Everything to know about your boiler pressure

19 September 2024

What should your boiler pressure be? Most of us don’t consider it until our heating systems play up. Then you start wondering what’s too little or too much. Here are some simple steps to ensure your boiler is at the correct pressure to work effectively.

EASY AS HACK

To stay on top of your boiler pressure, check your boiler reading or gauge once a month. But even once or twice a year in the summer/autumn could save you money and avoid a shivering shower or cold home in the winter.

AT A GLANCE

Your boiler pressure should be around 1.0 to 2.0 bar with the heating on or 1.0 to 1.5 bar when the heating’s off.
If the pressure is too high or low, turn off the boiler and allow it to cool.
Check the filling valves, then bleed the radiators if the pressure is too high.
Depending on your boiler type, open filling valves, levers or taps a little if the pressure is too low until the gauge rises.
Contact a Gas Safe registered gas engineer if your boiler has a leak or is still not operating correctly.

Everything to know about your boiler pressure

If your boiler pressure is too high or low, it can cause issues ranging from minimal heating and no hot water to leaks from burst pipes.^[1]

Knowing what your boiler pressure should be when it’s on and off helps you identify when it’s running too high or too low.^[2]

What is boiler pressure?

Boiler pressure describes the pressure of hot water circulating in your sealed central heating system and boiler.
Water pressure is different. It’s the pressure of water coming through your taps.^[3]

What should my boiler pressure be?

The correct boiler pressure will vary depending on whether your heating system is on or off.

Heating is on:

When the heating is on, your boiler pressure should be between 1.0 and 2.0 bar. 7 Each boiler manufacturer will recommend a different level, with the ideal pressure in the green zone.^[4] You should keep it below the upper-pressure limit of 2.75 and out of the red zone.^[5]

The boiler pressure increases when you turn your heating on because water expands as it heats, resulting in rising pressure within your central heating system.^[6]

Heating is off:

When the heating is off, your boiler pressure should be between 1 and 1.5 bar. The gauge’s needle should always be above your boiler manufacturer’s lower limit.^[7]

Boiler manufacturers specify the pressure for each model. So, the correct pressure depends on what’s in the manual – it’s vital to see what it says first^.[8]

How to check your boiler pressure

Most modern combi or system boilers will have a digital reading on the front, or a pressure gauge in a round dial at the front of the boiler or under a control panel.

Pressure gauges are likely to have a green and red zone to indicate the correct pressure.

Standard boilers with a water tank often in the loft may not have a pressure gauge. But they frequently have an expansion vessel or tank (it looks like a red metal balloon) with a pressure gauge on the pipework on the front.^[10]

Check your instruction manual, or google your boiler make and model, if you can’t find the pressure gauge. It should show its location and the boiler pressure the manufacturer recommends.

How to fix high boiler pressure

Look for signs that your boiler pressure has increased too much. Start by checking your pressure gauge.^[11]

Symptoms your boiler pressure is too high:

The pressure gauge is over 2.75 bar^[12]
Your central heating system has shut down
Water may be coming out of the safety discharge pipe
Filling valves are open or too loose^[13]

Reasons your boiler pressure is too high:

Your boiler pressure could be too high for the following reasons.

Repressurising has added too much water to the system
The pressure release valve (PRV) or expansion vessel is faulty^[14]
Excess water because the filling loop (the silver flexible pipe underneath your boiler) is open^[15]
The boiler is over 10 years old, and age is affecting its performance^[16]

Fixing your high boiler pressure

To assess what to do next, check all the above signs of high boiler pressure before turning off the heating and allowing your radiators to cool.^[17]

Make sure the filling valves are tightly secured, then proceed with bleeding your radiators to reduce the pressure. Use a radiator key to release excess pressure and a little water from the valve by twisting the key anti-clockwise.^[18]

Learn how to bleed a radiator.

Bleeding the radiators should help rebalance your boiler pressure and calm things down. Afterwards, you must re-check your pressure gauge to see whether it’s at the right level.^[19]

The gauge should be in the green zone and within your boiler’s pressure range. Then you can start operating the heating and hot water again. However, you should monitor the gauge over the next few weeks. If you haven’t resolved the issue or it worsens, contact a Gas Safe registered gas engineer.^[20]

You should also call an engineer if the pressure gauge is stuck or you don’t think it’s working^.[21]

How to fix low boiler pressure

Your boiler may have lost pressure if the gauge says it’s below 1.0 bar. But there are other signs that you could be losing water that needs replacing.^[22]

Symptoms your boiler pressure is too low:

No running hot water
Your radiators are cooling fast
A fallen pressure gauge^[23]

Reasons your boiler pressure is too low:

Someone has accidentally left the PRV open
A possible leak from the system, e.g. pipework or radiator valves^[24]
The open filling loop is open or broken, leading to water’s release^[25]

Fixing your low boiler pressure

You should turn off the boiler entirely and let it cool before fixing your low boiler pressure.^[26]

You’ll see a valve close to the red vessel and pressure gauge in a system or standard boiler. Open it a small amount and fill the system until the pressure gauge reaches 1.0 to 1.5 bar before closing it.^[27]

Underneath your combi boiler, you’ll find the filling loop valve with levers or taps on either side. Open the taps to allow mains cold water in, then wait until you reach 1.0 to 1.5 bar on the gauge. Close the valves one after the other.[^28]

Assuming no leaks or faults occur, you can turn the boiler on, let it run for a few hours, and stop it again to check the pressure.^[29]

How often should I check my boiler pressure?

It’s best practice to check your boiler pressure once a month to ensure it’s not too high or too low.^[30] Realistically, two or three times a year should be sufficient.

After bleeding your radiators, always check the pressure (in case it has dropped below 1.0).

Keep an eye on your boiler in the summer or autumn so it’s in good condition once the colder winter months arrive.^[31]

Ask about boiler cover when you get your home insurance quote today. Get a quote.

What is the rule of thumb for boiler pressure?

For combi, standard and system boilers, the rule of thumb for boiler pressure is between 1.0 and 1.5 bar.^[32]

What is the ideal boiler pressure?

The ideal boiler pressure is 1.0 to 1.5 bar because less than 1.0 bar on the pressure gauge means the pressure is too low. ^[33] More than 2.75 bar shows it’s too high.^[34]

Is a boiler with a high pressure dangerous?

A boiler with high pressure is not usually dangerous because your heating system’s pressure relief valve should kick in. However, high pressure can be hazardous if the relief valve isn’t working; for example, it’s stuck or faulty.^[35]

Does bleeding your radiators help with a high boiler pressure?

Yes, bleeding your radiators will help with a high boiler pressure by lowering the water pressure.^[36] Read: How to bleed a radiator.

How often should I increase my boiler pressure?

If you’re wondering how often you should increase your boiler pressure, it’s once or twice a year.^[37]

Why does your boiler pressure drop?

Your boiler pressure drops for several reasons – water leaks someone in the heating system, after bleeding radiators, and faults with the pressure relief valve or expansion vessel.^[38]