Boiler Leaking Water? Where It Is Coming From and What to Do

If your boiler is leaking water, the position of the leak tells you most of what you need to know: a drip from the small copper pipe outside is almost always the pressure relief valve, water pooling underneath the casing usually means a failed seal or a corroded joint inside, and a leak that only appears when the heating is running points at the pump or the heat exchanger. None of them fix themselves, and all of them get worse.

The good news is that a leaking boiler is rarely an emergency in the gas sense. It is a water problem, and the sensible first steps are things you can do yourself in a few minutes. Here is how our engineers read a leak on repair visits across Deal, Dover, Sandwich and Thanet, what you can safely do now, and what has to wait for a Gas Safe registered engineer.

Why is my boiler leaking water? Start with where the water is

Before you ring anyone, look at exactly where the water is coming from. Put a sheet of kitchen roll under the boiler, wait ten minutes and see where it goes dark. That one observation saves a lot of guessing.

Water dripping from the copper pipe outside

Every sealed-system boiler has a pressure relief valve that opens at around 3 bar and dumps water through a copper discharge pipe to the outside wall. If that pipe is dripping, or there is a green or white stain on the brickwork beneath it, the valve is either lifting because the pressure is genuinely too high, or it has been lifted once and is now failing to reseat.

The usual cause is a flat expansion vessel. When the vessel loses its air charge it can no longer absorb the expansion of the water as it heats, so pressure climbs from 1 bar cold to 3 bar hot and the valve does its job. People then top the system up, which makes the problem worse the next time the heating runs. If your gauge swings sharply between cold and hot, read our guide to a boiler losing pressure - it is the same fault seen from the other side.

Boiler leaking water from underneath

Water at the bottom of the casing is the vaguest symptom because everything drains downwards. On our visits the most common culprits, roughly in order, are:

• A weeping pipe joint where the flow, return or mains pipes enter the boiler. Compression fittings loosen over years of heating and cooling, and a slow weep leaves a green crust on copper long before it drips.
• The condensate trap or its joint. Condensing boilers produce a couple of litres of mildly acidic water every hour they run. If the internal trap cracks or the push-fit joint to the plastic condensate pipe has crept apart, you get a clear, cold drip that appears only when the boiler is firing.
• The pump. Pump seals wear, and a pump on its way out often leaks before it stops turning.
• The heat exchanger. On older units, or on boilers that have spent years on East Kent’s hard water without a filter or inhibitor, scale and corrosion eventually pinhole the heat exchanger. This is the leak nobody wants, because on most boilers it is the single most expensive part.

A leak that only appears when the heating is on

If the boiler is dry when it is idle and wet when it runs, the leak is pressure- or temperature-related: the relief valve, a seal that opens up as the metal expands, or the heat exchanger. Note when it happens and tell us.

Leaking from the pressure gauge or filling loop

A drip at the filling loop is a favourite. The braided hose is meant to be disconnected after use, and a loop left connected with a tired valve weeps quietly into the cupboard below. Check both valves are fully off before you assume the boiler itself has failed.

What to do right now

1. Turn the boiler off at its own switch or the fused spur, not at the consumer unit for the whole house. Water and live electrics inside a casing are not a combination to leave running.
2. Catch the water. A tray or towel under the boiler stops it reaching the floor, the kitchen units or the ceiling below.
3. Check the pressure gauge. If it reads above 2.5 bar cold, the leak is almost certainly the relief valve doing its job, and the problem is upstream of it. If it is at zero, the leak has been running a while.
4. Look at the discharge pipe outside for a drip or a stain.
5. Do not keep topping up. Repeatedly refilling a leaking system feeds fresh, oxygenated water into it, which corrodes radiators and the heat exchanger from the inside.
6. If the boiler is locked out with a code on the display, look it up before you ring. Low-pressure lockouts are common on Baxi units, which show E119, and on Glow-worm boilers, which give F22. The code is a symptom of the leak, not a separate fault.

What not to do

Do not open the casing. Everything inside a gas boiler, from the pump and the diverter valve to the relief valve and the heat exchanger, is a Gas Safe job by law, and a leak repair that disturbs the gas side has to be tested afterwards. It is not a matter of skill; it is who is legally allowed to break the seal.

Do not pour a leak sealer from the shop into the system. Used at random they block valves, sensors and heat exchangers, and we spend more time undoing them than the original leak would have taken.

Is a leaking boiler dangerous?

Water on its own is not, but two things around it can be. Water tracking onto the electronics can trip the fuse or damage the control board, turning a cheap repair into a dear one, and a leak that has run for weeks can corrode parts on the combustion side. If you ever smell gas, or the boiler makes an unusual noise, put out any flames, open the windows, leave the boiler off and ring the National Gas Emergency line on 0800 111 999 before anyone else.

What a leak repair typically costs

Repairs are charged by the hour, not a fixed price: £90 + VAT for the first hour, which is always billable, then £45 + VAT per half hour, plus parts. Current rates are on our pricing page. As a guide, from what we see on East Kent jobs:

• A weeping joint, a filling loop or a condensate trap is usually the first hour and a small part.
• A pressure relief valve or an expansion vessel recharge or replacement typically runs from around £120 to £250 in total, depending on the boiler and access.
• A pump is typically in the £200 to £350 range all in.
• A heat exchanger is the one to think hard about. On a boiler over ten years old the part alone can approach the cost of a new appliance, and we will tell you honestly when a replacement makes more sense. You can price one in a few minutes on our instant quote.

Stopping it happening again

Almost every leak we attend gave notice first: a slowly falling gauge, a stain on the wall outside, green crust on a fitting. An annual service catches them, and it costs £90 + VAT. Our boiler servicing page sets out what the visit includes.

If you would rather not think about it, BOYLA HomeCare plans start at £9 a month and include the annual service. Paid plans carry no call-out fees and emergency response within 24 hours, Monday to Friday, for members. Plans start after a £108 inc VAT inspection.

Boiler leaking now?

Turn it off, catch the water, check the gauge, then call us on 01304 381999. A real person answers the phone, we work across Deal, Dover, Sandwich, Canterbury and Thanet, and same-day visits are usually available within working hours. Every engineer is Gas Safe registered (2836), so whatever we find inside the casing, we are allowed to fix it. More on how we handle faults is on our boiler breakdown repairs page.