Water-based radiators heat homes with a passive system that does not dry air in the same way a forced air blower does in typical furnaces. Leaks, however, can develop and should be repaired as quickly as possible before damage to floors or walls occurs. Leaking valves can usually be repaired or replaced without shutting off heat to the entire system. The process is similar to replacing any other plumbing joint, only there's no need to sweat pipe with a torch.
Locate the water supply valve leading to the radiator. This is usually a valve along the water line leading into the room. Note this will be a valve other than the one at the base of the radiator.
Grip the valve handle firmly and turn it counterclockwise until it will no longer turn.
Place a dripping pan underneath the radiator drain and turn the drain stem counterclockwise to open the drain. There should just be a few drops or a slow trickle from the radiator.
Turn the breather knob at the top of the radiator counterclockwise. This releases the vacuum inside the radiator, and water should start flowing into the dripping pan. Note that if the pan starts to fill up before the radiator is empty, you can close the drain valve, dump the water then repeat the process until the radiator is empty.
Use an adjustable pipe wrench on the nut that connects the faulty valve to the radiator.
Apply downward pressure on the wrench to loosen the nut. Continue turning the nut by hand until it is completely free of the threads.
Repeat Steps 1 and 2 for the nut that connects the valve to the water supply line.
Remove the valve and nuts.
Scour the threads on the radiator with steel wool or a wire brush. This helps to ensure a watertight seal when installing the new valve.
Wrap PFTE tape around the threads on the radiator and the new valve three times.
Press the nut against the adaptor on the new valve. Grip the nut and twist up while pressing against the radiator threads. Continue to twist until the nut no longer spins by hand.
Use the adjustable pipe wrench to secure the nut snugly. Apply upward pressure on the pipe wrench until the nut no longer turns.
Repeat Steps 2 through 4 for the nut along the water supply line.
Grip the newly installed valve and turn it clockwise until the stem no longer spins.
Close the drain valve and breather valve then turn on the water supply to the room. If you shut off the entire system, turn on the boiler and the water to the entire building.
Open the breather valve a 1/4 turn--just enough for air to escape.
Monitor the valve for water. As soon as a steady stream of water runs through the breather valve, close it immediately.
Continue to monitor the valve for water. If any leaks persist, the radiator itself could require replacement. Replacing a valve will solve only one of potentially several problems that could be causing leaks around a radiator. These include corrosion inside the radiator, worn pipe threads on the radiator or a loose nut.