Have you been having problems with low boiler pressure? Most people take their boiler for granted. Each year it comes around to autumn, thousands of central heating systems are switched on for heating for the first time in months – and nothing happens. Speak to any Gas Safe certified plumbing engineer, and they will describe how busy it gets at “switch-on time”.

An obvious solution to this potential headache is an annual service in the summer months, which can catch, and deal with, many of the problems that would otherwise prevent the boiler from heating our homes.

One such problem that could be prevented with regular service is low pressure. For water to move around the system of the boiler, there needs to be a set amount of pressure pushing it through.

When this pressure drops, the water cannot flow through adequately, and you don’t get your hot water. The consequences of low pressure are a matter of inconvenience rather than any safety concern, and the solutions are relatively simple, so don’t panic!

Here we offer a detailed guide to low pressure in your boiler and what work may need completing.

low boiler pressure taps

What is boiler pressure?

An appropriate level of pressure in your boiler is needed to keep the water flowing effectively around the house. It’s the force that propels hot water through your boiler system. It is totally separate from your water pressure, which pushes water through your pipes to your taps.

If your boiler pressure is low, water will still come out of your faucets – it will just be cold water. Your boiler needs to push the heated water into the household system, and it needs pressure to do this.

Problems with pressure

Boiler pressure can be too high, as well as too low. High pressure will put strain on the system and eventually result in a breakdown. If the pressure gets too high, the boiler locks down to prevent harm and switches itself off. Low boiler pressure is a more likely consequence of boiler malfunction and will need intervention from an engineer eventually.

Checking your pressure

You can check the pressure of your boiler by looking at the small dial on the front of the system. It might be under the front flap panel, where the reset controls will also be hidden. The pressure is measured in bars, and the dial will show a scale of 0 – 4. The standard operating pressures of systems tend to be similar, but you can check your own boiler’s manual to be 100% sure!

A guide to pressure readings

If the pressure is at 0, it means that your boiler is switched off or has locked down. You might believe your boiler is on and should be showing some reading on this dial. If this is the case, then you have a problem that requires an engineer.

If the boiler is on but cold, then the reading should be somewhere between 1 and 1.5. It may vary slightly from model to model, but the normal optimal pressure when cold is 1.3 bar.

As the boiler comes to life and begins to warm the water, you should see a gradual increase in this pressure. The changes in pressure should be gradual. If there are spikes, there might be an issue.

When the water is hot, the pressure will not be significantly different. You are looking for somewhere between a 0.3 and 0.5 change on the dial. Your boiler should have a pressure of about 1.8 and certainly no higher than 2.

The pressure increase comes from the expanding of water as it heats and so naturally fills up more space. If it goes above 2, you may have a problem with your expansion vessel or the pressure release valve, which will need the intervention from a plumber.

You should never attempt to maintain or repair the boiler itself! This should only be left to heating engineers. Read on to find out a few things you can try yourself, however!

boiler switch
Photo credit: Digital Buggu on Pexels

Why is my boiler losing pressure?

First, low boiler pressure is nothing to worry about. There is no danger, and the most pressing problem you face is that you are cold. You might find your heating is not pumping out enough to warm your house and your showers are lukewarm at best.

However, the longer a boiler runs at low pressure, the more strain is being put on the system and the more problems that might arise. Therefore, it is best to find out why the pressure is low and resolve this.

Boiler loses pressure when heating is off.

If your boiler loses pressure when the heating is off, there could be a leak in your relief valve, or there’s air in your system. There may also be a small leak somewhere else in your system that means the problems will have started slowly and may get worse over time.

Boiler loses pressure when heating is on.

Again, loss of pressure when the heating is on could be a problem with the pressure relief valve. However, it is possible that there is an issue with an expansion vessel, you have a leak, or your system needs bleeding.

