Boiler losing pressure: is it a hidden leak?
A sealed heating system should hold its pressure for months. If yours drops every few days, water is getting out somewhere.
Check the visible parts first: radiator valves, the pipework around the boiler, and the pressure relief pipe that runs outside. A dripping relief pipe means the boiler is letting out water to relieve high pressure, usually because of a failed expansion vessel, a faulty relief valve or an overfilled system. That needs a Gas Safe registered engineer, or an OFTEC technician for an oil boiler. If nothing visible is wet, the relief pipe is dry and the pressure still falls, the leak may be inside the boiler or hidden under a floor or in a wall.
What normal looks like
Most combi boilers sit at around 1 to 1.5 bar when the system is cold, rising a little when it is hot. Your boiler manual gives the exact figure. A slow drop over many months is common. Needing to top up every few days or weeks is not.
Checks you can do yourself
- Radiators and valves. Run a dry tissue around each valve and joint. Look for green or white crusting, a sign of a slow weep.
- Around the boiler. Look for drips or staining on the pipes below it.
- The pressure relief pipe. A copper pipe usually runs from the boiler through the wall outside. If it drips or there are stains below it, the boiler is releasing water through its safety valve. That usually means a failed expansion vessel, a faulty valve or an overfilled system, and it needs a Gas Safe registered engineer.
- Floors and ceilings. Warm patches on a solid floor, damp patches near pipe runs, stains on a ceiling below radiators.
When the leak is hidden
Heating pipes often run inside floor screeds, under floorboards and within walls. A pinhole in one of those can lose enough water to drop the pressure without leaving a mark for weeks. Left alone, a heating leak keeps soaking whatever it sits under, and each top-up adds fresh water to the system.
ADI's engineers find heating leaks with thermal imaging, pressure testing and tracer gas, marking the spot so only a small area needs opening.
Worth knowing: ADI's No Find No Fee terms do not cover central heating leaks. Ask on the phone for the price of a heating leak survey before you book.
Common questions
Can I keep topping up the pressure?
It keeps the heating running, but each top-up adds fresh water and oxygen, which speeds up corrosion inside the system, and the leak keeps soaking whatever it is under. Get it found.
Does insurance cover a heating leak?
Escape of water cover usually includes heating systems. Trace and access cover, where you have it, can pay for finding the leak. Check your policy.
More leak guides
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Signs of a hidden leak
Damp patches, a meter that never stops, a boiler that keeps losing pressure, and the other clues that water is escaping somewhere you cannot see.
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What to do when you find a leak
The first hour: stopping the water, keeping safe, the photos your insurer will want, and who to ring in which order.
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High water bill? Check for a leak
How to test your meter in half an hour, who is responsible for which pipe, and why a leak on your supply pipe can cost you twice.
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Water coming through the ceiling
Where the water is most likely coming from, when a stained ceiling becomes dangerous, and what insurance tends to cover.