boiler pressure too high

Boiler Pressure Too High? How to Reduce it the Right Way

Last updated on May 16th, 2024

Having high boiler pressure is a problem. There are a few different causes and a few different fixes.

When the pressure keeps rising too high it is usually followed by it dropping too low after the pressure relief valve lets the water out of the heating system.

I have been repairing boilers and heating systems for over a decade and have fixed many system pressure problems.

Why is The Pressure Too High?

Your boiler pressure is too high most likely for one of these reasons:

  • Somebody repressurised the boiler too much: Reduce pressure in the boiler by letting water out
  • The expansion vessel has lost air or the diaphragm inside is split: Recharge or replace the expansion vessel
  • The filling loop is not closed fully or damaged: Close the filling loop fully or replace it
  • Pinhole leak on plate heat exchanger on a combi boiler: Replace the plate heat exchanger

Pressure Too High When Heating On

The pressure will always rise when the heating is on.

It’s perfectly normal for it to rise from 1.5 bar to 2 bar when the heating is on. If it’s rising close to 3 bar then you have a problem.

All pressurised heating systems (combi boilers and system boilers) must have an expansion vessel.

An expansion vessel takes some of the expansion of the heating system when the heating is on using air and a rubber diaphragm inside.

If your pressure is too high when the heating is on, this will be an expansion vessel problem.

This means there is no air in the expansion vessel so the heating system is taking all the expansion of the heated water and causing the pressure to rise too high.

Pressure Too High Then Too Low

If your boiler pressure is too high then too low, this means the expansion vessel does not have any air in it which is causing the pressure to rise.

When the boiler pressure is too high the pressure relief valve (PRV) kicks in and does its job.

This means your boiler will keep losing pressure no matter how many times you repressurise it.

A PRV is a safety valve on all pressurised heating systems which safely lets the water outside when the boiler pressure is too high (usually 3 bar).

combi boiler pressure relief valve
PRV on a boiler

To fix this you will need to either recharge the expansion vessel using a pump or replace the expansion vessel if the diaphragm inside is split.

You can find out if it’s split by pushing the Schrader valve in on the expansion vessel, if water comes out that means it’s split and the expansion vessel needs to be replaced.

boiler expansion vessel valve testing
Checking expansion vessel Schrader valve

How to Reduce The Pressure

To reduce the pressure of your boiler you must let some water (or air) out of the heating system.

Bleed a Radiator

The easiest way to do this is to bleed some water out of a radiator using a radiator bleed key.

How to bleed a radiator:

  1. Put the bleed key on the square bleed valve screw
  2. Turn the bleed key anti-clockwise until the water starts coming out
  3. Let the water stream into a bucket or catch it with towels
  4. Watch the pressure gauge
  5. Close the bleed valve when the pressure is at 1.5 bar.

This will reduce the pressure in the system but it can get a bit messy sometimes.

How to Reduce it Without Bleeding Radiators

Here are a few different ways to reduce pressure without bleeding radiators:

  • Open drain off valve
  • Open pressure relief valve
  • Crack a nut
  • Magnetic filter

Open a Drain-Off Valve

Most central heating systems have at least one drain-off valve. You can find them on radiator valves, radiator pipes, and on the bottom of boilers. They are for draining water out of the central heating system.

Drain off valve on a Baxi boiler
Drain off valve on a Baxi boiler

You can connect a hose to a drain-off valve using a hose clamp and run the pipe to an outside drain or down the toilet or sink, then open up the square nut using an adjustable spanner until the water starts coming out.

If you don’t have a hose, you can open the drain-off valve, catch the water in a bucket or tub and keep emptying it.

This might take a while. Just keep checking the pressure gauge on the boiler until it has dropped to around 1 to 1.5 bar.

Open the Pressure Relief Valve

Another way to reduce boiler pressure without bleeding radiators is to open the pressure relief valve. They are on every central heating system that has a pressure gauge.

They are usually inside the boiler but could potentially be in the hot water cylinder cupboard.

If your pressure gauge is on the boiler then the pressure relief valve will be inside the boiler. They are typically brass with a red knob.

You should see it when you remove the casing from the boiler but you might need to check the manual or Google it to find the exact location on your model of boiler.

Once you’ve found it, you simply turn it anti-clockwise (or pull the lever on some boilers), and the pressure will drop very quickly.

boiler pressure relief valve pulled
Pressure relief valve handle being pulled

This is the easiest way to reduce boiler pressure without bleeding radiators and the most common way I do it when I’m on a job.

The only problem with opening the pressure relief valve is that once they’ve opened they might not close again properly.

They can get a little bit of debris stuck in them so they never quite close fully and you are left with a tiny slow leak coming out of the blow-off pipe outside.

This will tend to happen on dirty systems but 9 times out of 10 it’s fine. If it does happen, you need to replace the PRV.

Crack a Nut

Cracking a nut is another way to lower the pressure without bleeding radiators when the pressure rises too high. This is one of the messiest ways to do it but it is possible.

You can put towels and a bucket under a nut on the boiler, radiator valves, or anywhere on the central heating pipes.

Then crack the nut until water starts coming out and keep an eye on the pressure gauge.

How to reduce boiler pressure
Reducing pressure by cracking nut

Magnetic Filter

It is also possible to reduce the pressure via the central heating magnetic filter.

You can place a towel and bucket underneath the magnetic filter then open the nut on the bottom (if it has one) or you can open the bleed valve with a bleed key.

Let the water come out into the bucket and close it when the pressure gauge has dropped to around 1 to 1.5 bar when the system is cold.

Is high boiler pressure dangerous?

High boiler pressure is not dangerous. Pressurised heating systems have a pressure relief valve (PRV) in them so when the boiler pressure reaches 3 bar, the PRV will open and let the water out to stop the pressure from building up to a dangerous pressure.

If the PRV is not working or the PRV pipe is capped off, then high boiler pressure can potentially be dangerous but I have never come across a dangerously high-pressure situation in over a decade of being a heating engineer.


To sum things up, having your boiler pressure too high is quite common and most of the time it’s because there is no air (or not enough air) in the expansion vessel.

The most common fix by far in my experience is to recharge the expansion vessel. Recharging the expansion vessel is a simple job for a heating engineer but very tricky when doing it for the first time.

If you have let the pressure rise too much when repressurising the boiler, then reducing the pressure by letting water out of the system is an easy fix.

Feel free to ask me any questions in the comment section below and I’ll try my best to help.

Please share this post if you find it helpful.


  • Steven Reid

    I am a full-time plumber and Gas Safe registered engineer. I incorporated Housewarm Ltd. in 2011 to provide heating and plumbing services to homes in Newcastle upon Tyne, England. I now blog about what I've learned over the years to help DIYers and plumbers.

    View all posts
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