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The valve at the bottom of a radiator is opened and closed a few times a year, and after decades the packing that seals its stem dries out. The result is a drip from under the handle, a rust stain on the floor, and, on a steam system, a hiss when the heat is on. It is one of the few heating repairs that is often fixed with a single turn of a wrench.
Where the water is coming from
Dry the valve and watch.
- From under the handle, around the stem: the packing. The usual case.
- From the large nut where the valve meets the radiator, the union: the union's seal has failed.
- From the joint where the pipe enters the valve: the threaded connection.
- From the body of the valve itself: a crack. The valve must be replaced.
Tightening the packing nut
- Let the system cool. On a steam system, wait until the radiator is cold.
- Find the packing nut: the nut directly under the handle, around the stem. On some valves the handle must come off first, held by a screw on top.
- Hold the valve body with one wrench and tighten the packing nut with another, a quarter turn at a time. Clockwise, gently. Overtightening makes the valve hard to turn and can crush the packing.
- Run the heat and check.
If a quarter turn or two does not stop it, the packing is finished and needs replacing: back the nut off, wind a few turns of graphite packing string around the stem in the direction the nut tightens, and retighten the nut. On a hot water system, the system pressure will push water out while the nut is off, so have a cup ready and work quickly, or have the plumber drain that radiator first. On a steam system with the boiler off, there is no pressure, and this is easy.
A leaking union
The union is the big nut that joins the valve to the radiator. Tightening it with two wrenches, one on each side, sometimes stops a weep. If not, the union has to be separated and its seat cleaned or the gasket replaced. On a steam system this is straightforward when cold. On a hot water system the radiator must be isolated and drained first.
Steam radiator valves: a warning
A one-pipe steam valve must be fully open or fully closed. Half open, it traps condensate in the radiator, which causes banging and spitting at the vent. If a valve will not open fully, or its handle turns without doing anything because the stem has stripped, the valve needs replacing. Replacing a steam valve involves separating the union and the pipe thread, and is best left to a plumber who works on steam.
Replacing a valve
For hot water, the system is drained to below the valve, the old valve unscrewed from the pipe and the union, and a new one fitted. Consider a thermostatic radiator valve, which lets you set each room's temperature. For steam, only a valve made for steam will do. Both are plumber's work unless you are experienced.
When to call a plumber
- Packing that will not stop leaking
- A union that weeps after tightening
- A cracked valve body
- A steam valve that will not open fully
- Any valve replacement
Find a licensed plumber near your town.