Boiler Leaking Water From Underneath

What's happening?

Water coming from the bottom of a boiler comes from one of four places and each has a different meaning. Clear water with falling pressure is a leak from the sealed heating circuit: a pump seal, a pipe joint or a valve inside the casing. Water dripping from the small pipe outside is the pressure relief discharge, not a leak inside. Slightly cloudy, slightly acidic water from a plastic pipe is condensate and means a cracked trap or a joint on the condensate line. Water running from the hot water side on a combi is a leaking plate heat exchanger or a fitting. All of them put water inside a box that carries mains electricity, which is why the first move is to isolate it.

What usually causes it?

• A worn pump seal or a pump head that has lost its gasket
• A pipe joint, compression fitting or sealing washer inside the casing that has corroded
• A pressure relief valve that has lifted and not reseated
• A cracked condensate trap or loose condensate pipe joint
• A leaking auto air vent or expansion vessel connection
• A plate heat exchanger or hot water fitting leaking on a combi

Is it dangerous?

Yes, because the water is next to the boiler's electrics. Isolate the boiler first. A slow weep is less urgent than a steady drip, but a boiler that is leaking and tripping the electrics must stay off. A leak that is only condensate is mildly acidic and should still be cleaned and dealt with, not ignored.

Can I fix it myself?

Switch the boiler off at the fused spur, put a container underneath, and note whether the gauge pressure is falling and whether the water is clear or cloudy. Mop up and look at where it first appears. Do not take the casing off and do not wrap or seal a leak inside it. Leave the boiler off at the spur until the engineer has seen it.

When should you call a heating engineer?

Call a Gas Safe registered engineer the same day for any leak from inside the casing or the base. If the leak is only the pipe outside, note when it drips and ask for the expansion vessel and relief valve to be checked. Call out of hours if water is reaching sockets or the fuse box.

What will the heating engineer actually do?

• Isolate the boiler and trace the leak to the pump, joint, valve or condensate trap
• Replace the failed pump seal, washer, fitting or valve
• Check the plate heat exchanger and hot water side for a leak on a combi
• Dry the boiler and check the circuit board and wiring have not been affected
• Repressurise and watch the gauge through a heat cycle
• Check the pressure relief and expansion vessel if water is also leaving outside

Who fixes this

• Boiler repair

Questions people ask

Sources & References

1. Vaillant: what does a pressure relief valve do?
2. Vaillant: what is an expansion vessel?
3. Vaillant: getting ready for winter, your condensate pipe
4. HSE: domestic gas safety, homeowner FAQs