What should the boiler pressure be when the heating is on?

What is the right boiler pressure?

Your boiler must be at just the right operating pressure for your heating to work properly.

If the boiler pressure is too high this may cause your boiler to shut down or ‘lockout’.

If the boiler keeps losing pressure it could mean there is a leak in the system.

But, what is the right amount of pressure for your boiler? 

We’ve answered the top FAQs to help you understand what pressure your boiler should be at and what to do if it gets too high or falls too low.

In this blog we will discuss:

  • How to check the boiler pressure 
  • What the boiler pressure should be when the heating is on and off
  • How to increase and reduce boiler pressure 
  • When to call in the professionals

How to check the boiler pressure

Checking the boiler pressure is really simple. All you need to do is look at the pressure gauge and make sure that the dial is in the correct zone. The pressure gauge looks like a round dial usually numbered from 0 to 4 with green and red zones.

Most homes in the UK have modern combi boilers with a pressure gauge. You will find the boiler pressure gauge on the front of the boiler or under the control panel cover. If you have an older boiler, you might find the pressure gauge underneath the unit near the pipes. 

On Baxi boilers, the pressure gauge is under the control panel cover. There are also some instructions printed on the back of the cover to explain the central heating system pressure. 

However, if you do not have a Baxi do not worry. Most boiler brands have similar pressure gauges and normal boiler pressure requirements. 

Struggling to find the pressure gauge? Take a look at your boiler instruction manual. So what is normal boiler pressure and what are the correct zones?

What is normal boiler pressure?

According to the Baxi control panel, normal operating boiler pressure is between 1 and 2.5 bars. This normal range is marked on the pressure gauge as the green zone.

However, the ideal point within the green zone is around 1.3. So the dial should normally be in the lower end of the green zone on the pressure gauge.  

If the system pressure rises above 2.5 or falls below 0.5, the dial will move into the red zone. This indicates that the boiler pressure is not normal and requires attention. 

What is considered normal boiler pressure also changes when the heating is turned on and turned off. So what is the difference?

boiler pressure instructions on the boiler

What should the boiler pressure be when the heating is on?

When the heating is turned on your boiler pressure should be between 1.5 and 2 bar.

Most boilers will highlight this area on the pressure gauge in green to help you see if the needle is pointing to the correct amount of pressure.

Some boilers will also have the critical zones below 1 and above 2.5 marked in red to help you identify any issues.

What should the boiler pressure be when the heating is off?

If your central heating system is turned off the boiler pressure should be between 1 and 1.5 bar. This means that the needle should still stay in the green area of the pressure gauge.

Depending on the make of your boiler, the recommended optimal working pressure by most manufacturers is 1.3 bar.

We would always advise checking your boiler manual to make sure your boiler is operating at the optimal working pressure.

Is boiler pressure the same as water pressure?

No. Boiler pressure and water pressure are separate and perform different functions. Boiler pressure is the pressure of the hot water inside your central heating system. Whereas, water pressure determines the force and speed at which water flows from the taps and shower. 

Water pressure can vary depending on several factors such as the condition of your pipework, how much water is being used in your own home and in the local area, and how far away your hoe id to the water storage tank.

running water from a tap

How do I know if my boiler has low pressure?

Keep an eye out for these signals that indicate the boiler has low pressure:

  • The pressure gauge is in the lower red zone
  • The heating will not turn on and radiators won’t heat up
  • There is no hot water from the taps or shower
  • The boiler shows an error code
  • The boiler goes into lockdown mode

What happens if boiler pressure is too low?

Fortunately, low boiler pressure is not a dangerous issue. It is normal for boilers to lose pressure over time and you might find that you need to top it up once or twice a year.

If the pressure drops to a critical low point below 0.5, the boiler may lock out or shutdown and leave you without heating. Of course this isn’t ideal in the winter! So make sure to check on the pressure once a month. 

If the boiler pressure continues to drop regularly, it will need resolving as it may mean there is a bigger problem to solve than simply topping up. Here are a couple of potential causes of severe low boiler pressure.

water leaking out of a radiator onto the floor

What causes low boiler pressure?

System leaks

Low boiler pressure occurs when there is not enough water in the central heating system. This often means that water is escaping from somewhere because of a leak in the boiler, pipework or radiators. If the pressure keeps dropping, take a look around your plumbing fixtures and check for any leaks, flaking paint or wallpaper, or signs of damp.

Recently bled radiators

If you have recently had to bleed your radiators, this can create a drop in boiler pressure because bleeding removes air from the heating system. The pressure may correct itself after bleeding, if not it will need to be topped up.  

Open pressure relief valve

The pressure relief valve (PRV) is a safety feature that controls the boiler pressure to ensure that is does not reach a critical level. If the pressure relief valve is accidentally left open, the pressure will quiclly drop. You should be able to find the relief valve at the top of the water heater. To close the valve, simply push the lever down to stop the water flow.

