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Why is my boiler losing pressure?
It's one of the most common boiler problems we get called about — you glance at the pressure gauge and the needle has dropped into the red. The good news: low boiler pressure is usually easy to understand, and often easy to fix. Here's what's going on and what to do about it.
What pressure should your boiler be?
Most modern combi boilers are happiest sitting between 1 and 1.5 bar when the heating is off. When the heating fires up and the water warms, the pressure naturally rises a little — that's normal. If the needle sits below 1 bar (often marked in red), your boiler is low on pressure and may lock out or stop heating altogether.
The common causes
1. You've recently bled your radiators
Letting air out of a radiator also lets a little pressure out of the system. If you've bled your rads recently, a small drop is completely normal — just top the boiler back up and you're done.
2. A small leak somewhere in the system
This is the most common reason for a boiler that keeps losing pressure. Leaks are often tiny and slow — a weeping radiator valve, a joint under the floor, or a drip from the boiler itself. Have a look around radiator valves and pipework for damp patches or light staining. A leak won't fix itself, and left alone it can cause real damage.
3. A failed expansion vessel or pressure relief valve
Inside your boiler, the expansion vessel absorbs the extra pressure as water heats up. When it fails, pressure can swing about or escape through the pressure relief valve — you might spot water dripping from a small pipe outside the house. These are jobs for a Gas Safe engineer.
Quick check: Is there water dripping from a small copper pipe poking through your outside wall? That's often the pressure relief valve discharging — a sign something needs looking at properly.
The fix you can try yourself
If the pressure has dropped once and there's no sign of a leak, you can usually re-pressurise the boiler yourself:
- Turn the boiler off and let it cool.
- Find the filling loop — a silver braided hose with a valve (or two) underneath the boiler.
- Open the valve(s) slowly. You'll hear water flowing and see the gauge rise.
- Close the valve(s) when it reaches about 1.2 bar.
- Turn the boiler back on and check it fires up.
Always follow your own boiler's manual — the exact steps vary by make and model.
When to call an engineer
If the pressure drops again within days, or keeps falling no matter how often you top it up, stop topping it up and give us a call. Repeated pressure loss means there's a leak or a faulty part that needs finding and fixing properly — and chasing it early is far cheaper than dealing with water damage later.