Radiator valve repair, from a weeping stem to a dead vent

What is actually leaking

"The radiator is leaking" almost always turns out to be one of four specific things, and which one it is determines whether this is a twenty-minute job or a morning.

The valve stem. Water weeping from around the handle spindle when the valve is operated. This is packing that has dried and shrunk with age, and on most older valves it is repackable, the gland nut is loosened, new packing goes in, and the valve carries on for another decade. This is the most common radiator leak and the cheapest fix.

The union nut. The connection between valve and radiator, which loosens with thermal cycling and building movement. Sometimes a careful tighten, more often a strip and re-make with fresh jointing.

The radiator body. Corrosion pinholes, usually along the bottom of a section or at a seam. On cast iron this is often repairable at a joint; a rusted-through section generally is not, and the radiator needs replacing.

The vent or trap. On steam, a spitting air vent is a failed vent, not a leak in the plumbing sense, the vent should close as soon as steam reaches it, and one that keeps discharging water and steam has failed and needs replacing.

Steam valves versus hot water valves

They look similar and they are not interchangeable, which matters because fitting the wrong one causes problems that are then blamed on everything else.

One-pipe steam valves are large-bore, angled, and designed for two-way flow, steam in, condensate out, through the same opening. They must be fully open or fully closed. They have no packing arrangement suited to being throttled and no capacity to handle counterflow through a restricted opening, which is exactly why half-open one-pipe valves cause the banging people complain about.

Two-pipe steam valves can be throttled because condensate leaves through the separate return, via the trap.

Hot water valves are smaller bore, single-direction, and perfectly happy being partially closed for balancing. Many hot water radiators also have a lockshield valve on the return, set once during balancing and then left alone.

When a valve is replaced, it must match the system type and the pipe size, and on steam it must be pitched so condensate drains back. A valve installed level or pitched the wrong way on a one-pipe system will hold water and hammer.

Air vents and why they fail

On one-pipe steam, the air vent is the most important part of the radiator and the most neglected. Its job is to let air out as steam arrives, then close tight the instant steam reaches it. When it works, the radiator heats fully and quietly. When it fails, one of two things happens.

Failed closed, clogged with sediment or corroded shut. Air cannot leave, so steam cannot enter, and the radiator stays cold no matter what the thermostat does. This is the single most common cause of a cold radiator in a working steam system.

Failed open, the internal element no longer closes on contact with steam, so the vent spits water and steam into the room, marks the wall and wastes heat. Unpleasant but harmless, and a five-minute replacement.

Vents are inexpensive and are genuinely consumable, a system that has not had its vents replaced in twenty years has mostly dead vents. Replacing them all at once, and choosing vent rates deliberately rather than fitting identical ones everywhere, is the core of steam balancing and often transforms how a building heats.

Main vents matter even more. Large, fast vents at the end of each steam main let steam reach the risers quickly; seized main vents make the whole system slow and uneven. They are frequently original and frequently dead.

Steam traps on two-pipe systems

A steam trap sits at the radiator outlet on a two-pipe system and does one job: let air and condensate pass into the return while holding steam back. They contain a thermostatic element or a float, and they wear out.

When a trap fails open, steam blows straight through into the return line. That pressurises the return, prevents other radiators from draining properly, causes hammering in the return piping, and wastes a significant amount of fuel. One failed trap can unbalance an entire building, which is why chasing individual cold radiators without checking traps is usually wasted effort.

When a trap fails closed, condensate backs up in the radiator and it goes cold from the bottom.

On a system that has never had traps replaced, expect most of them to be failed. Renewing traps across a building is a project rather than a call-out, but it is frequently the single highest-value thing that can be done to an old two-pipe system.

Thermostatic radiator valves

A thermostatic radiator valve regulates flow to an individual radiator based on room temperature, giving genuine room-by-room control without rewiring anything. On hot water systems they are straightforward and well worth fitting in rooms that consistently overheat.

On steam they are different animals. A one-pipe steam TRV does not throttle the inlet valve, it controls the air vent instead, closing the vent when the room is warm so steam stops entering. That is the only way to regulate a one-pipe radiator without causing hammering, and it works well. Two-pipe systems can use conventional inlet-side TRVs.

Practical notes: do not fit a TRV in the room with the main thermostat, or the two controls fight each other. Do not enclose a TRV head behind curtains or furniture, because it senses the air immediately around itself. And on any system, TRVs need occasional exercising, a head left on one setting for years can seize, which is why it is worth running them through their range at the start of each season.

Seized, painted and broken handles

Decades of paint, corrosion and disuse leave a lot of radiator valves immovable. Forcing a seized handle is how you snap a stem or crack a cast iron body, which turns a nuisance into a genuine repair.

A technician will free it properly, releasing the packing nut, penetrating oil, controlled heat where appropriate, and working the stem gradually. Where the stem is already damaged, the valve is replaced. Where the handle alone is broken or missing, a replacement handle or a bonnet swap often solves it without touching the body.

Worth knowing: a valve that has not been operated in years may weep once it is finally moved, because the packing has moulded itself around a stem in one position. That is normal, and it is repacked at the same visit rather than being a new problem.

Replacing a radiator

Sometimes the valve is fine and the radiator is finished, rusted through, cracked from a historic freeze, or so heavily painted it barely emits heat.

On steam, a replacement must be matched by radiation capacity, not by physical size, because the boiler was sized to the total connected radiation of the whole system. Fitting a substantially smaller or larger radiator changes the system balance and, across several changes, can leave the boiler mismatched to its load.

On hot water, sizing is by heat output for the room, which is more forgiving.

Cast iron radiators, incidentally, are worth keeping where possible. They are extremely durable, they radiate gently and comfortably, and reconditioned units are readily available. Stripping and repainting an original radiator with a thin coat, thick paint genuinely reduces output, usually beats replacing it with modern panel.

Related: steam heat service, baseboard service, and leak detection if water is appearing and you cannot see where from.