Steam and Hot Water Heating
Water-based heating systems have proven to be a reliable source of quiet, even heating for many years. There are two basic type 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) to provide heat in the individual rooms of a house. The most common type fuels used with residential boilers are gas and oil.
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 while the pressure within the system increases. In a teapot, the excessive pressure is vented through a hole in the spout. In a steam heating system, the steam rises under low pressure (less than 5 psi) through the pipes to the radiators.
On each radiator, an air vent opens as the steam increases the air pressure within the radiator. When the steam reaches the vent, it closes. As the heat of the radiator radiates to the room, the steam eventually cools and condenses to water, and, by gravity-flow returns to the boiler to be reheated.
All steam boilers require a pressure relief valve and a low-water cut off. Most steam systems have a manual feed valve to allow for the periodic addition of water to the system. A “sight” gauge on the side of the boiler indicates the water level. Water normally needs to be added every few weeks in order to maintain the water level at the one half to two thirds full level.
A steam system should operate without any excessive noise. If there is a “knocking” noise in the system, it may indicate partially closed or malfunctioning valves or sagging pipes. Constant discharging of the air or steam indicates a vent malfunction.
Modern forced hot-water heating systems utilize one or more pumps to quickly and quietly circulate the heated water through pipes to convectors (or radiators). 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 calls for heat, an oil or gas burner is activated and begins to heat up the water in the boiler. Depending on the particular system, the circulator pump will either activate to begin circulating the water throughout the house when the thermostat calls for heat or when the water reaches a pre-set temperature.
Most modern hot water boilers have an automatic feed system. As the water is heated, it expands. The extra volume is forced into an expansion tank, which helps prevent the build up of excessive pressure. A pressure relief valve is also required to ensure the boiler pressure does not reach excessive levels. Boiler temperature and pressure can be observed on a gauge mounted on the unit. Under normal operating conditions temperatures will range from 160° to 220°F (70-105°C), with water pressure in the 12-18 psi (pounds per square inch) range. Readings outside these parameters generally indicate that the unit should be checked.
The original hot water systems operated by gravity flow, with the heated water rising by convection through the distribution system. This type system also uses a manual feed valve and open expansion tank, usually installed in the attic or an upper closet. Gravity systems are rarely found in their original state; most have been replaced or have been modernized.
Steam and hot water systems have an economic service life of approximately 20-25 years; many operate beyond that time frame. Regardless of the age or type system, annual servicing is recommended. One attractive 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. This is done by way of multiple zone valves and one or more circulator pumps.
Another type distribution system occasionally found with hot water boilers is referred to as a radiant system. Plastic or metal pipes are installed in the concrete floor slab or in tracks in a wood floor system then covered with finished flooring. There are no radiators; the 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 and limitations on the type floor coverings used. While carpeting can be used with radiant heat, heavy padding and/or carpeting can lower system efficiency.
With older heating systems, there is a possibility the piping and/or boiler is wrapped with an asbestos-containing insulation. With this type insulation, precautions are advised; generally any repair, encapsulation work or removal should be performed by a qualified specialist. The Home Owner Information Guide on “Asbestos in the Home” contains additional information.