet-fired)


Fossil fuel heat production (gas- and oil-fired)


Biofuel heat production (biodiesel- and vegetable oil-fired)


Electric resistance heat production


Hydronic heat delivery (can supply an air delivery system)


Can be used for domestic hot water heating

System components


Combustion chamber (or resistance elements for electric units)


Exhaust chimney (not required for electric units)


Heat exchanger


Direct vent (sealed) intake and exhaust (for combustion units)


Circulation pump


Temperature and pressure relief valve, expansion tank, air bleeder as required


Controls as required

How the system works

gas boiler diagramWhen heat is required, a burner ignites (or electric elements are powered). A water jacket heat exchanger places water in the path of the heat in a configuration for optimal heat absorption. When the water has reached the desired temperature, a circulating pump moves it through the system, providing a constant flow of heated output water.

Direct heatThe heated water from the boiler is provided directly to the heating system. The boiler cycles on every time there is a call for heat. This system is common for space heating systems.

Indirect heat The heated water from the boiler is supplied to one or more storage tanks with a capacity based on expected heat requirements, and the storage tank is maintained at a set temperature by the boiler. When heat is called for it is drawn from this tank rather than directly from the boiler. This system is common if the boiler is supplying both the space heating and domestic hot water systems, multiple units or domestic hot water alone. These systems do not require the boiler to cycle on each time there is a call for heat.

Condensing boiler Many newer combustion units (both direct and indirect) are condensing boilers. Exhaust gasses carrying waste heat are used to warm incoming water, putting more of the heat generated to use. The incoming pipes are carrying cooler water, and condensation can form on the pipes in the presence of the hot exhaust gasses. Condensate is removed through a drain. This type of boiler is more efficient than non-condensing models.


System output

Residential boilers have outputs that range from 50,000–300,000 Btu/hr. Efficiency rates for condensing boilers range from 90–98 percent, with many units at 95–96 percent.

In most cases, the expressed efficiency is not the “true” thermal efficiency, which could be measured at an ideal steady state and not reflect real world performance. The accepted standard of efficiency is “annual fuel utilization efficiency” (AFUE). The method for determining the AFUE for residential furnaces is the subject of ASHRAE Standard 103. Ensure that rated output for comparison between units is the AFUE.

Environmental impacts: high

The impacts for boilers are directly linked to the fuel source used to fire them. When considering the impacts of a fuel source, it is important to be as thorough as possible. Calculations for impacts must consider the entire life cycle, from extraction to processing, transportation, combustion and by-products. Effects on habitat, social fabric, geopolitical and financial systems and resilience must be taken into account, along with more easily quantifiable (though likely controversial) numerical assessments such as carbon contributions and air pollutants (see Ranking Fuel Sources sidebar).


Material costs: Moderate to high

Unit type and output will affect costs.


Labor input: moderate

Boilers are relatively straightforward to install for trained professionals.


Skill level required for the homeowner

Installation — Difficult. Most boiler installations require a qualified professional.

Use — Easy.

Maintenance — Easy to Moderate. Annual inspection of temperature and pressure readings, burner flame (if required) and water pH.


Sourcing/availability: Easy

There are sufficient numbers of boiler system designers, suppliers and installers that competitive quotes should be available in most regions.


Code compliance

Boiler systems are an accepted solution in all codes. New technologies like pellet and biofuel boilers may require alternative compliance efforts.


Durability: moderate

Modern boilers can last fifteen to forty years, depending on use and maintenance. Manufacturers offer warranties of five to ten years on heat exchangers and pumps


Indoor air quality

All combustion devices can create IAQ issues, as they consume air and produce exhaust. Modern, code-approved devices have “sealed” combustion, meaning that combustion air is drawn directly from outside and exhausted back to the outside, and usually drawn by a fan to ensure proper flow. There are many possible points of leakage in these systems; be sure to check installations for issues. The combustion chamber is never completely sealed from indoors, and some amount of indoor air will be drawn into the unit.

Solid fuel-burning devices will generate ash as a by-product, and in most cases the ash is stored in the unit and will need to be manually emptied. This can introduce a lot of dust into the home. The movement of firewood and, to a lesser degree, pellet fuel will also introduce dust and allergens.

Hydronic delivery systems result in gentle air movement and minimal circulation of dust and allergens.

Forced-air delivery systems can circulate a lot of dust and allergens. Use the best filters possible to help reduce issues.

Adequate ventilation is required in any well-sealed home. Whether provided actively or passively, ensure that pre-warmed fresh air is introduced and circulated in the home during seasons when windows are closed.


Future development

Efficiency levels for boilers are approaching 100 percent, beyond which there is no further improvement. Refinement of controls and pumps will likely lead to small gains for entire systems.

A possible avenue for improvement is the incorporation of solar thermal heating systems that work in conjunction with boilers, resulting in systems that seamlessly use the least amount of non-solar energy possible.



Boilers can be operated in a low-energy scenario, but the entire system could not be manufactured and installed without significant energy input.


Tips for successful boiler heating


1. Many boilers have output capacities and temperatures that far exceed the needs of an energy-efficient home. Be sure to choose a unit that matches your heat loss calculation and required temperature closely.

2. Units with stainless steel or aluminum/silicon alloy heat exchangers can be a worthwhile investment, inhibiting corrosion of vulnerable components.

3. Boilers are proprietary units and each will have unique installation instructions that should be followed.

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