Hydronic boilers

Hydronic boilers are used in generating heat typically for residential uses. 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 is normally heated by natural gas and is currently the most economical and therefore the usual choice. The fluid is in an enclosed system and circulated throughout by means of a motorized pump. Most new systems are fitted with condensing boilers for greater efficiency. The name can be a misnomer in that, except for systems using steam radiators, the water in a properly functioning hydronic boiler never actually boils. These boilers are referred to as condensing boilers because they condense the water vapor in the flue gases to capture the latent heat of vaporization of the water produced during combustion.

Hydronic systems are being used more and more in new construction in North America for several reasons. Among the reasons 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. (Copper heats mostly through conduction and radiation, whereas forced-air heats mostly through forced convection. Air has much lower thermal conductivity and higher specific heat than copper; however, convection results in faster heat loss of air compared to copper. See also thermal mass.)
• They do not dry out the interior air as much.
• They do not introduce any dust, allergens, mold, or (in the case of a faulty heat exchanger) combustion byproducts into the living space.

• Safety valve: used to relieve pressure and prevent possible explosion of a boiler
• Water level indicators: to show the operator the level of fluid in the boiler, a water gauge or water column is provided
• Bottom blowdown valves provide a means for removing solid particulates that condense and lay on the bottom of a boiler. As the name implies, this valve is usually located directly on the bottom of the boiler, and is occasionally opened to use the pressure in the boiler to push these particulates out.
• Hand holes are steel plates installed in openings in the boiler shell to allow for inspections of the interior of the boiler.
• Low- water cutoff is a mechanical means (usually a float switch) that is used to turn off the burner or shut off fuel to the boiler to prevent it from running once the water goes below a certain point. If a boiler is "dry-fired" (burned without water in it) it can cause rupture or catastophic failure.
• Surface blowdown line provides a means for removing foam or other lightweight non-condensible substances that tend to float on top of the water inside the boiler.
• Circulating pump is designed to circulate water back to the boiler after it has expelled some of its heat.
• Feedwater check valve or clack valve: a nonreturn stop valve in the feedwater line. This may be fitted to the side of the boiler, just below the water level, or to the top of the boiler. A top-mounted check valve is called a top feed and is intended to reduce the nuisance of limescale. It does not prevent limescale formation but causes the limescale to be precipitated in a powdery form which is easily washed out of the boiler.

References: Wikipedia