Do this first
- Switch the boiler off at its control and at the fused spur beside it. If the spur is wet, leave it and turn the circuit off at the consumer unit instead.
- Close the heating isolation valves under the boiler if you can reach them safely, or the internal stopcock if you cannot.
- Put a container under the drip and towels around the base. Boiler water is usually black and it stains everything it touches.
- Do not remove the casing. It is not a cover, it is part of a sealed combustion enclosure, and it is unlawful for anyone who is not registered to work on the appliance.
- Note when the water appears. Only after the boiler fires, or continuously? That single observation decides which of the two families of fault you have.
- Look at the discharge pipe outside, the small pipe through the wall near the boiler. If it is dripping or limescale-streaked, the system is shedding water there.
- If you smell gas, leave the building, do not touch switches, and ring the national gas emergency service on the number printed on your gas bill.
Two families of fault, and the observation that separates them
Water that appears only after the boiler fires is a pressure fault: the system is being asked to hold more volume than it has room for, and the safety valve is doing its job by letting some out. Water sitting under the boiler constantly is a mechanical failure: a joint, a pump seal, a diverter or a heat exchanger. Only the second one empties the system while you sleep.
| When and where | Likely cause | Who fixes it |
|---|---|---|
| Outside discharge pipe drips shortly after the boiler fires; gauge swings high then low | Expansion vessel has lost its charge, or the pressure relief valve is passing | Heating engineer at the appliance |
| Steady drip from a pipe or compression joint below the casing, cold or hot | A joint on the pipework outside the appliance | Us |
| Water pooling under the centre of the boiler, constant, often rusty | Pump seal, diverter valve, internal seals or heat exchanger | Gas Safe registered engineer only |
| Clear water from a plastic pipe on a condensing boiler, in freezing weather | Blocked or frozen condensate, backing up | Often self-clearable |
| Nothing visible at the boiler but pressure keeps falling | A leak elsewhere on the circuit, often under a floor | Us, by detection |
The pressure relief valve is worth understanding rather than fearing. Its job is to release water if pressure climbs too far, so an occasional discharge is the valve working. A constant or heavy discharge means the system is repeatedly reaching high pressure, which is a symptom of something deeper, usually a perforated expansion vessel diaphragm. If the gauge is the thing that worries you most, start with a boiler that keeps losing pressure, or with pressure dropping with no visible leak if you have already looked everywhere obvious.
What the repair involves
Where the leak is on the pipework outside the appliance, which is a large share of them, the joint is cut out and remade, the circuit refilled, vented and pressure tested, and the cabinet, floor and wall behind dried and made good. Where the gauge keeps falling and nothing around the boiler is wet, the circuit is isolated and pressure tested, then traced with thermal imaging and tracer gas to find the loss under a floor. That is boiler and heating leak repair, and the whole of it including the making good.
What we do not do is work inside the appliance. If the test points at the pump, the diverter or the heat exchanger, we say so and you book a Gas Safe registered engineer for it, rather than paying us to confirm it twice.
When it is not us
- Anything behind the casing. Gas Safe registered engineers only, by law. That includes taking the front off to look.
- The boiler is in warranty. Use the manufacturer's scheme first, or you risk the claim.
- Frozen condensate in cold weather. A white plastic pipe running outside, a boiler locking out, and no fire: thawing that pipe with warm, not boiling, water from a kettle and a cloth often restores it. That is a homeowner job.
- The discharge pipe drips a little occasionally. That is the safety valve doing what it is for. It is worth mentioning at the next service rather than calling anyone out, and the boiler pressure hub covers the fault codes that go with it.
- You rent. Report it in writing immediately. The heating installation and the annual gas safety check are the landlord's responsibility.
- Communal plant in a block. A landlord's boiler or heat interface unit is instructed by the managing agent, not by you.