Why is my boiler leaking water?

A leaking boiler is one of the few plumbing problems where the correct householder action is almost entirely observation. Everything behind the casing is regulated, and much of it is either live or carrying gas. But a good five minutes of observation from outside turns a vague call into a precise one, and an engineer who knows what they are coming to is an engineer who arrives with the part.

Safety

What you'll need

• A container and towels
• A torch
• A phone camera (optional)

Step by step

1. Switch the boiler off and contain the water.Off at its own isolating switch — usually a fused spur beside it. Put a container under the drip and towels down. If water is running onto electrical accessories or a consumer unit below, switch that circuit off at the consumer unit too.
2. Do not remove the casing.This is the boundary. The casing on a modern boiler is part of the sealed combustion arrangement as well as an electrical enclosure. Removing it is Gas Safe work and there is nothing behind it you can lawfully do.
3. Look at the pipework below the boiler.Five or six pipes drop out of the bottom of most boilers: flow, return, gas, cold in, hot out and condensate. Dry each one and watch. A drip forming repeatedly at one compression joint is a plumbing repair, not a boiler fault.
4. Check the filling loop.A weeping filling loop under the boiler is one of the most common causes of what people report as a leaking boiler. Check the valves are fully closed and dry the loop and its unions to see whether they reappear wet.
5. Check the pressure gauge and the relief valve pipe.If the gauge is at the top of its range and the small discharge pipe on the outside wall is wet, the relief valve is doing its job because of a pressure fault, and the water may be tracking back inside. That is an expansion vessel or overfilling problem.
6. Check the condensate pipe and its connections.A plastic pipe leaving the boiler, usually white or grey, carrying clear water when the boiler runs. If it is disconnected, cracked or blocked, condensate escapes at the boiler and looks exactly like a leak. This one is often a simple repair.
7. Note whether it leaks hot or cold, running or idle.Water only when the boiler is firing points at the heating side or the heat exchanger. Water all the time points at the mains side or a joint. Water only when a hot tap runs points at the domestic hot water side of a combi. These distinctions are exactly what the engineer needs.
8. Isolate the water if it is more than a drip.The service valves on the pipes under the boiler let you isolate it without draining the house — usually small lever or slotted valves on the flow, return and cold inlet. If you cannot identify them, close the main stopcock and switch everything off.
9. Report what you saw.Where the water appears, whether it is hot or cold, when it runs, the pressure gauge readings cold and hot, and whether the relief valve pipe outside is wet. Photographs help. That description frequently halves the visit.

Tips

• Photograph the pipes under the boiler while everything is dry, and again when wet. It is the clearest possible way of showing an engineer where it comes from.
• Rusty water below a boiler is a bad sign — it means the leak has been going for a while and is attacking the boiler's internals.
• If the boiler is old and has an internal leak, ask about repair cost versus replacement before authorising work. Internal leaks are often the point at which replacement becomes the sensible option.
• Keep the service record and the model number to hand. Parts availability for older boilers is a real factor in the repair-or-replace decision.

If it doesn't work

Questions people ask