Here, we find two kinds of systems, both of them operating on the principle that water, when heated, expands, and in expanding becomes lighter and rises.
The "closed" system
The other kind of system is called the "closed" system, and here the expansion tank is located at the base of the system, together with the boiler.
The expansion tank has no overflow pipe but is completely airtight. When the water in the system expands, it compresses the air in the tank and puts the water in the entire system under pressure, making possible the maintenance of a higher water temperature, In the "closed," or pressure, hot water system, the purpose is to force the temperature of the water as high as possible without forming steam.
Needless to say, there is a limit to the amount of pressure that the system can stand, and a relief valve must be installed somewhere in the line to relieve excess pressure, should it be necessary.
While the normal water temperature in an open system will be approximately 180 degrees, in a closed system it may be well over 200 degrees.
Another advantage of the closed system is that smaller pipes can be used than are required for the open system.
Hot water systems have two sets of pipes to each radiator. One carries hot water from the boiler, and the other returns the cool water from the radiator to the boiler.
Difficulties in hot water heating can usually be attributed to air in the lines and radiators, the water level too low in the system, or pipes and fittings that are too small or have been incorrectly installed.
Radiator air valve
Each radiator is equipped with an air valve that can be opened to allow air inside the radiator to escape and water to fill the radiator. This valve should be kept open until water begins to flow out, indicating that all the air has been displaced by water. If air is left inside the radiator, it will not heat properly.
The water level
A hot water system will work well only when the entire system is full of water. The water level can be checked in several ways. In open systems, where the expansion tank is located in the attic or some high point above the highest radiator, the amount of water in the tank is indicated by a glass gauge on the side of the tank. Or you may look into the tank itself.
Most systems require the expansion tank to be about one-third full when the water is cold. If the tank has a gauge, it should be marked off for the proper water level.
A needle gauge on the boiler, called the altitude gauge, also indicates when the system is properly filled. This gauge has two needles, one black and the other, generally, red. The red needle is set at the proper water level for the system. The black needle indicates the true water level and varies with the water level changes. When the red needle is over the black, the system is properly filled with water.
In pressure systems, the amount of water is automatically controlled by a valve. When the water level drops, and with it the pressure, the valve opens and allows water to enter. Reading the altitude gauge is a check, to see that the valve is operating properly. If, for some reason, the altitude gauge reads higher than it should, have the valve examined and the malfunction corrected before turning on the furnace.
Pipes and fittings
Like other heating systems, much of the difficulty encountered in a hot water mechanism is due to poorly installed pipes and fittings. A steam fitter or plumber should make the necessary changes, as the home mechanic usually does not possess the tools required for the work. The pipes used must be large enough to allow the passage of sufficient water.
Resistance caused by small pipes will prevent the system from operating efficiently.
To avoid replacing the pipes, pumps may be installed at various points, and these will increase the volume of water flowing through the pipes. These pumps are electric, and will increase the circulation as well as the volume of water through the pipes, forcing water to any part of the house which is not well heated.
Ninety degree angles in the pipes will also cause resistance in the system. These should be replaced, where possible. with forty-five degree fittings.
Many hot water systems have been installed which do not give satisfactory heating because the lower radiators do not function as well as those on the upper floors. This condition can be remedied by installing larger main lines, removing excess branch lines, and other rearrangements.
The cost of having a hot water system redesigned is high, but so long as it is faultily installed, the system will not function well. The homeowner must decide whether to continue to put up with a troublesome heating system which wastes fuel, or pay for a redesigned system which will, in time, pay for itself in reduced fuel bills.
Filling the system
The important fact to remember in filling a hot water system is that air in the pipes and radiators will prevent the circulation of hot water through the system, and the radiators will not heat.
The first step in filling the system is to open all the radiator shut-off valves and close the air valves. After this has been done, open the shut-off valve to the water supply. As water enters, open the air valve on the radiator nearest the boiler to allow the air inside to be replaced by the incoming water.
Keep the air valve open until water comes out of it, then close it. Repeat this operation on each radiator, working from the lowest to the highest, until the air in each has been replaced by water. After this has been done, check to see that the water level in the expansion tank is correct.
In a closed pressure system, leave the water supply shut-off valve to the boiler open, as the pressure valve will allow the passage of water when necessary. When the air valves on the radiators are opened, the air will escape and the pressure will drop. This decrease in pressure opens the intake valve and allows the proper amount of water to enter.
Fresh water always contains a certain amount of air, and this becomes trapped in radiators when the system first goes into operation. The air should be expelled by opening the air valve, in the manner employed for filling. After the air is out and the valve closed, more water will have to be added. Should it be necessary to add water at frequent intervals, continue to clear the radiators.
In some instances, the position of the colored needle on the altitude gauge of an open system will not give the correct reading when the system is full. This needle can be reset to conform with the water level in the expansion tank by removing the glass cover plate on the gauge and positioning the needle manually.