Hot Water Boiler Systems 101

Boilers are central heating systems in that the heat is generated in one location in the house and distributed via piping to the various rooms.

The word boiler is somewhat confusing in that hot water systems do not actually boil the water. They heat it to about 160 degrees Fahrenheit, typically. Some hot water systems have been converted from steam.

Closed Systems

Modern hot water boiler systems are closed systems. The water in the boiler, in the piping, and in the heat source within each room, is under pressure. The pressure within the system is normally a few pounds higher than what is required to force water up to the highest level within the house. In a typical house, this pressure would be 12 to 15 pounds per square inch.

As the system heats up, the water expands and the pressure builds. An expansion tank, or cushion tank, normally located near the boiler, has air trapped in it. As the water expands, it begins to fill the expansion tank, compressing the air. This prevents excessive pressure build-up in the system, as the water gets hotter.

Closed systems normally have a circulating pump to force the water through the system.

Open System

Before closed or pressurized systems, boilers were open system. The water within an open system is not under pressure. There is an expansion tank; however, it is not a pressure vessel. The expansion tank (or gravity tank) on open systems is located above the highest radiator within the house. It is normally found in a closet on the top floor. When the system is cold, the expansion tank has very little water in it.

As the system heats up, the water expands and begins to fill the tank. The tank is designed to easily handle all of the extra volume that is created by expansion. Expansion tanks on open systems usually have a sight glass on their exterior. This allows one to determine the level of the water within the system. If the system is low on water, it is simply added until the tank begins to fill, when the system is cold. If the tank is overfilled when the system is cold it will overflow as the system heats up. An overflow pipe emerges from the top of the expansion tank. It typically passes through the attic and discharges onto the roof, or down into a floor drain.

Open systems don't have circulating pumps. The water is moved through the system by gravity.

Heat Exchanger and Burner

With the exception of electric boilers, all boilers have two major components: a heat exchanger and a burner. The heat exchanger contains the burning fuel on one side and the water to be heated on the other. The flame heats up the metal and the metal in turn heats the water. Heat exchangers are made of cast iron, steel or copper.

To burn fuel within the heat exchanger, a burner is required. The most common fuels are natural gas and oil. The burners on hot water systems are very similar to those on warm-air furnaces.

In addition to the major components, closed systems often have automatic water make-up devices to add water to the system as needed. On an open system, water is added manually by opening a valve on the pipe between the plumbing and heating system, until water is visible in the bottom of the expansion tank. On a closed system, a cold water plumbing pipe is connected to the boiler, with a special valve between. It is not a direct connection because the plumbing water pressure fluctuates and is too high for the heating system. Therefore, a pressure reducing valve is installed so that the pressure within the heating system does not exceed the right amount. If the plumbing system were ever drained, there would be the possibility of impure water from the heating system draining back into the plumbing system. Consequently, in recent years, automatic water make-up systems have contained a back-flow preventer.

Safety Features

There are also safety features built into a boiler. The first safety device ensures that ignition has taken place. Without proper ignition, the oil burner or gas valve, is shut down. The second safety device is a high temperature limit. Should the water within the system exceed a safe temperature (about 200 degrees F.) the system shuts down.

On closed systems, a pressure relief valve is provided. Should the pressure in the system exceed 30 pounds per square inch, the pressure relief valve discharges water. Open systems don't require a pressure relief valve because they're not pressurized, and excess water simply overflows from the top of the expansion tank.

Some larger boilers have low water level safety devices to shut down the system if there is insufficient water in the boiler.

Radiators

Once the water in the boiler is heated, it is distributed through the house. Radiators are the most common form of hot water heating distribution. Radiators are constructed of cast iron. Most radiators have a control valve at one end. This valve allows the water to the radiator to be shut off and, theoretically, it allows the amount of water flowing through the radiator to be adjusted so that the radiator gives off more or less heat. Seldom are radiator valves throttled down. They are normally wide open, and commonly leak if turned.

A small bleed valve is located near the top of the radiator. This allows air which is trapped within the radiator to be removed. This is normally done annually. It is not unusual to find this valve obstructed with paint.

Hot water convectors are an alternative to radiators. Convectors are either cast iron or are tubing (usually copper) fitted with aluminum fins. Convectors take up less space because they are usually less than 12 inches high. Radiators are typically 24 to 36 inches high. However, convectors have to be longer than radiators to produce the same heat, and because they heat up and cool down more quickly than radiators, can result in uneven heating.

Some hot water heating systems employ piping buried in floors or ceilings. The piping heats the floor or the ceiling which in turn radiates the heat to the room. The pipes have traditionally been galvanized steel, black steel, or copper, although recently, flexible plastic tubing has been used. The pipes are usually buried about 3 inches below the surface and are 8 to 16 inches apart.

Isolating Valves

Isolating valves are simply valves for the water leaving the boiler which, if turned off, shut off water to a portion of the house. These valves are not very different than valves in a plumbing system. Most are manually operated, although sophisticated multi-zoned houses with more than one thermostat may have electrically operated valves.

The most common problems are leakage and seizing. Since the valves are not used often, they tend to seize. These valves are not functionally tested during an inspection as the first time a valve has been turned, after sitting for any length of time, it is prone to leakage.

Sears Home Services can help you with any repair or installation issues.