Hydronic boilers are used in generating heat for residential and industrial purposes. They are the typical power plant for central heating systems fitted to houses in northern Europe (where they are commonly combined with domestic water heating), as opposed to the forced-air furnaces or wood burning stoves more common in North America. The hydronic boiler operates by way of heating water/fluid to a preset temperature (or sometimes in the case of single pipe systems, until it boils and turns to steam) and circulating that fluid throughout the home typically by way of radiators, baseboard heaters or through the floors. The fluid can be heated by any means...gas, wood, fuel oil, etc., but in built-up areas where piped gas is available, natural gas is currently the most economical and therefore the usual choice. The fluid is in an enclosed system and circulated throughout by means of a pump. The name "boiler" can be a misnomer in that, except for systems using steam radiators, the water in a properly functioning hydronic boiler never actually boils. Some new systems are fitted with condensing boilers for greater efficiency. These boilers are referred to as condensing boilers because they are designed to extract the heat of vaporization of the flue gas water vapor. As a result of the lower flue gas temperatures, flue gas water vapor condenses to liquid and with dissolved carbon dioxide forms carbonic acid. The carbonic acid would damage a typical boiler by corroding the flue and fireside boiler heating surfaces. Condensing boilers solve this problem by routing the carbonic acid down a drain and by making the flue exposed to the corrosive flue gas of stainless steel or PVC. Although condensing boilers are becoming more popular, they are still less common than other types of hydronic boilers as they are more expensive.
Hydronic systems are being used more and more in new construction in North America for several reasons. Among those are:
- They are more efficient and more economical than forced-air systems (although initial installation can be more expensive, because of the cost of the copper and aluminum).
- The baseboard copper pipes and aluminum fins take up less room and use less metal than the bulky steel ductwork required for forced-air systems.
- They provide more even, less fluctuating temperatures than forced-air systems. The copper baseboard pipes hold and release heat over a longer period of time than air does, so the furnace does not have to switch off and on as much. (Hydronic systems heat mostly through conduction and radiation, whereas forced-air heats mostly through forced convection. Air has much lower thermal conductivity and volumetric heat capacity than copper, so the conditioned space warms up and cools down more quickly than with hydronic. See also thermal mass.)
- They tend to not dry out the interior air as much as forced air systems, but this is not always true. When forced air duct systems are air-sealed properly, and have return-air paths back to the furnace (thus reducing pressure differentials and therefore air movement between inside and outside the house), this is not an issue.
- They do not introduce any dust, allergens, mold, or (in the case of a faulty heat exchanger) combustion byproducts into the living space.
Forced-air heating does have some advantages, however. See forced-air heating.
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