Quick answer
A radiator leaking water is usually leaking at a joint, not through the metal: the valve packing nut, the union nut between valve and radiator, or the bleed valve. Those are a tighten or a re-pack. A pinhole or rust weep in a cast-iron section or steel panel means a replacement radiator. Close the radiator valve, catch the drip and watch the boiler gauge for falling pressure.
If your radiator is leaking water, dry it off with a towel and find exactly where the first new bead forms. Water runs along pipes and down the back of a radiator before it drips, so the puddle is rarely under the leak. Where the water starts tells you whether this is a quarter-turn with a wrench or a new radiator.
The five places a radiator leaks
- Valve packing nut. The small nut under the valve handle squeezes packing around the stem. Packing dries out over years of sitting in one position and weeps when the handle is turned for the first time in a decade. Water appears at the stem, right under the handle.
- Union nut. The large nut joining the valve to the radiator is a metal-to-metal seat. It loosens as the radiator expands and contracts through heating cycles, or after the radiator has been shifted for flooring or painting. Water appears at the joint between valve and radiator.
- Bleed valve seat. The small air bleeder at the top corner drips from its centre if it was not closed firmly after bleeding, or if its seat is scored. Water tracks down the end section.
- Pinhole corrosion. A rust-stained weep on the face of a steel panel radiator or on the body of a cast-iron section is the metal failing from the inside. A weep between two cast-iron sections means the nipple joining them has corroded.
- Steam radiators: a spitting air vent. On a one-pipe steam system the vent on the side of the radiator is supposed to pass air and close on steam. A vent that spits or dribbles has failed, or the radiator has settled and no longer pitches back toward its supply valve, so condensate pools inside. A steam valve left partly open does the same; it should be fully open or fully closed.
Stopgap: stop the drip without making it worse
- Close the radiator's valve by turning the handle clockwise until it stops. If there is a second valve at the other end (a lockshield, under a plain cap), close it too and count the turns so it can be reopened to the same setting.
- Put a tray and a towel under the leak. Heating water is often black with iron oxide and stains flooring.
- Do not open the bleed valve on a hot system to relieve pressure, and keep hands clear of water that may be 70 °C or more.
- Look at the boiler pressure gauge and note the reading. Check it again in a few hours.
- Leave the heat on for the rest of the house unless the leak is fast. In freezing weather a cold house does more damage than a slow drip.
Closing the hand valve stops a leak at the bleed valve or in the radiator body only when both ends are shut. A packing nut leak may keep weeping with the valve closed, since the stem can still be under system pressure; that one needs the nut snugged.
Why the boiler gauge matters when a radiator leaks
A leaking radiator lowers the pressure in the whole system. Most boilers have an automatic fill valve that quietly replaces the lost water, which hides the leak and does harm of its own: fresh water carries dissolved oxygen and, in Halton and Hamilton, hardness, and both attack the boiler and the other radiators from inside. If the fill valve is closed or stuck, the pressure falls until the upstairs radiators go cold or the boiler locks out. A gauge that keeps dropping means the leak is bigger than the drip you can see, and why a boiler loses pressure explains where else to look.
Which leaks are a tighten or re-pack, and which mean a new radiator
- Packing nut: tighten clockwise an eighth to a quarter turn while holding the valve body with a second wrench. If it still weeps, the valve is re-packed with new graphite or PTFE packing, a short job.
- Union nut: a small tightening often seals it. If not, the joint is opened, the seats cleaned and re-made. A pitted seat means a new valve, which requires draining that radiator or the system.
- Bleed valve: close it firmly. If it still drips from the seat, a new bleeder threads in once the pressure is let down.
- Steam air vent: replace the vent and re-pitch the radiator with shims under the far legs.
- Pinhole in a steel panel: replace the radiator. Panels are thin, and one pinhole is followed by others.
- Weep in a cast-iron section or between sections: epoxy and sealants are short-lived patches. Replacement is usually the sound repair, either with a salvaged cast-iron radiator of similar output or a new panel sized to the room.
The decision rule: a leak at a nut is a repair, and a leak through the metal is a replacement. Old valves with seized nuts are the grey area. Forcing a seized union on a 90-year-old radiator can crack the fitting, so if a nut will not move with moderate effort, stop.
Call now, or book a visit
- Call now: water is spraying or running, not dripping; the leak is at a pipe inside a wall or ceiling; the boiler pressure is falling toward zero; or water is reaching electrical wiring.
- Book a visit: a slow drip at a packing nut, union or bleeder that a tray can hold, with steady boiler pressure.
ZK Mechanical repairs and replaces radiators in the older brick neighbourhoods of Hamilton, Burlington, Oakville and west Toronto. There is no flat price for a radiator leak, since a re-pack and a cast-iron replacement are different jobs; you get a written quote first, and the $90–$150 diagnostic counts toward the work. See radiator repair, radiator installation for replacements, or boiler repair if the gauge is the bigger worry, and book a visit here.