Understanding boiler pressure (sealed systems)
Quick answer
Boiler pressure is the water pressure inside a sealed central heating system, shown on a gauge or digital display. Recording it when cold and again after the heating has run gives useful clues if you’re dealing with lockouts, a leak, or discharge. This guide explains what the readings can suggest — for guidance on rising or falling pressure specifically, see our dedicated guides.
Safety first
- Never remove boiler covers or attempt any internal repair — this is work for a Gas Safe registered engineer.
- If you smell gas or suspect carbon monoxide, leave the property and call 0800 111 999.
- Avoid repeatedly topping up or discharging water before you understand the pressure pattern, as this can mask the real fault.
Full checklist: Safety guidance
What you may notice
- A pressure reading roughly in the range shown on your gauge or manual when cold — is generally considered normal, though this varies by system — always check your manual rather than assuming a figure.
- Pressure that rises noticeably once the heating has been running — can suggest expansion-related behaviour as the water heats up.
- Pressure that falls steadily over hours or days, even when cold — may point to a leak, a discharge event, or an issue with the filling loop.
- Pressure at zero, or the boiler refusing to operate — often means the system needs attention before it can run — see our guide on boiler pressure dropping.
These are possible explanations, not a confirmed diagnosis. An engineer can confirm the exact cause on inspection.
Safe checks to make, in order
- Check and note the pressure when the system is cold. Do this with the heating off and the system fully cooled, so you have a reliable baseline.
- Run the heating for a while, then check and note the pressure again. Comparing the two readings helps show whether any change is linked to heat (expansion) or is a steady loss.
- Look outside for any sign of discharge from the pressure relief pipe. This is usually a copper pipe low on an outside wall. Dripping or staining can be a useful clue.
- Compare your readings against your manufacturer’s guidance, if you have your manual to hand. Recommended pressure ranges vary between boiler makes and models.
What you can do safely
- Recording the pressure when the system is cold.
- Checking the pressure again after the heating has run for a while.
- Looking outside for any discharge from the pressure relief pipe.
- Using those readings to judge whether any problem is steady, hot-only, or occasional.
When to call a professional
- Pressure behaves unpredictably or changes rapidly with no clear pattern.
- There’s water discharging outside from the pressure relief pipe.
- You need to top up frequently, or the boiler is locking out.
What to tell the engineer
- Your cold pressure reading and your reading after the heating has run.
- Any discharge noticed from the outside pressure relief pipe.
- Roughly how often, if at all, you’ve needed to top up.
- Your boiler make and model, if you have it to hand.
FAQs
What’s a normal boiler pressure reading?
This depends on your specific boiler and system, so it’s best to check the marked range on your gauge or your manual. An engineer can also confirm the correct pressure for your installation.
Why does pressure rise when the heating is on?
Water expands slightly as it heats up, which can push the pressure reading up a little in a sealed system. A small rise is often normal, but a sharp or repeated rise is worth investigating.
How do I top up the pressure myself?
Many combi boilers have a filling loop with clear instructions in the manual. If you’re not confident, or you find yourself topping up often, it’s worth getting a professional to check for an underlying cause instead.
What’s the difference between a sealed system and an open-vented system?
A sealed system is a closed loop with a pressure gauge, common in most modern boiler installations. An open-vented system uses a header tank instead and doesn’t use the same pressure gauge readings — if you’re not sure which you have, your manual or an engineer can confirm.
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