Resolving low-pressure issues

If your boiler has stopped working it’s not the end of the world! It is possible that the solution is straightforward and won’t cost you a thing. Let’s take you through some steps, which you can do yourself, which might avoid an engineer call-out charge and repair bill. It must be said only to attempt the below if you’re 100% confident in what you’re doing.  If you’re in any doubt, call a heating engineer!

boiler pipes
Photo credit: jjawei cui on Pexels

Before trying any remedy involving the boiler

You only need to leak a small amount of water for your pressure to drop, so it might be a good idea to run an inspection of your pipes and bleed your radiators before you attempt any remedy.

  • Check the external pipework as well as those running into your radiators.
  • Feel around the area for any damp spots.
  • You may wish to call an engineer to deal with any leak you find or else use some compound sticky tape to seal the area and check to see if this makes a difference.
  • Then, using the key you would have received upon moving into your property, move around the radiators turning the valve at the end.
  • Air will escape for a short period, followed by water.
  • This bleeding of the radiators will remove air from the system that will be preventing the water from expanding, and so increasing water pressure.

First steps: Reset

If the dial continues to show one or lower, you may have an issue of low pressure. It might be that the pressure becomes so low that the boiler will lockout. This means that the boiler will stop working and will need to be reset.

If a simple reset does not work, there may be other interventions required. The reset button is usually on the front of the boiler; it may be hidden under the flap near the pressure dial.

It could be as simple as this. We are merely suggesting that you do the equivalent of switching it off and switching it back on again. Your boiler will then fire into life, and you can wait to see if the pressure dial moves to the optimal position of 1.3.

The next simple step: Add water

If this has not worked, then you may still be able to repressurise your boiler yourself by adding more water to the system.  Water may be lost over time, and eventually, most boilers will need a top-up. How often you need to add will depend on the age and condition of your heating system.

To add more water, you need to find the filling tube and use your mains water supply to top up your boiler. There are two types of filling tube: built-in and external. You should be able to work out where yours is by looking in your operating manual.

Step 1: Turn off the boiler.

You need the boiler switched off and the water cool before you start to add water to the system.

Step 2: Check the levels

There will be a sight glass near the filling tube. The appropriate level for the water is one inch off centre on this sight glass.

If, when you look at the sight glass, it is empty, it is essential to wait for a time for the boiler to be cold.

Step 3: Add water

There will be a water feed valve. Pull this back to allow water to flow into the boiler. You should keep a close watch on the water level in the sight glass – and when it is at the appropriate level, you should push the valve to the closed position.

Make sure that both ends of the filling tube are securely attached before opening the valve. Then, when you have completed all three steps, press reset on the boiler.

Finally: Call a Gas Safe Certified Engineer

Anything that requires you to remove the outer casing of the boiler is work that is not appropriate for you to be doing.

You should work with a registered plumber. Not all plumbers can work on gas appliances. They have to go through a Corgi certification process, or similar, to permit them to maintain or repair your gas appliance for you.

Although you might feel like changing a pressure valve or checking an expansion vessel sounds simple, there are inherent dangers when working on any gas appliance.

Therefore, once the simple checks and tests are done – the reset, bleeding the radiators, checking for leaks, refilling the system – and the pressure is still low, you need a Gas Certified Engineer to come and do the work.


Prevention is better than cure. Organising an annual service from June to September will help identify and rectify any potential issues with pressure. The engineer will check all the essential components as part of this service and will offer assurance that the heating will work at that moment of the first switch on in the autumn. You will avoid an emergency call out charge too, which is a bonus!

Then, if the boiler pressure is too low, you can go through the simple steps above to remedy the problem yourself.

  • Bleed the radiators
  • Check for leaks
  • Reset the boiler
  • Add water to the system

If at this point, the pressure is still low or the boiler keeps losing pressure, call for a Gas Safe Certified Engineer to work on the problem.

We hope you’ve found this article informative and helpful. If you’re interested in replacing your boiler, be sure to get in touch with us!


Charlie is a content writer at HomeSage. Having left Oxford University he joined HomeSage in 2020 to help create the UK's best resource in the home improvement sector. In his spare time he enjoys walking his dog Zeus and playing cricket.