Boilers heat water, which distributes heat via radiators or radiant-floor tubing. Alternately, boilers can supply a forced-air distribution system if it has a hot-water heat exchanger. Heating systems that use water or steam to transfer heat are much less common than forced-air systems, accounting for only about 2% of the U.S. total in single-family homes built in 2012.
No matter how the heat is distributed, you’ll find improved efficiency in modern boilers. Government regulations require boilers to have a minimum AFUE of 80%, but some are available with efficiencies greater than 95%. Natural gas is the most common fuel, but high-efficiency models that burn No. 2 fuel oil are also available.
Condensing boilers use the same technology as condensing furnaces to capture waste heat. Combustion gases are routed through a stainless steel heat exchanger where water vapor condenses and releases more heat.
Boilers can produce domestic hot water, but can’t provide air conditioning or dehumidification. That probably makes boilers with hydronic distribution a better bet in regions where air conditioning and dehumidification aren’t top priorities.