HOME CLINIC; HOW RADIATORS WORK, AND WHY THEY SOMETIMES DON'T
By BERNARD GLADSTONE
Published: January 25, 1987
HOMES heated by steam or circulating hot water have radiators. For maximum economy and efficiency it is essential that the homeowner make certain that the radiators are operating properly.
When radiators are not working properly, or when they are not putting out as much heat as they should, some parts of the house will be colder than others. It's also likely that a lot of heat is being wasted because the hot water circulates through the system and back to the boiler without ever losing all of its heat to the air inside the house. When this happens, people tend to turn the thermostat higher than normally required, thus adding considerably to the heating bill.
After the hot water heats the radiator, heat is supposed to be radiated into the room. The heat actually warms the air in two ways: by radiation and convection. Radiation accounts for only a small percentage of the heat given off by the radiator; a much higher percentage of the heat is distributed by means of convection - as the radiator warms the air next to it this heated air rises, drawing cool air into and through the radiator from underneath. This movement of air sets up vertical currents that will distribute heated air throughout the room.
It is essential that the air be able to flow freely over, under and around each radiator. When radiators are inside an enclosure, there must be plenty of venting at top and bottom. Even baseboard radiators recessed into the wall will have plenty of openings along the floor, as well as along the top of the radiator's baseboard housing.
Decorative cabinets used to enclose a free-standing cast-iron radiator should also be designed with plenty of openings. Never install heavy curtains or draperies in front of a radiator, and try to leave a space of several inches between the radiator and any large piece of furniture in front of it. Even then it is best not to position large pieces in front of a radiator when these pieces go right down to the floor (couches, tables, and other pieces with legs will not restrict air flow as much).
Older-style free-standing radiators of cast iron consist of multiple sections that allow air to flow through and around. The cast iron stores heat, which will continue radiating out into the room for some time after the heat goes off. However, they also take longer to heat up. Modern convector-type baseboard radiators have a great many metal fins attached to the outside of the pipes that carry the hot water from the boiler. As these fins heat up, they warm the air flowing around them so that a current of warm air starts circulating soon after the heat comes on.
Because air circulation is essential for efficient dispersal of heat, it is important that radiators be cleaned regularly. Radiators and radiator fins should never be allowed to accumulate layers of dust or lint. Such accumulations will not only keep air from flowing freely, they will also act as insulation that will slow the transmission of heat.
Since radiators are often mounted against an outside wall, much heat can be wasted through the cold wall directly behind the radiator (this is especially true if the radiator is recessed.) One technique for reducing this waste, as well for improving efficiency, is to slide a sheet of aluminum-faced foam insulation board behind the radiators. This type of insulation is sold in many large hardware stores and home centers, but if you cannot find it you can make your own by gluing aluminum foil to the face of some foam-core board, which can be bought in most art supply stores and hobby shops.
In a steam system it is important that the radiators be level - or they can slope very slightly toward the inlet valve through which the steam enters. If a radiator slopes the other way then water will get trapped, causing gurgling and banging noises and often keeping the radiator from heating up properly. That's because as the steam cools and condenses into water it has to run out through the same pipe that the steam entered.
To check, place a level on top of the radiator to see if it is tilted the wrong way. If so, you can slip a thin piece of wood under the end to raise it slightly as shown in the drawing.
Banging in steam radiators or pipes can also be caused if the pipes are sagging or not sloping properly, often a problem after structural alterations or caused by sagging of structural members. Just as radiators must drain easily, so too must pipes in a steam system be pitched slightly so all of them slope down toward the boiler. Otherwise water can get trapped in low places (where the pipe has sagged). This can not only cause banging or hammering noises each time the steam comes up, it can also prevent radiators from getting as hot as they should.
In a hot-water system, air trapped inside a radiator can keep hot water from filling the radiator so it will not get hot all the way across. To correct this, radiators usually have small vent valves mounted near the top (usually at the end opposite the inlet valve) that will allow you to bleed out the trapped air. To do this you open this valve while the heat is on and the hot water is circulating (some need a special key that you can buy in hardware stores; others can be opened with a screwdriver). Leave the valve open until all the air escapes and water starts to flow freely in a solid stream, then close it tightly.