What should you do first?
- Turn the boiler off at its controls.
- Isolate the power at the boiler’s fused spur switch, but only if it is dry and you can reach it safely.
- Catch the water with a bowl or towels and move anything valuable away.
- Note where it is coming from: under the boiler, from a pipe, from the outside wall or from inside the case.
- Check the pressure gauge and note the reading.
If the leak is heavy and coming from the pipework rather than the boiler, closing the isolating valves under the boiler can slow it, if you know which they are. If not, leave them.
Where do boiler leaks usually come from?
- Pressure relief valve. This valve opens to release water if the system pressure gets too high. Water may appear on the copper discharge pipe outside, or the valve itself may drip once it has opened a few times. The cause is often an expansion vessel that has lost its charge, letting pressure spike when the heating warms up.
- Pump seals. The circulating pump inside many boilers has seals that wear over time and weep.
- Joints and valves under the boiler. Compression fittings can loosen with years of heating and cooling, and isolating valves can seep around the spindle.
- Heat exchanger. Corrosion can eventually pierce the heat exchanger, mostly on older boilers. This is usually the most expensive cause and can tip the decision towards replacement.
- Condensate pipe and trap. A blocked or frozen condensate pipe can back water up and spill it from the trap. In winter, see frozen condensate pipe.
Is it the boiler or the heating system?
Leaks from radiators, radiator valves and pipe joints around the house also drain the boiler’s pressure, and they are often easier to fix. If the boiler itself is dry but the gauge keeps dropping, see boiler losing pressure.
What will the engineer do?
The engineer will find the source, check the pressure relief valve and expansion vessel, and test the pump and internal joints. Many leaks are a single part such as a relief valve, a seal or a fitting. You will be told the cause and the price before any work, and if the heat exchanger has failed on an older boiler you will get honest advice about whether repair is worth it.