Boiler Pressure Rising To 3 bar: Here’s Why and How to Fix

You should keep a regular check on your combi boiler to prevent issues later on.  One thing to look at is the boiler pressure gauge.  In the green zone or around 1.3 – 1.5 bar is good.  Sometimes the pressure can get too high, and keep rising to 3 bar.  Although not catastrophic, it can indicate a bigger problem about to happen.

What can cause a combi boiler pressure to rise to 3 bar and above?

There are many reasons why you might see the boiler pressure rising to 3 bar and above.

1. Too much water in the system.
2. Valves failure.
3. Expansion vessel has lost charge.
4. Undersized expansion vessel.
5. Failing boiler parts.

Is 3 bar boiler pressure too high?

High boiler pressure rising to 3 bar, although not dangerous can indicate another problem that may occur. 

If the pressure gets too high, the boiler has a built-in safety feature called the pressure release valve (PRV) to stop it from damaging the boiler.  When the pressure goes down, the boiler should fire up again as normal.

The PRV is there as a failsafe in case the pressure within the combi boiler system exceeds 3 bar. In which case it will discharge before any damage is done to the internal workings of the boiler.

What is normal boiler pressure?

The ideal boiler pressure is between 1 to 1.5 bars when the heating is off.  The pressure will rise anywhere up to 2.5 bars with the heating on and this is normal for most boilers. The manufacturer’s user manual will also state the ideal pressure and they usually mark the safe zone on the pressure gauge in green.

How to check boiler pressure

First, locate the pressure gauge on the front of a combi boiler. It is a round dial usually found under the control panel. The dial shows the pressure reading in bars from 0 = no pressure, to 4 – very high pressure. The needle will point to the pressure reading both when the boiler is running and when it is idle.

Symptoms of high boiler pressure

If boiler pressure gets too high, it goes into the red zone and the boiler can develop faults due to the extra strain on the system.  Certain parts within the boiler become prone to cracks and seals start breaking, resulting in water leaks.  The boiler, if still running, becomes inefficient and has to work harder to get to the required temperature.

Boiler overflow pipe dripping outside house

A sign of pressure build-up is the boiler overflow pipe outside the house dripping water. The overflow pipe is usually made of copper and goes through the outside wall. It can leak when the pressure release valve it is connected to in the boiler is either faulty due to sludge buildup or is not fitted correctly.

How can boiler pressure rise to 3 bar?

1. Too much water in the system

When the pressure goes down in the central heating system for whatever reason, and you use the filling loop to re-pressurise it, the system can be overfilled, especially as you top up the water when the system is cold.

The filling valves may have been left open slightly by accident which lets water continue filling the system slowly.

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2. Valves have failed

The valves used for filling could themselves fail, even if you have shut the handles tight.  One of them could not be shutting down properly and the other letting water by which would go through the filling loop and pressurise the boiler.

3. Expansion vessel problem

Even after the pressure comes down on the boiler, if you find it goes back up again, this would indicate a problem with the expansion vessel inside the boiler.  It may need servicing by being charged up or even replacing and a gas safe registered engineer will need to look into it. 

The pipe that goes into the vessel can also become blocked leading to high boiler pressure and this should be checked too.

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4. Undersized expansion vessel

A large house with a big heating system usually requires a bigger expansion vessel.  Normally, those within boilers don’t have a very large capacity and so may be undersized for the house. 

You may need to install an additional EV outside of the boiler otherwise the high pressure problem can reoccur.

5. Failing boiler parts

Although the whole system is connected, the main boiler parts that closely relate to boiler pressure that could have become faulty and cause high pressure are the pressure release valve, expansion vessel, diverter valve and filling loop. 

The engineer will check these closely when called out to this fault finding job.

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What should boiler pressure be when heating is on?

The pressure gauge on a boiler will show anywhere between 1 and 2.5 bar when the heating is turned on. The upper limit will be shown on the green zone of the pressure gauge . It is important the boiler doesn’t stay over this limit. Each manufacturer sets its own upper limit but most go up to 2.5 bar.

What should boiler pressure be when heating is off?

Boiler pressure between 0.75 and 1.5 bar is correct when central heating is turned off. Boilers have a green zone marked on the pressure gauge where the needle should stay within at all times. Don’t let it go below this level. The user manual for the boiler will tell you exactly what they consider low pressure.

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How to reduce boiler pressure

The following are the simple steps you can take to reduce your boiler pressure:

Use the magnetic filter on your boiler system

• Close the filling loop properly so no more water enters into the heating system
• Check the gauge to see if it’s showing pressure 2 bar or higher
• Turn off the boiler completely and let the system cool down
• Locate the valve at the bottom of the filter, and place a bucket under it
• Open the valve to let water out which will reduce the pressure on the gauge
• When pressure goes down to 1 bar, close the valve

The above method is the quickest way to get water out of the system.  But not all boilers have a filter with them so the next method is also quick to do.  You will probably need a spanner to open the valve:

Use the drain off valve

1. Close the filling loop properly so no more water enters into the heating system
2. Check the gauge to see if it’s showing pressure 2 bar or higher
3. Turn off the boiler completely and let the system cool down
4. Locate the drain off valve which is usually fitted on the pipework near a radiator at the lower part of the heating system, usually downstairs in a hallway near a door.
5. Put a container underneath the drain off valve ready to collect water
6. Undo the valve (might need a spanner) on the drain off to release water
7. When pressure reaches 1 bar, close the valve

The next method is mostly used to get rid of trapped air in the radiators but can also be used to drop the boiler pressure.  It is a slower method overall though

Bleed a radiator to reduce boiler pressure

• Close the filling loop properly so no more water enters into the heating system
• Check the gauge to see if it’s showing pressure 2 bar or higher
• Turn off the boiler completely and let the system cool down
• Find the most convenient radiator to work with and place a container underneath the bleed valve
• Open the valve with a radiator key but resist the urge to open the valve too much as it can actually shoot out with all the water in the system, causing a mess
• When pressure goes down to 1 bar, close the valve

When the boiler pressure has come down and the heating is off

With the system cold, you should be aiming for 1bar pressure.  The pressure may rise a bit as the system heats up and the water expands but it should only go up a small amount, to 1.3 or 1.4bar. 

If it carries on rising to 2 or 3 bar, that indicates a problem with the expansion vessel, which may need servicing.

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