Is Your Boiler Pressure Too High?

Your heating system relies on a delicate balance of water and air.

High pressure wrecks this balance and puts unnecessary strain on your boiler’s internal joints.

Over time, this can lead to (expensive) boiler problems. 

How urgent these problems are depends partly on how high the pressure is.

Read on to learn:

• What normal boiler pressure is
• What causes high pressure
• How you can safely reduce pressure yourself
• When it is time to call a professional.

What normal boiler pressure is

Boilers’ pressure gauges show how much water pressure is in the system. These will be a physical dial with a needle or a digital display screen.

Here is how you should interpret your gauge’s reading:

• Under 1 bar: Your pressure is too low. The boiler may display a lockout error code and refuse to operate.
• 1 to 1.5 bar: This is the ideal range when your boiler is turned off and cold.
• 1.5 to 2 bar: This is the normal range when the central heating is active and water expands under heat.
• Above 2.5 bar: This is high pressure. If the needle creeps above 2.5 bar, you should take steps to reduce it.
• 3 bar and above: This is the critical red zone. At this point, safety controls will trigger, or your pressure relief valve will discharge water.

Signs of high boiler pressure

You may have high boiler pressure if you notice any of the following:

• Your pressure gauge is in the red zone. If digital reading or manual dial needle sits above 2.5 bar when the heating is off
• Water dripping from the discharge pipe outside. If the small 15mm copper pipe that runs through your external wall behind the boiler is releasing water, it means your pressure release valve (PRV) has activated
• Your boiler keeps cutting out 
• Banging noises from pipes. These boiler noises usually mean excess pressure is causing water to hammer through pipes and radiators

Common high pressure fault codes by boiler brand

Most modern boilers display a fault code when pressure is too high and lock out. The most common codes by brand are:

• Vaillant boilers (ecoTEC series): F.47: high water pressure fault. 
• Baxi boilers (200, 400, 600 series): 117: high system pressure
• Worcester Bosch boilers (Greenstar series): 117: the safety lockout code that often accompanies high pressure. 

(Note: codes may vary by model. Ideal boilers do not typically display a fault code for high pressure because the PRV will activate mechanically instead.)

If your boiler is displaying one of these codes, excess water pressure is the cause. Work through the steps below to reduce it.

If the fault code still returns after you have bled your radiators, call a Gas Safe engineer.

High pressure in modern boilers is usually not dangerous

High pressure in a modern or old UK boiler is not usually a safety hazard and will not cause it to explode.

Modern boilers are equipped with multiple safety controls. They also often have a dedicated pressure relief valve to automatically discharge excess water if pressure levels get too high.

Older boilers’ safety controls might be corroded or untested, which means they’re more likely to fail. Even so, the main risk here is damage to your heating system rather than danger to your household.

3 common causes of high boiler pressure

1. Too much water in the system

This is the most common cause of high pressure. It usually happens if you have recently topped up your system pressure with too much water.

2. A filling loop left slightly open

A filling loop is the silver braided hose underneath your boiler that connects your heating system to your mains water pipe. 

It has either one or two black taps. If you don’t close them both completely after repressurising your boiler, mains water will continue to leak into your system, causing the pressure to rise.

3. A ruptured expansion vessel membrane

Inside your boiler is an expansion vessel. It’s a sealed container divided into two chambers by a rubber diaphragm. One side holds compressed air whilst the other holds water from your heating system.

If the diaphragm wears away, or the vessel loses its air charge, there’s no space for the water to expand. This causes your system pressure to rise when the heating is on.

How to reduce your boiler pressure in 3 steps

You must always turn your boiler off and allow the system to cool down completely before attempting any maintenance.

Working on a hot boiler can cause severe scalding from heated water.

1. Check your filling loop taps

Before doing anything else, inspect the silver braided filling loop underneath your boiler casing.

What to check 

Ensure both plastic taps are turned completely off.

How to do it 

Verify that the black lever handles are positioned at a 90-degree angle to the pipework, which indicates they are closed. If a tap is left slightly open, tighten it firmly to stop the flow of mains water.

2. Bleed your radiators

If your filling loop is closed but the pressure is still too high, you must release some of the excess water from your system by bleeding your radiators.

What to check 

Confirm that your heating is switched off and your radiators are completely cold.

How to do it 

Hold a small container under the radiator’s bleed valve (it’s typically situated at one of the top ends). 

Insert a radiator bleed key and turn it slowly anti-clockwise. You will hear air hissing out first, followed by water. Let dirty water run into your container to actively reduce the system’s water volume.

What to do if there is a problem 

Check your boiler’s pressure gauge periodically. If you are working alone, bleed one radiator for 30 seconds, then check the gauge. 

If you have a helper, they can watch the gauge and tell you to close the valve once the pressure needle drops back to 1.2 bar. 

Once reached, tighten the radiator bleed screw clockwise, taking care not to overtighten and snap the pin.

3. Switch the boiler back on

Once the cold pressure is stable between 1 and 1.5 bar, you can test the system.

What to check Ensure all radiator bleed valves are tightly closed.

How to do it 

Turn your boiler power back on and set your central heating running. Monitor the pressure gauge closely as the system heats up. 

It is completely normal for the pressure to rise slightly (up to 2 bar) due to thermal expansion. But if it creeps back into the red zone towards 3 bar, you have an ongoing fault.

When to call a Gas Safe registered engineer

While releasing excess water is a straightforward DIY task, certain high-pressure symptoms require a certified professional.

You must book a Gas Safe engineer if:

• The pressure continues to rise constantly: If your filling loop is tightly closed and you have bled your radiators, but the pressure needle continues to climb when the system is cold, your filling loop valves are likely letting water past and need replacing.
• The pressure spikes only when the heating is on: If your pressure is perfect when cold but climbs into the red zone the moment your heating warms up, your internal expansion vessel has failed. An engineer must recharge the vessel or install a replacement.
• Your boiler is leaking water: If water is pooling underneath your boiler casing or dripping from the copper discharge pipe outside, a seal has failed or your pressure relief valve is stuck open. Only a certified technician can legally remove the boiler cover to replace these components.

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