Blog · Troubleshooting
Why Is My Boiler Losing Pressure? Common Causes and Fixes
If your boiler pressure gauge keeps dropping below 1 bar, the boiler may lock out and leave you with no heating or hot water. The good news: some causes are a five-minute fix. Others need a Gas Safe registered engineer — here is how to tell them apart.
What boiler pressure should be
Most sealed (combi and system) boilers run at 1 to 1.5 bar when cold, rising a little as the system heats up. Below about 0.5 bar many boilers will shut down as a safety measure. A pressure gauge that slowly creeps down over days or weeks is the classic sign something is escaping.
The common causes
Pressure loss almost always comes down to one of these:
- A small leak somewhere on the pipework, a radiator valve or the boiler itself
- Recently bled radiators (bleeding releases pressure — just top it up)
- A failed or waterlogged expansion vessel
- A faulty pressure relief valve letting water out through the overflow pipe outside
What you can safely check yourself
Look around radiator valves and visible pipework for damp patches or green/white staining. Check whether the small pipe that exits through an outside wall (the pressure relief discharge) is dripping. If you have just bled radiators, simply re-pressurise using the filling loop — our guide on how to bleed a radiator explains where to find it.
Top the pressure back up to around 1.2 bar when cold. If it holds, great. If it drops again within days, you have a genuine fault that needs tracing.
When it needs an engineer
A failed expansion vessel or a leaking heat exchanger inside the boiler is not a DIY job — it involves working on a sealed, pressurised gas appliance and should only be done by a Gas Safe registered engineer. Repeated pressure loss is also worth weighing up against the age of the boiler; our guide on repair vs replacement can help you decide.
If you are anywhere in our service area, we can find and fix the cause — book a visit or get a fixed price.
Quick questions
Is it safe to keep re-pressurising my boiler?
Topping up occasionally is fine. But if you are doing it more than once a month, you have a leak or a component fault that should be diagnosed rather than masked.
Can low boiler pressure damage the boiler?
Running persistently low can stop the boiler working and, over time, stress components. Modern boilers usually lock out to protect themselves, which is why you lose heating and hot water.
Why does my pressure rise too high instead?
Pressure that climbs above 2.5–3 bar usually points to a failed expansion vessel or a filling loop left slightly open. Both need looking at by an engineer.
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Sources & further reading
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