Your boiler pressure should be between 1 and 1.5 bar when the heating is off, rising to roughly 1.5 to 2 bar while the heating is running. Under 1 bar is too low and over 2.5 bar is too high. Those figures apply to almost every modern combi and system boiler sold in the UK.
We’re Clever Energy Boilers, Gas Safe registered (no. 554138) and accredited Worcester Bosch installers based in Bingley. We’ve installed more than 25,000 heating systems across Yorkshire, the North West and the Midlands, and checking pressure is one of the first things our engineers do on every visit.
What is normal boiler pressure?
Normal is 1 to 1.5 bar cold and up to about 2 bar hot. Here’s how to read your gauge at a glance.
| Reading | Heating off | Heating on | What to do |
|---|---|---|---|
| Under 1 bar | Too low | Too low | Top up the pressure |
| 1 to 1.5 bar | Normal | A little low but fine | Nothing |
| 1.5 to 2 bar | Slightly high | Normal | Nothing, unless it keeps rising |
| 2 to 2.5 bar | Too high | On the high side | Keep an eye on it |
| Over 2.5 bar | Too high | Too high | Bleed a radiator or call an engineer |
Your boiler manual is the final word, as a few models suggest slightly different figures.
How do I read my boiler pressure gauge?
Most boilers have either a dial gauge or a digital display. A dial gauge usually sits on the front or underneath the boiler, with a needle and a green zone that marks the safe range. Some also have a red zone for too high or too low.
Digital displays show the pressure as a number, often with “bar” next to it. On some boilers you’ll need to press a button or scroll through a menu to see it. If you can’t find it, check the manual for your model.
Why does boiler pressure go up when the heating is on?
Water expands as it heats up, and in a sealed system that pushes the pressure up. A rise of around 0.5 bar is normal. Your boiler’s expansion vessel absorbs most of that extra volume, so the needle shouldn’t climb dramatically.
If pressure jumps well over 2.5 bar once the radiators are hot and then drops low when they cool, the expansion vessel has probably lost its charge. That needs a Gas Safe engineer.
Is boiler pressure different for combi, system and regular boilers?
Combi and system boilers are sealed and pressurised, so they work in the 1 to 1.5 bar range above. Regular (heat-only) boilers are different. They’re fed by a cold water tank in the loft, so the pressure depends on the height of that tank and there’s usually no gauge to watch.
Our guide to the different types of boiler explains how each one works.
What pressure should Worcester, Ideal, Vaillant and Glow-worm boilers be?
All four brands recommend the same general range: around 1 to 1.5 bar cold. The way they show it differs.
- Worcester Bosch: most models have a dial gauge underneath or a reading on the display. As accredited Worcester Bosch installers, we set pressure correctly on every boiler we fit.
- Ideal: Logic and Vogue boilers show pressure on a gauge or the display, and flag low pressure with an F1 code. See our Ideal boiler installation page.
- Vaillant: ecoTEC models show pressure on the display and use F22 for low pressure. See Vaillant boiler installation.
- Glow-worm: Glow-worm is Vaillant’s sister brand, so the readings and codes are similar.
What if my boiler pressure is too low?
Top it up through the filling loop until the gauge reads 1 to 1.5 bar. Our guide on how to repressurise a boiler takes you through it. If it keeps dropping, read why a boiler keeps losing pressure.
What if my boiler pressure is too high?
Bleeding a radiator usually brings it down. If it climbs back up, the filling loop may be letting water in or the expansion vessel may need attention. Our post on high boiler pressure has the full checklist.
Does pressure that won’t settle mean I need a new boiler?
On a boiler over 8 years old, pressure that swings up and down usually points to worn parts like the expansion vessel or pressure relief valve. Fixing one often leads to another, so replacing the boiler tends to work out cheaper over time.
We offer boiler replacement at a fixed price. Ask us about the long, market-leading warranties you can add through Clever Energy.
Boiler pressure FAQs
Is 2 bar boiler pressure too high?
Not with the heating on. 2 bar is normal for a hot system. If it reads 2 bar when the boiler is cold, it’s a little high and you could bleed a radiator to bring it down.
Is 0.5 bar boiler pressure too low?
Yes. Most boilers lock out somewhere below 1 bar, so top it up to between 1 and 1.5 bar.
What should combi boiler pressure be?
The same as any sealed system: 1 to 1.5 bar cold, up to about 2 bar hot.
Why is my boiler pressure gauge reading zero?
Either the system has lost almost all its water through a leak or the PRV, or the gauge itself has failed. Try topping up. If the needle doesn’t move while you can hear water going in, the gauge is faulty.
Should boiler pressure drop when the heating is off?
A small drop as the water cools is normal. A steady fall day after day isn’t, and usually means there’s a leak.
