What to do if your boiler loses pressure

If you’re having problems with your boiler — like the dreaded 'no hot water' — loss of water pressure might be to blame. The good news is that low pressure is relatively easy to diagnose and, in some cases, you can put it right yourself. To help you stay calm under (low) pressure, here’s our handy guide to the most common boiler pressure problems, and their fixes.

1. What is boiler water pressure?

Your boiler heats cold water that flows around a circuit of pipes and radiators through your home. For it to work as efficiently as possible, your water pressure needs to be stable. Pressure in most modern combination boilers is maintained by a 'filling loop', which connects to your cold water pipe. We’ll come back to this!

How to bleed a radiator

2. What causes a drop in pressure?

Gas engineers can help with boiler maintenance and boiler servicing

There are two leading culprits when it comes to low water pressure:

Leaks

You may have a leak somewhere in your boiler system. It may only be a tiny leak but, if it’s gone unnoticed for some time, it could lead to a gradual drop in pressure. Take a look around your home for signs of damp patches, around pipes, radiators and the boiler itself. You should never look for leaks inside your boiler, though. Only a Gas Safe engineer should open this. If you do find a leak, or need to top up the water pressure regularly, give one of our engineers a call and they’ll be on-hand to help.

Bleeding radiators

If you bled your radiators recently, you may have lost some pressure. That’s because, when you bleed a radiator, air is released, which lowers the pressure in your boiler system.

3. How can I check my boiler pressure?

Usually, on the front of your boiler, you’ll find a water pressure gauge. If you have a hydraulic pressure gauge, you’ll see low and high pressure indicated by red sections on the dial. The position set when the boiler was installed is sometimes shown by a red indicator needle.  On most digital gauges, you’ll see a flashing pressure reading, if there’s a low (or high) pressure warning.

If your boiler pressure reads less than 1 bar, it’s possible that you might have lost water from the system, which needs to be replaced. This is where that filling loop (or filling link) comes in again!

If the pressure gauge on your boiler indicates high pressure (above 2.75 bar), then you may need to bleed a radiator to bring the level down to around 1.5 bar. This can be tricky and, occasionally, dangerous; due to high water temperatures. We recommend an engineer take a look if you are experiencing high pressure – this could be a result of a fault inside the boiler and may need an engineer visit to rectify. Additionally, reducing pressure by bleeding a radiator may take some time, and you will need some method of catching the water to prevent damage to wall and or floor coverings. Above all, be careful. This water is very hot, so maybe get an engineer to do it.

What to do if your boiler is leaking

When at this point there's not enough pressure

How to fix cold radiators

When the radiators are cold this is normal pressure

What to do if you have no hot water from your boiler

When the radiators are hot this is normal pressure

What to do if you have no heating or cold radiators

When at this point there's too much pressure

4. Can I fix low boiler pressure myself?

What to do if your boiler has no pressure

Boiler pressure systems differ, so it’s always best to check your manual, to see if you can re-pressurise it yourself. Your boiler may also have instructions on the rear of the control panel. (If you need any tools to remove this panel, don’t touch it! Get in touch and we’ll have a Gas Safe engineer do this for you).

Top tip!

Visit your boiler brand’s website, to see if they have helpful tutorials and videos about topping up pressure on their systems.

Re-pressurising your boiler means allowing more water to enter the system, from the water mains supply, via the filling loop. The two main types of filling loop are 'built in' and 'external'.

Filling loops may vary in design, but here’s the basic re-pressurising process:

  • Switch off and allow your boiler to cool
  • Double-check that both ends of the filling loop are securely attached
  • Open both valves, to allow cold mains water into the system (you should hear it)
  • Wait for the pressure gauge to read 1.5 bar
  • Close both valves, one after the other
  • Switch the boiler back on and, if needed, press the reset button
  • Undo both ends of the filling loop and remove. Be careful to catch any water spillage, and keep the filling loop in a safe place!

5. Do I need to call a Gas Safe engineer?

If you’re in any doubt about how to re-pressurise your boiler system, or you’ve already done so according to the user manual but are continuing to have pressure problems, our expert engineers will diagnose and fix the problem.

If you’re already a HomeCare customer, we’ve got you covered! Book an engineer anytime from your online account.

You don’t need to be a British Gas customer to book one-off boiler repairs though, and you’ll always get a fixed price.

How to check boiler pressure

And if you’re not a British Gas customer, you can book one-off boiler repairs, and you’ll always get a fixed price[1]

[1] Terms and conditions apply. British Gas is a trading name of British Gas Services Limited which is authorised and regulated by the Financial Conduct Authority. Registered in England and Wales (No. 03141243). Registered office: Millstream, Maidenhead Road, Windsor, Berkshire SL4 5GD.