A boiler that keeps losing pressure is one of the most common complaints we deal with. The good news is that some causes are minor and easy to understand. The less good news is that a persistent drop usually points to something that needs fixing properly. Here’s how to make sense of it.
What boiler pressure actually is
Most modern sealed-system and combi boilers have a pressure gauge on the front, usually showing a reading between 1 and 1.5 bar when cold. This is the water pressure in your heating system. If it drops too low, the boiler may shut down as a safety measure and refuse to fire up.
A small drift over many months is normal. A noticeable drop every few days is not.
Common causes
1. A leak somewhere in the system
The most frequent reason for ongoing pressure loss is a leak. Have a look around:
- Radiator valves and the radiators themselves
- Visible pipework, especially under floors near radiators
- Around the boiler itself
Look for damp patches, staining, or small green/white deposits around joints. Even a tiny weep can drop the pressure over time. Never look for leaks inside the boiler casing yourself — that’s a job for a Gas Safe registered engineer.
2. Recently bled radiators
If you’ve just bled your radiators to clear cold spots, it’s normal for the pressure to drop afterwards. In this case it’s a one-off and topping the system back up should fix it.
3. A faulty pressure relief valve (PRV)
The PRV is a safety valve that releases water if the pressure gets too high. If it’s faulty or has been triggered, you may see water dripping from a small pipe outside the house. A leaking PRV will cause a slow, steady loss and needs an engineer to diagnose.
4. A failed expansion vessel
The expansion vessel absorbs the changes in pressure as your heating heats up and cools down. When it fails, you’ll often see the pressure swing up high when the heating is on and drop low when it’s off. Re-pressurising the vessel or replacing it is a job for an engineer.
What you can safely check and do
- Top up the pressure. Most boilers have a filling loop (a small silver braided hose with valves underneath). Your boiler’s manual shows how to use it to bring the pressure back to around 1.5 bar. Do this with the boiler switched off and cold.
- Note how quickly it drops. If you top it up and it falls again within days, make a note — this is exactly the information an engineer needs.
- Check for obvious leaks around radiators and visible pipes.
When to call an engineer
Call in a professional if:
- The pressure keeps dropping despite topping up
- You can see water leaking from the boiler or the outside PRV pipe
- The pressure swings high and low with the heating
- You can’t find an obvious cause
Bettess Gas carries out boiler repairs across Watton and the surrounding Norfolk area, and persistent pressure problems are bread-and-butter work for us. If yours keeps dropping, it’s worth getting it looked at before it leaves you without heating — and a routine service often catches these issues early.