What to do straight away
A leaking boiler is rarely an emergency on its own, but water and electricity do not mix, and a slow drip can damage floors and ceilings if you leave it. Take these steps first.
- Turn the boiler off at its controls or programmer.
- If the fused spur (the switched socket that powers the boiler, usually near it) is dry and you can reach it without touching water, switch it off. If there is water on or around the spur or any sockets, do not touch them. Switch off at the consumer unit if it is safe to do so, or call an electrician.
- Put a bowl, tray or bucket under the drip and lay towels on the floor.
- Take a photo of where the water is coming from and note the reading on the pressure gauge. This helps the engineer diagnose the problem.
- Do not remove the boiler casing to look inside.
If you smell gas at any point, do not use electrical switches, open the windows and call the National Gas Emergency Service on 0800 111 999 straight away.
If the leak has drained water from the heating system, the pressure will drop and the boiler may show a low pressure code such as Vaillant F22 or Baxi E119. Do not keep topping up a boiler that is actively leaking, as you will simply push more water through the leak.
Where boiler leaks usually come from
Pressure relief valve
The pressure relief valve lets water out when the system pressure reaches about 3 bar. It discharges through a copper pipe to the outside. If the pressure is too high, or the valve is worn and not sealing, it will drip. You may see water at the end of the pipe outside rather than inside the house. High pressure is often caused by a faulty expansion vessel, which we explain in our guide to boiler pressure that is too high.
Pump seals
The pump pushes water round the heating circuit. Its seals and joints can wear with age, letting water seep out. Pump leaks are usually inside the casing, so you tend to see water dripping from the bottom of the boiler itself.
Heat exchanger
The heat exchanger is the part that transfers heat from the burner into the water. Corrosion, often from poorly treated system water, can eventually cause it to leak. This is the most expensive leak to repair, and on an older boiler it can make replacement the better option.
Expansion vessel
The expansion vessel does not usually leak water itself, but when it loses its air charge the pressure rises sharply as the system heats up. That pushes the relief valve open, so the drip you see outside is often a symptom of a vessel problem rather than a fault with the valve. Recharging or replacing the vessel is an engineer job.
Pipe fittings and joints
Compression joints on the pipes under the boiler can loosen over time, especially where pipes expand and contract as they heat and cool. You can often see a drip or a crusty green or white deposit on the fitting.
Condensate pipe and trap
A blocked or frozen condensate pipe can cause water to back up and escape from the boiler. The external pipe is something you can check, and our guide to a frozen condensate pipe explains how. The condensate trap inside the boiler is an engineer job.
Which checks you can do and which need an engineer
You can look at anything outside the casing. Anything inside it is for a Gas Safe registered engineer only.
| Leak source | Typical signs | Safe to check yourself? | Who fixes it |
|---|---|---|---|
| Pressure relief valve | Drips from the copper pipe outside, pressure drops | Yes, look at the pipe outside | Engineer |
| Pipe fitting under the boiler | Drip or deposit on a visible joint | Yes, visual check only | Engineer or plumber |
| External condensate pipe | Blocked or frozen pipe, gurgling, lockout in cold weather | Yes, thaw with warm water | You, or an engineer to reroute |
| Pump seals | Water dripping from inside the casing | No | Engineer |
| Heat exchanger | Water inside the casing, rust marks, repeated pressure loss | No | Engineer |
| Condensate trap | Water inside the casing | No | Engineer |
Do not try to tighten fittings with a spanner while the system is under pressure. An overtightened compression joint can crack and make the leak much worse.
Typical boiler leak repair costs in the UK
Prices depend on your region, the boiler make and model, and how much work is needed to reach the part. These are typical ranges including parts and labour. If the engineer finds more than one fault, ask for an itemised quote so you can see what each repair costs.
| Repair | Typical UK cost |
|---|---|
| Remaking or replacing a leaking pipe fitting | Typically £80 to £150 |
| Replacing the pressure relief valve | Typically £150 to £250 |
| Recharging or replacing the expansion vessel | Typically £80 to £400 |
| Replacing pump seals or the pump | Typically £200 to £400 |
| Clearing or replacing the condensate trap | Typically £80 to £150 |
| Replacing the heat exchanger | Typically £400 to £900 |
Is it worth repairing a leaking boiler?
Most leaks, such as a fitting, a relief valve or pump seals, are worth fixing on almost any boiler. The harder decision is a leaking heat exchanger on an older model. If your boiler is well over ten years old, has had several other faults and needs a new heat exchanger, ask the engineer for a quote for both the repair and a replacement boiler so you can compare. A new boiler also comes with a fresh manufacturer warranty.
If the boiler is still under warranty, contact the manufacturer before booking an independent engineer. Unauthorised repairs can affect your cover.
How to reduce the chance of leaks
- Have the boiler serviced every year so small weeps are caught early.
- Keep the pressure between 1 and 1.5 bar when cold. Constant high pressure strains seals and valves.
- Ask your engineer to check the level of corrosion inhibitor in the system, as it helps protect the heat exchanger and radiators from internal rust.
- Keep the outside condensate pipe clear and insulated before winter.