The short answer
If your boiler is leaking water, switch it off at the mains, place a container under the drip, and avoid touching electrical parts. Leaks come from failed seals, corrosion, excess pressure or pump problems, and they worsen if ignored. Book a Gas Safe engineer promptly.
What the law requires
What should I do right now?
Switch the boiler off at its electrical supply, usually a fused spur beside it. Water and live electrics belong apart.
Place a container under the drip and lay a towel to protect flooring and anything below.
If water is flowing rather than dripping, close the mains cold water stopcock to slow it while you arrange help.
Do not open the boiler casing and do not tighten fittings inside it. Gas appliance internals are legally Gas Safe territory, and an overtightened fitting cracks.
Note where the water appears (underneath the casing, at a pipe joint, from the outside discharge pipe) and book an engineer. That one observation shortens the diagnosis considerably.
A leak is rarely a same-minute emergency, but it is never a leave-it-for-spring problem. Water finds electronics, corrodes what it sits on and lifts flooring, and the small drip you meet in October is the failed PCB you pay for in December.
Why is my boiler leaking?
Location tells most of the story. Water from underneath the casing usually means an internal seal or gasket has failed, or a component (pump, diverter valve, heat exchanger) has begun weeping; internal corrosion in older boilers produces the same symptom with a worse prognosis. Water at a visible pipe joint near the boiler is often a fitting or valve, one of the simpler fixes on the list. Water from the small copper pipe on your outside wall is the pressure relief valve discharging, which points at excess pressure: an overfilled system, a failed expansion vessel, or a relief valve that no longer reseals; our boiler losing pressure guide explains that trio in full. And a leak that appears only when the heating runs suggests a joint that opens with heat expansion, the intermittent kind that hides from casual inspection and shows itself to a pressure test.
Is a leaking boiler dangerous?
The water itself is heating system water, unpleasant but not hazardous. The risks sit beside it: water reaching the boiler’s electrics can trip circuits or damage the control board, sustained leaking damages the fabric of your home, and a leak from the heat exchanger on an older appliance is occasionally the visible sign of a fault an engineer needs to assess properly. Switching the boiler off removes the electrical risk immediately, which is why it is step one. If you ever smell gas alongside a leak, that is a different emergency: leave the property and call the National Gas Emergency line free on 0800 111 999 first.
Leaks are priority visits across London. Tell us where the water appears and we will usually name the likely cause on the phone.
What does a boiler leak repair cost?
London ranges for the common culprits: a pipe fitting or valve repair £95 to £180, an internal seal or gasket replacement £110 to £220, a pressure relief valve £110 to £190, pump-related leaks £200 to £350, and a weeping heat exchanger £400 upwards, which on an older boiler is where the repair-or-replace conversation honestly begins; our repair or replace guide covers the maths. Every quote is fixed after diagnosis, and catching a leak early keeps you at the cheap end of this table, which is the entire argument for booking promptly.
Leaking boiler, quick answers
Can I keep using a boiler that is leaking slightly?
Switch it off and treat it as urgent rather than emergency. Running it risks the electrics and worsens whatever is failing.
Why does my boiler only leak when the heating is on?
Heat expands metal and opens marginal joints; it is a real leak that hides when cold, and a pressure test finds it.
Is water from the outside pipe a leak?
It is the pressure relief valve doing its job, but doing it repeatedly means a pressure fault that needs fixing before the valve wears out.
Will my home insurance cover the damage?
Many policies cover resulting water damage but not the boiler repair itself; check your escape-of-water terms, and photograph any damage before cleanup.