A boiler that keeps losing pressure is one of the most common heating problems we're called out to across North and East London. The good news: low pressure is often something you can top up yourself in a couple of minutes. But if it keeps happening, it usually points to a fault worth getting checked.
What pressure should my boiler be?
Most modern combi and system boilers sit at around 1 to 1.5 bar when the heating is off (cold). You'll see this on the pressure gauge — a round dial or a digital readout — on the front of the boiler. When the system is hot, pressure rises a little, often to around 2 bar. That's normal.
If the needle sits below 1 bar (often in a red zone), your boiler is low on pressure and may shut down or fail to heat properly.
Common causes of pressure loss
A tiny, gradual drop over many months can be normal. A noticeable or repeated drop usually has one of these causes:
- A leak somewhere in the system — radiators, pipework or joints (even a pinhole leak loses pressure over time)
- Bleeding radiators recently, which releases water and pressure
- A failed or waterlogged expansion vessel inside the boiler
- A faulty pressure relief valve (you may see water dripping from an outside overflow pipe)
When to call a Gas Safe engineer
Topping up occasionally is fine. But if you're re-pressurising more than once a month, or you can see water leaking from the boiler or an outside overflow pipe, there's an underlying fault — commonly a leak, a failed expansion vessel, or a worn pressure relief valve. These need a Gas Safe registered engineer.
Never attempt to open up or repair the sealed parts of a gas boiler yourself — it is illegal and dangerous to carry out gas work unless you are Gas Safe registered.
How to re-pressurise your boiler
If your pressure is low, you can usually top it up using the filling loop — the small braided hose or keyed valve underneath the boiler:
- 1
Turn off your boiler and let it cool
Switch the boiler off and wait for it to cool down before you start.
- 2
Find the filling loop
Locate the filling loop under the boiler — usually a silver braided hose with a valve at each end, or a built-in keyed valve.
- 3
Open the valves slowly
Open both valves to let cold mains water into the system. You'll hear water flowing.
- 4
Watch the gauge
Keep an eye on the pressure gauge and close the valves once it reaches about 1.2–1.5 bar. Don't overfill.
- 5
Close the valves and check
Close both valves fully, turn the boiler back on and check it fires up. If the pressure keeps dropping or climbs too high, call an engineer.