TL;DR
A small rise in boiler pressure while the heating is running is normal. Water expands as it heats, and the expansion vessel takes up the extra volume. The gauge settles back once the system cools. Pressure that falls while the heating is off and the boiler is cold is the pattern to worry about. It usually points to a leak or a failed expansion vessel, rather than normal behaviour. Track the gauge over a day or two, and if there’s a steady decline, a Gas Safe registered engineer is who to call — check the Gas Safe Register to find one.
This distinction trips a lot of homeowners up. A rising gauge feels alarming, and a falling one can go unnoticed for days. In fact, it’s usually the other way round in terms of what actually matters.
Pressure rising when the heating is on: normal
When the boiler fires, the water in the system heats up and expands. A correctly working expansion vessel absorbs that extra volume. So you’ll typically see the gauge climb a few tenths of a bar above its cold reading. Once the heating switches off and the water cools, the vessel releases what it took up. The pressure eases back down towards the original cold figure. This rise-and-fall is expected and nothing to worry about. It just needs to return to roughly the same baseline each time the system goes fully cold again.
Pressure dropping when the heating is off: the one to watch
A genuine problem shows up as pressure loss that persists while the boiler is cold, and the heating has been off for a while. In that state there’s no thermal expansion to account for it. So any decline points to water leaving the circuit somewhere — a corroded pipe, a loose joint, or a faulty radiator valve are common causes. It can also mean the expansion vessel has lost its own charge. Either the internal bladder has failed, or the air pressure inside it has leaked away. In that case, the system pressure drifts down over time, even without a leak elsewhere. Either way, a cold-system pressure drop is a sign something needs looking at, not normal behaviour to shrug off.
How to check this yourself over a day
- First thing in the morning, with the heating off and the boiler fully cold, note the exact reading on the gauge.
- Run the heating for at least half an hour to an hour, then switch it off.
- Wait until the system is properly cold again — this can take another hour or two depending on the size of the property.
- Note the gauge again and compare it with your morning reading.
Say the reading is down from 1.3 bar to 1.0 bar or less. If the same pattern repeats over consecutive days, you have a measurable loss worth investigating. A single lower reading on one day isn’t necessarily cause for alarm. Gauges aren’t perfectly precise. However, a repeating pattern across several mornings tells you something is genuinely wrong.
When to get a professional pressure test
Three things are reasons to call in a heating engineer: a steady day-on-day decline, a cold pressure that’s fallen below your manual’s range, or a gauge that never returns to the same baseline. An engineer will typically isolate sections of the circuit, to measure loss under controlled conditions. They’ll also check the expansion vessel’s charge via its Schrader valve. Tracing visible pipework and radiator joints for weeping or corrosion is part of the same process. Our guide to boiler losing pressure but no leak covers this in more depth, and is worth reading if you’ve checked everything visible already. Thames Water manages the mains supply, but a leak inside your own sealed circuit is a separate matter for a plumber, not the water company.
Catching this pattern early avoids unnecessary water loss and keeps the boiler running efficiently. It also cuts the risk of damage to nearby plaster or flooring. If your gauge has shown the drop-when-cold pattern above, a Gas Safe registered engineer is the right person to test it — the Gas Safe Register can help you find one nearby.
Frequently Asked Questions
Why does my boiler lose pressure when the heating is off? This is the pattern worth paying attention to. With the heating off and the system cold, there’s no thermal expansion happening. So a genuine drop points to water leaving the system somewhere — a leak or a failed expansion vessel — rather than normal behaviour.
Is it normal for boiler pressure to rise when the heating is on? Yes. As the water heats it expands, and the expansion vessel absorbs that extra volume. So a small rise of a few tenths of a bar while the heating is running is expected. It should settle back to the cold reading once the system cools. If it stays high instead, that’s covered in our guide to boiler pressure that’s too high.
Should my boiler pressure stay exactly the same overnight? With the heating off and the system cold, yes. The gauge should read roughly the same in the morning as it did the night before. A steady day-on-day decline is the sign to get it checked, rather than something to wait out.
Book a pressure check
If your own reading matches the pattern above, it’s worth a proper test rather than more guessing — a Gas Safe registered engineer is who to call for that (check the Gas Safe Register). If the pressure loss turns out to be a leak in the pipework rather than the boiler, get in touch online and our leak detection & repair work can take it from there. We cover Staines-upon-Thames, Uxbridge and Maidenhead, plus Slough and the surrounding towns.
