What boiler pressure actually is
Your central heating is a sealed system — the water in your radiators and pipes is held under a gentle, constant pressure so it circulates properly. ‘Boiler pressure’ is simply a measure of that, shown on a gauge or display on the front of the boiler.
This guide is for modern combi and system boilers — the sealed type with a pressure gauge. An older ‘conventional’ boiler with a tank in the loft works differently and won’t have a pressure reading to worry about.
When cold, most boilers are happiest between 1 and 1.5 bar. As the system heats up the pressure rises a little — often towards 2 bar — which is normal. The problem is when it drifts down below where it should be.
What happens when the pressure gets too low
Below roughly 0.5 bar, most boilers have a safety feature that stops them firing until the pressure is topped back up. That’s why a loss of pressure so often shows up as no heating and no hot water — the boiler hasn’t broken, it’s protecting itself.
Topping up will usually get things running again. But topping up treats the symptom, not the cause. If you’re doing it again and again, something is genuinely wrong and needs looking at.
Why your boiler is losing pressure
There are only a handful of reasons, and the way it drops is a big clue.
A small leak in the system
By far the most common reason for a slow, gradual drop is a small leak — a weeping radiator valve, a loose joint or a tired seal. Often too small to spot, but enough to drop your pressure over days or weeks.
A faulty expansion vessel
If the pressure drops suddenly with no sign of a leak, the expansion vessel is often the culprit. It absorbs the extra pressure as the water expands, and over time its internal charge can fail.
A stuck pressure relief valve
The safety valve lets water out if pressure climbs too high — but sometimes it doesn’t fully close again and quietly lets pressure escape. A telltale sign is water dripping from a small pipe outside.
When it’s time to call an engineer
A single top-up now and then is nothing to worry about. It’s worth getting a Gas Safe engineer out if you’re topping up more than once or twice a year, the pressure drops quickly, you can see water leaking, or you’re simply not confident doing it yourself — which is completely fair enough.
Boiler still losing pressure and you can’t get to the bottom of it? If you’re in South Manchester or Cheshire, our Gas Safe engineers will find the cause and fix it for good — no hard sell, no jargon.
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