Many older homes are heated with steam or hot water radiators. Newer homes may have convectors. You can make minor repairs to radiators, but leave problems with a boiler or piping to a professional.
A steam radiator has a small air vent near the top that emits bursts of steam when the radiator heats. Steam systems cycle between hot and cold. Most are connected to only one pipe at the floor, but some have two. You can dismantle and service a steam radiator valve if the boiler is off. It can be difficult to work on, however, because the high heat tends to seize up the pipe joints.
Hot water radiators are connected to two pipes at the floor. They maintain a constant warmth, rather than cycling between hot and cold. Before you dismantle a hot water radiator valve, you must drain the system.
About two hours, plus time for finding the parts
Groove-joint pliers, pipe wrench, screwdriver
Familiarity with your system
Turn down the thermostat so the boiler will not come on while you work
Radiator valve replacement parts or a new valve, strand packing, pipe-thread tape
If water leaks from beneath the handle, turn down the thermostat and wait for the radiator to cool. Tighten the packing nut (just under the handle) using groove-joint pliers and tighten the larger union nut using a pipe wrench. If this does not solve the problem, move on to the next step.
With a hot water system, water must be drained from the radiator. Turn down the thermostat. Attach a hose to the boiler's drain valve and run the hose to a floor drain; open the valve to drain the system. Starting at the top floor of your house, open the bleeder valves of all radiators.
Unscrew the packing nut and remove the stem, first by unscrewing and then by pulling it out. If the leak originates just under the handle, wrap the stem with strand packing and reinstall. If the leak is lower or if this does not solve the problem, replace the valve (Step 4).
Unscrew the union nut that attaches the valve to the radiator, then unscrew the valve from the pipe. Take the old valve to a plumbing and heating supply store to find an exact replacement; look carefully to be sure it will fit. You also may need to replace the short pipe that emerges from the radiator.
A steam radiator valve must be turned either all the way on or all the way off. To adjust the heat, an adjustable air vent is used. Turn down the thermostat and use pliers to unscrew the old air vent. Buy a compatible adjustable unit. Wrap the threads with pipe-thread tape and screw the new unit in place.
If a hot water radiator is not heating enough, air may be trapped inside. Turn the thermostat up and wait for the radiator to get warm. Hold a cup under the bleeder valve and open it with a bleeder key, long-nose pliers, or screwdriver. Spluttering water or hissing air may come out. Once water flows in a steady stream, tighten the valve.