Quick answer: Most boilers should read between 1 and 1.5 bar when cold. If it's lower, top it up with the filling loop, then restart the boiler. If you need to top up every week or so, there's probably a leak.
This guide is for sealed systems, which covers most combi and system boilers. If you have a water tank in the loft feeding your heating, your system works differently and this pressure gauge won't apply.
What's the right pressure?
Look at the gauge on the front of your boiler, or on the display, when the heating has been off for a while and the system is cold. On most boilers it should be between 1 and 1.5 bar. It will rise a little when the heating is on, which is normal. Your user manual gives the exact range for your boiler.
- Below 1 bar: low. The boiler may stop working or show a fault code.
- Above about 2.5 bar when cold: high. See the section on high pressure below.
How to top up the pressure
Most boilers have a filling loop: usually a silver braided hose with a valve at each end, under the boiler. Some newer boilers have a built-in lever or a key instead, so check your manual if you can't see a hose.
- Turn the heating off and let the system cool down.
- Make sure both ends of the filling loop are firmly attached.
- Open the valves slowly, one and then the other. You'll hear water running into the system.
- Watch the gauge. When it reaches about 1.2 to 1.5 bar, close both valves.
- If your filling loop is designed to be removed, take it off afterwards, with a towel underneath to catch drips.
- Turn the boiler back on. If it shows a fault, reset it once as your manual describes.
Why does my boiler pressure keep dropping?
Topping up once in a while is normal. Needing to do it every week or so isn't. The most common reasons are:
- You've just bled your radiators. Letting the air out lowers the pressure. This is a one-off, so top up and carry on. How to bleed a radiator.
- A small leak. Look for damp patches, rust or drips around radiator valves, exposed pipes and under the boiler.
- The pressure relief valve. If the pressure went too high, this valve lets water out, usually through a copper pipe that runs outside. A drip from that pipe means something needs looking at.
- A problem inside the boiler, such as the expansion vessel. This needs an engineer.
If you can't find a leak and the pressure keeps falling, book a Gas Safe registered engineer.
What if the pressure is too high?
First, check the filling loop valves are fully closed. If the pressure is only slightly high, bleeding a radiator will lower it. If it stays above about 2.5 bar when the system is cold, or water is coming out of the relief pipe outside, call an engineer.
Never take the casing off your boiler or try to repair gas parts yourself. That work must be done by a Gas Safe registered engineer. With boiler cover, your provider sends one for you. Compare boiler cover.
Have boiler cover?
Call your provider before paying an engineer yourself. A repair like this is exactly what cover is for.
No cover yet? A new plan won't pay for a fault you already have, because problems that exist when you join aren't covered and there's a waiting period at the start. It can protect you next time.
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Low-pressure fault codes: Vaillant F22 · Ideal F1 · Baxi E119
Last updated 30 October 2026