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INFORMATION SERIES: Steam & Hot Water

Water based heating systems have proven to be a reliable source of quiet, even heating for many years.  There are two basic types of systems to consider, steam and hot water.  While these systems differ in their operation and controls, they both utilize a boiler as the vessel to heat the water and a piped distribution system connected to radiators (or convectors) in the individual rooms of the house.  The most common type fuels used to heat residential boilers are gas and oil.

As the heated water expands, it is forced up into the expansion tank to prevent excessive pressure build-up in the system. A pressure relief valve is also required for all hot water systems. Under normal operating conditions, depending on the particular boiler, temperatures will range from 160- to 220"F (70-105`C) and
pressure should be in the 12-18 psi (pounds per square inch) range. Readings beyond these parameters generally indicate that the unit should be checked.

Since forced hot water systems are closed systems, most hot water boilers will have an automatic feed system to maintain the required water and pressure levels. This feature utilizes a pressure reducing valve which reduces normal water supply pressures to the level required for the particular boiler. Gravity systerns would have a manual teed.

The original type hot water heating systems operated by gravity flow as the heated water rose by convection through the distribution system, then, as it cooled, returned by gravity flow to the boiler. This type system also had an open expansion tank, usually installed in the attic or an upper closet, to allow for the expansion of the heated water, and a manual feed valve. Gravity systems were generally installed prior to the 1930s and are rarely found in their original state; most have been replaced or have been modernized.

One feature of forced hot water systems is the ability to break the systems up into zones allowing for the separate control of the heating in the individual rooms or areas of the house. Zoning is accomplished by way of individual circulator pumps or zone valves.

Another type distribution system occasionally found is referred to as a radiant heat system. With this type system, plastic or metal pipes are embedded in the concrete floor slab (less often between the joists of a wood frame floor.) There are no radiators as heat radiates from the floor as the concrete is heated. The advantage of this type system is that it provides uniform heating at the lowest or generally coldest point in the house. Disadvantages with radiant systems include difficulty in determining or locating leaks in older systems and limitations on the type floor coverings used. While carpeting can be used with radiant heat, heavy padding and/or carpeting can restrict the efficiency of the system. On older heating systems, there is a possibility that your piping and/or boiler is wrapped with an asbestos-containing insulation. With this type insulation precautions are advised; the   HouseMaster Information Guide on "Asbestos in the Home" contains additional information on this subject.

In a steam boiler, water is heated in much the same way as it is in a whistling teapot. As water in the partially filled teapot or boiler is heated to 212°F (100°C), it turns to steam, and pressure within the system increases. In a teapot, the excessive pressure is vented through a hole on the pouring spout; while in a heating system, the steam rises under low pressure (less than 5 psi) through the pipes to the radiators. On each radiator an air vent will open as air pressure builds up within it. When the steam reaches the vent, the vent closes. As heat is radiated to the room, the steam in the radiator condenses to water, and, by gravity flow returns to the boiler to be reheated.

Most steam systems have a manual feed valve to allow for the periodic addition of water to the system. This normally needs to be done once a month in order to maintain the water level gauge at approximately the one half to two thirds full level. The water level can be determined by observing a "sight" glass or water gauge mounted on the boiler.

All steam boilers should have a pressure relief valve and a low water cut of. A steam system should operate without any excessive noise. If there is a "knocking" in the system, it may indicate partially closed or malfunctioning valves or sagging pipes. Constant discharging of the air vent indicates a malfunction of the
vent.  

Forced hot water heating systems utilizing a pump to quickly and quietly circulate the heated water throughout the house have been commonly used since the 1940s. This type system is a closed system, relying on the physical relationship of pressure, temperatures and volume for safe and efficient operation. When the thermostat is adjusted and calls for additional heat, the burner is activated and begins to heat up the water in the boiler. When the water reaches a certain temperature, a control will activate the circulator pump(s) to begin circulating the water throughout the distribution system.  

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