Open filling loop

The filling loop is the small mechanism that is used to top up the boiler pressure. If the filling loop is left open or broken, the boiler pressure will rise into the critical zone. When the pressure rises above 3 bar, the water will be released outside through the PRV pipe as the system is filled with too much water and pressure. This will cause the boiler to constantly lose pressure and drop to 0 bar when the water is drained.

Damaged expansion vessel

All pressurised central heating systems, including combi boilers, have an expansion vessel to control the boiler pressure when the water heats up and expands. If your boiler keeps losing pressure after topping it up, it may be that there is a problem with the expansion vessel. Expansion vessels can be recharged by pumping them up. However, if the inside of the vessel is split it will need to be replaced. 

Faulty boiler

If you have not discovered a leak, tried topping up the pressure and exhausted all other options, it could mean that there is a problem with the boiler in general. Mechanical issues can be common in older boilers, so it may be time to call a Gas-Safe heating engineer and consider replacing the boiler. 

How do I stop my boiler from losing pressure?

Properly maintaining your boiler and radiators will help prevent the boiler losing pressure. 

Here’s how:

  • Keep inhibitor levels correct to avoid corrosion and leaks
  • Use leak sealer to repair small leaks
  • Book an annual boiler service

Why is the boiler pressure higher when the heating is on?

When you turn the heating on the water in the pipes and radiators heats up and expands. This water expansion causes the pressure to increase in the whole system.

So, when the heating is on you should notice the boiler pressure on the gauge to increase slightly but not above 2 bar.

Boilers are built to quickly control these jumps in pressure via the pressure relief valve. This means that you can safely use your central heating and run hot water at the same time.

Your boiler pressure should not increase by more than 1 bar above it’s optimal working pressure when you turn the heating on.

Is high boiler pressure dangerous?

High boiler pressure is not dangerous. There are plenty of safety devices in the heating system designed to prevent high boiler pressure becoming a dangerous problem. For example, the pressure relief valve (PRV) and expansion vessel perform the task of stabilising the boiler pressure and preventing any system damage. 

When the boiler pressure gets too high, these mechanisms will shut the boiler down and remove excess water through the external PRV pipe to lower the pressure. That means the worst case scenario with high boiler pressure is that the system is put into lockdown mode, leaving you without central heating. 

Do you have a faulty boiler?

If you do see a jump up by 1 bar or your pressure needle rises into the red section above 2 bar this could signal a fault with the pressure relief valve or expansion vessel. These kinds of faults will require an authorised and regulated service engineer.

The price to repair these issues can vary depending on the extent of damage. Most plumbers have fixed prices per hour, but we would always advise that you check their credentials and ensure that they are regulated by the financial conduct authority.

How often should I check my boiler pressure?

  • Check your boiler pressure once a month to make sure it’s not too high or too low. 
  • You should also check the pressure after bleeding your radiators in case it has dropped below 1 bar.
  • It’s a good idea to keep an eye on the boiler pressure just before the heating season starts so that your boiler is in top shape for winter.

How to increase boiler pressure

If your boiler pressure gauge needle is below 1 bar or in the critical red zone you will need to repressurise the system.

To increase the boiler pressure you will need to add mains cold water to the system using the external filling loop. Make sure that the boiler is turned off and completely cooled down before you start.

You should only have to increase the boiler pressure once or twice a year.

But, if your boiler keeps losing pressure this could mean that the water from the system is leaking. Please always contact a Gas Safe engineer to repair the boiler if you spot a leak – you can find an engineer near you with Gas Safe’s – Find An Engineer Tool.

Ready to top up? Head to our full guide on how to fill a pressurised heating system.

If you have a combi boiler like a Worcester Bosch, you can easily repressurise the system using the keyless filling link. at the bottom of the boiler.

external filling loop

How to reduce boiler pressure

If you notice that your boiler pressure is too high, it may be that you have recently topped up the boiler pressure by too much.

It may also mean that you have forgotten to close the pressure relief valve after topping up or accidently knocked the valve and loosened it.

When air goes in, the pressure goes up. So make sure to check that they are tightly closed before you start to lower the pressure further.

After this, if you still need to reduce the boiler pressure you will need to bleed your radiators.

Take a look at our simple run through of how to bleed your radiators and towel rails.

Things to remember about boiler pressure

  • Heating on – boiler pressure between 1.5 and 2 bar
  • Heating off – boiler pressure between 1 and 1.5 bar
  • To increase boiler pressure – top up with water
  • To reduce boiler pressure – bleed your radiators
  • Check your boiler pressure once a month
  • Always use a registered engineer for any boiler repairs
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