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Heating · RadiatorsRadiator Repair
Radiator repair is usually valve repair: most leaks are at the packing nut under the handle, the union where the valve meets the radiator, or the air vent, and all three are fixed without replacing the radiator. A pinhole or a weep between cast-iron sections is the exception and normally means a new radiator. Diagnosis is $90–$150, and most of this work falls between $150 and $800.
Find where the water is coming from before deciding anything. A drip from under the handle is the packing nut: the packing around the valve stem has dried out, and a small turn of the nut or fresh packing stops it. A drip from the large nut between the valve and the radiator is the union, which leaks when the radiator has been knocked out of line or the joint faces have corroded. A weep at a top corner is the bleed valve, and why a radiator leaks water helps you tell them apart.
A rust streak running down between two sections is a different matter. A cast-iron radiator is a row of sections joined by nipples or gaskets, and when a joint or the iron itself lets go there is no dependable field repair; sealants and epoxy buy a season at best. The practical fix is a replacement of matching output, either reclaimed cast iron or a steel panel. Until then, keep a pan under it and watch the boiler gauge, because a leaking radiator pulls fresh water into the system through the fill valve.
The other half of the job is a radiator that will not heat. On an old hand valve the stem seizes, or the disc drops off the stem so the handle spins and the valve stays shut. A radiator can also be air-bound, or half-blocked with sludge so it is warm at the top and cold along the bottom. We check the valve, bleed the radiator and feel the supply pipe before deciding which, and why one radiator is cold lists the checks you can make first.
Working on one radiator rarely means draining the house. If it has a valve on each side, both are closed and only that radiator is emptied. Many older systems have a single valve per radiator, so the system is drained to just below it, the repair made, and everything refilled, bled and brought back to pressure. Our radiator heating system service case study shows that on a real job, and boiler repair covers faults at the heat source.
Signs a radiator needs repair, not just bleeding
- Water or a green-white crust at the nut under the valve handle
- A drip from the union between valve and radiator, or a stained floor beneath it
- A valve handle that will not turn, or turns freely and changes nothing
- One radiator stone cold with its valve open while the others are hot
- A rust line or a bead of water between two cast-iron sections
How it works
Every joint on the radiator is dried and watched under pressure, the valve is worked through its travel, and the supply and return pipes are checked for heat. That separates a packing leak from a union leak from a failed section.
The radiator is valved off and emptied where it can be. Otherwise the boiler is shut down and cooled and the system drained to just below the radiator, with the floor and the finish underneath protected.
The valve is repacked or replaced, the union remade or the radiator swapped. The system is then refilled, bled from the bottom floor up and returned to its cold pressure, and the repair is checked hot before we leave.
Common questions
Is a leaking radiator an emergency?
A slow drip at a valve is not: put a container under it and book a visit. Water running or spraying, a leak from the body of the radiator, or a boiler gauge falling toward zero is urgent. Close that radiator's valve, switch the boiler off and call our emergency line, because a boiler firing on a system that is losing water can be damaged.
Can I tighten the packing nut myself?
Yes, carefully. Hold the valve body with one wrench and turn the small nut under the handle clockwise an eighth to a quarter of a turn with another. If the drip stops, you are done. If it does not, the packing needs replacing, and forcing the nut further only strips it. Leave the large union nut alone; overtightening that one cracks old fittings.
Should a leaking cast-iron radiator be repaired or replaced?
The rule is where the water comes out. A leak at the valve, the union, a plug or the bleed screw is a repair, and the radiator has decades left. A leak from the iron itself or from the joint between two sections means replacement, because the sections cannot be reliably resealed in place and one that has started weeping keeps going.
Why does my radiator bang or hiss?
On a one-pipe steam system, banging is water trapped by a valve left half open or a radiator that has settled the wrong way; those valves are meant to be fully open or fully shut. Hissing is the air vent, which should close once steam reaches it. On a hot-water system, ticking is the metal expanding and gurgling is air, and that calls for bleeding.
When is the best time of year to fix radiators?
Between May and September. The system can be drained without leaving anyone cold, several valves can be done on one drain-down, and there is time to source a matching radiator if one has to be replaced. Leaks that only show when the system is hot get noted in winter and fixed in spring unless they are getting worse.