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Why is my boiler pressure too high?
If the gauge is past 2.5 bar with the heating on, or above 2 with it cold, there's too much water in the system or something is letting it build up. Here's why, and how to bring it down.
- 2.5+bar with the heating on is too high
- 3bar: where the relief valve opens
- 1.2bar: what to bring it back to, cold
3 barrelief valve opens
High pressure is less common than low pressure, and usually easy to fix. It most often happens after the system has been topped up a bit too far.
If it keeps coming back after you've brought it down, there's a part letting it happen, and that needs an engineer.
What's normal
Read it cold first.
- Heating cold: between 1.0 and 1.5 bar
- Heating on: up to about 2 bar
The pressure rises as the water heats and expands. A little rise is normal. If it climbs past 2.5 bar, or water starts dripping from the copper pipe on the outside wall, the system is working too hard.
Cold, then hot
Why it goes too high
One of four things, most of the time.
It was overfilled
Too much water went in when it was last topped up. You can sort this yourself.
The filling loop is letting water in
If the valves aren't fully closed, or one is worn, mains water keeps creeping in. Check both are tight. If the pressure still rises, the loop needs replacing.
The expansion vessel has lost its charge
With no air cushion to take up the expansion, the pressure spikes every time the heating runs. It needs recharging or replacing by an engineer.
A worn part inside a combi
In some combis a worn internal part can let mains water pass into the heating. It's less common, and an engineer can test for it.
Expansion vessel
How to bring it down
If it was overfilled, you can do this with a radiator key and an old towel.
Heating off
Switch the boiler off and let the system cool completely.
Pick a radiator
Hold a towel under the bleed valve at the top corner of a radiator.
Let a little out
Open the valve slowly with the key. Air hisses, then water trickles out.
Stop at 1.2 bar
Watch the boiler gauge and close the valve when it reads about 1.2 bar.
Is it dangerous?
The boiler won't burst. The relief valve opens at 3 bar and lets water out. Running high for weeks does strain seals, valves and the pump, so it's worth sorting.
If it climbs back up
Then there's a fault.
If the pressure rises again within hours or days of bringing it down, the filling loop, the expansion vessel or a part inside the boiler needs looking at.
A repair visit is £120 inc VAT. If we can fix it on the visit, that's all you pay. If a part is needed, we tell you the price before fitting it.
Back to 1.2 bar
Which one sounds like you?
- Just topped it upBleed a radiator
Let a little water out.
- Rises every time the heating runsExpansion vessel
Recharge or replace.
- Rises with the heating offFilling loop
A valve letting by.
- Too low, not too highTop it up
Our losing pressure guide.
Common questions
What should boiler pressure be?
Between 1.0 and 1.5 bar with the heating cold, rising to about 2 bar when it's on.
Can high boiler pressure cause a leak?
It can. The relief valve lets water out at 3 bar, and long periods at high pressure strain seals and valves.
Why is water coming out of the pipe outside?
That's the relief valve doing its job. The pressure has reached about 3 bar. Bring it down, then find out why it got there.
Is it safe to bleed a radiator to lower the pressure?
Yes, with the heating off and cool. Let water out slowly and stop at about 1.2 bar.
The pressure goes up even with the heating off. Why?
Mains water is getting in, usually through a filling loop valve that isn't fully closed or is worn.
How much does it cost to fix?
A repair visit is £120 inc VAT. If we fix it on the visit, that's all you pay. Parts are priced before we fit them.
Related pages
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