Water leaking from the boiler
A wet boiler is not one fault but three, and you can tell them apart before anyone opens the case. What the water is decides both the urgency and the bill.
What it usually means
Three separate water circuits meet inside a combi case, and they are at different pressures, carry different water and fail for different reasons. Working out which one is escaping is most of the diagnosis, and it needs nothing but your eyes and the pressure gauge.
The sealed heating circuit runs at around 1.2 bar and contains the same treated water your radiators hold. It is the only one of the three whose escape shows on the gauge, so a needle that is falling as well as a puddle on the floor identifies the primary side immediately. That water has usually been round the system for years, so it is often discoloured and smells faintly metallic.
The cold mains and the hot tap side sit at whatever your incoming pressure is, which in most streets is somewhere between two and four bar — higher than the heating circuit, and unlimited. Mains water leaks do not move the gauge, do not stop, and produce far more water far faster, which is why they are the ones that reach a ceiling below.
The third is condensate, and it is not a leak in the plumbing sense at all. A condensing boiler pulls water back out of its own flue gases at a rate of a couple of litres an hour, and that water is mildly acidic — around pH three or four. It collects in a trap under the appliance and drains away through a plastic pipe. A cracked trap, a perished seal or a pipe that has been knocked off produces clear water that appears only while the boiler is running and does not touch the gauge.
What none of this is, is a job to open the case for. Mains electricity at 230 volts, a gas valve and the sealed combustion chamber are all within a few inches of the water, and everything behind that front panel is work the law restricts to Gas Safe registered engineers. The useful thing an owner can do is identify the water, isolate the appliance and be able to say which of the three it was.
How to narrow it down
Work through these in order.
Is the pressure gauge falling as well?
If it is, the water is coming from the sealed heating side, which narrows it to a handful of parts: the heat exchanger, the pump head gasket, the automatic air vent, the safety valve, or a joint on the pipework immediately below. If the gauge is holding steady, you are looking at mains water or condensate instead.
What does the water look and smell like?
Grey, brown or black water with a faintly metallic smell has been in the radiators and has inhibitor in it. Absolutely clear water that only appears while the appliance is running, and often smells slightly sharp, is condensate off the flue. Clear, clean water that keeps coming whether the boiler is on or not is mains.
Is it wet inside the case, or only on the pipes underneath?
Look along the copper and plastic below the appliance with a torch before assuming the worst. Compression joints, isolating valves and the filling loop connection are all under there, they all loosen with years of heating and cooling, and a nut nipped up during a visit is the cheapest outcome on this page.
Is it worse with the heating on, or with a hot tap running?
Water that appears only when the radiators are hot points at the primary circuit and its expansion. Water that appears only when somebody runs a hot tap points at the domestic hot water side — the plate heat exchanger and its seals, or the cold inlet feeding it — and that one will not show on the gauge at all.
Has it been freezing outside?
A blocked or frozen condensate pipe backs the water up into the trap and then out of it, so a boiler that has stopped in cold weather and is dripping is often reporting one fault rather than two. That has a remedy of its own and it is worth reading before booking anybody.
Is water reaching the electrics, or can you smell gas?
Either of those changes what you should do next. Switch the appliance off at the fused switch beside it rather than at the boiler controls, and turn off the water feeding it. If you can smell gas, do not touch any switch at all: leave the property and ring the National Gas Emergency Service on 0800 111 999 from outside, or 0800 002 001 in Northern Ireland.
Codes commonly shown with this
- F.77VaillantThe condensate pump or the flue non-return flap has not responded. The boiler cannot clear its condensate or confirm the flue, so it has shut down.Get it checked
- CE 207Worcester BoschSystem water pressure has dropped too low for the boiler to run safely. Topping the system back up to 1.2 bar usually clears it in five minutes.Get it checked
- D5Worcester BoschThe condensate pump has failed or cannot discharge. Acidic water will back up inside the appliance, so it needs attention promptly.Get it checked
- A9ViessmannOn a 100-W, 111-W or 050-W, A9 is a communication error with the OpenTherm room controller: the boiler carries on heating without it. On a 242-F it means the internal pump is blocked.Usually simple to fix
- 108AristonThe heating circuit does not contain enough water to run safely. Refill through the filling loop to about 1.2 bar and the code clears.Usually simple to fix
- F37FerroliSystem water pressure is outside the correct range. Because this is an F code it clears by itself once you top the system back up.Usually simple to fix
When it is urgent
Take care
What it costs
Typical repair cost
£90–£650
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Get an engineer to look at this
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.