Home / Guides / Boiler pressure
What pressure should my boiler be?
Between 1 and 1.5 bar on the gauge when the heating is cold, rising to about 2 bar when it's running. Here's how to read it, and what to do if it's too low or too high.
- 1 to 1.5bar on the gauge with the heating cold
- About 2bar is normal with the heating on
- 3bar: where the relief valve lets water out
1.2bar: a good cold reading
Your boiler's pressure is the pressure of the water inside your heating system. Too little and the boiler shuts itself down to protect itself. Too much and a safety valve starts letting water out.
Most of the time it's a two minute check you can do yourself, and often a two minute fix too.
What the gauge should read
Check it with the heating off and cold.
With the heating cold
Between 1.0 and 1.5 bar. This is the reading that matters, so check it before the heating comes on in the morning.
With the heating on
Up to about 2 bar is normal. Water expands as it heats, so the needle rises a little and drops back as the system cools.
Where to find the gauge
On the front of the boiler or just underneath it. Some are a dial with a green zone, some show the pressure on the boiler's display.
Reading the gauge
If the pressure is too low
Below 1 bar when cold.
Under 1 bar the boiler may stop working and show a fault, often below about 0.5 bar. Low pressure codes include F22 on a Vaillant, E118 on a Baxi and F1 on an Ideal.
- No heating or hot water
- The boiler won't fire up
- A low pressure fault code on the display
You can usually top it up yourself in a few minutes. The steps are below.
Checking for drips
Keeps dropping?
If you're topping up more than once a month, water is going somewhere. Our losing pressure guide covers the usual causes.
How to top up the pressure
Most combis have a filling loop underneath, a short silver braided hose with a valve at each end. Some Worcester boilers have a built-in filling key instead, and the boiler's manual shows where it is.
Heating off
Switch the heating off and let the system cool, so you get a true reading.
Find the loop
Find the filling loop or filling key under the boiler.
Open slowly
Open the valves slowly. You'll hear water going in. Watch the gauge.
Close at 1.2 to 1.5
Close both valves fully at 1.2 to 1.5 bar, then switch the boiler back on.
If the pressure is too high
Above 2.5 bar with the heating on.
If the gauge goes past 2.5 bar when the heating is on, or heads towards 3, the system is under strain. At 3 bar the relief valve opens and lets water out through a copper pipe on the outside wall.
- Too much water went in when it was last topped up
- The filling loop valves aren't fully closed, letting mains water in
- The expansion vessel has lost its air charge
If it was overfilled, bleeding a radiator brings it back down. If it keeps climbing after that, there's a fault an engineer needs to look at.
Letting a little out
Which one sounds like you?
- Below 1 barTop it up
Step by step, with the filling loop.
- Keeps droppingFind the leak
A repair visit finds where it's going.
- Above 2.5 barBring it down
Usually a radiator bleed.
- Pressure fine, still no heatCheck the fault code
What your display is telling you.
Common questions
What should my boiler pressure be?
Between 1.0 and 1.5 bar on the gauge when the heating is cold, rising to about 2 bar when it's running.
Is 2 bar too high?
Not with the heating on. If it reads 2 bar or more with the system cold, it's been overfilled and you can bring it down by bleeding a radiator.
Is it safe to top up the pressure myself?
Yes. Topping up with the filling loop is a normal homeowner job and the makers explain it in the manual. If you're not sure, call us and we'll talk you through it.
Why does the pressure go up when the heating is on?
Water expands as it heats. The expansion vessel takes up most of that, so a small rise is normal. A big rise points to a vessel that has lost its charge.
Is high boiler pressure dangerous?
The boiler won't burst. The relief valve opens at 3 bar and lets water out. Running high for a long time does strain seals and valves, so it's worth sorting.
How much does it cost to get pressure fixed?
A repair visit is £120 inc VAT. If the engineer can fix it on the visit, that's all you pay. If a part is needed, we tell you the price before fitting it.
Related pages
Not sure what you need? Give us a shout.
Book online in a couple of minutes or give the office a call, Mon to Fri, 9am to 5pm. We'll tell you what we think, and if the answer is to leave things as they are, we'll say that too.
Worcester Bosch Accredited Installer
Guarantees of up to 12 years on the boilers we fit. Gas Safe registered. Fife Trusted Trader. 400+ customer reviews.