What to do if your boiler pressure is too low

14 September 2023 | Celia Topping

Most of the time, fixing a problem with your boiler’s pressure is quick and easy. And you don’t always need to call out an engineer. Follow our step-by-step guide on how to get your boiler pressure back on track.

What is boiler water pressure? 

Your boiler keeps your home warm by heating up water, and sending it through all of the pipes. It needs the right amount of pressure to get the heat to where it needs to go.

For a gas boiler to be efficient, it relies on a balance of water and air in its system. The right balance creates steady, stable pressure, which means there’s hot water and heat whenever you need it.

The pressure is usually managed by your boiler’s filling loop, which is connected to the cold water pipe. If the pressure (or water level) drops too low, the system has to work harder to keep things moving. This could mean higher heating bills for you. 

What happens if boiler pressure is too low?

Here are some of the things you might notice if your boiler has low pressure:

  • You have no hot water
  • Your radiators aren’t heating up properly
  • The dial on the front of your boiler is showing low pressure

Is low boiler pressure dangerous?

No. Low water pressure isn’t dangerous, and shouldn’t cause damage to your boiler. But it might mean you’ll pay more for your heating, so it’s worth getting sorted as soon as you can.

What should boiler pressure be? 

Your boiler pressure should be between 1 and 1.5 bar. But all boilers are different, so have a look at your manufacturer’s instructions to be sure.

To check your boiler’s pressure, open up its front panel so you can see the buttons and dials on the control board. Then follow these steps:

  1. Find the circular pressure gauge. This is usually marked with red and green zones.
  2. For most boilers, the needle should be in the green zone – between 1 and 1.5 bar. But it can vary, so double check your manual.
  3. If the needle is below 1 (in the red zone), your boiler might have lost pressure. When the pressure is low, your central heating system won’t be working efficiently.
  4. If the pressure is above 1-1.5 bar when the boiler’s off, you need to bring it down to around 1.5 bar. You can do this by bleeding the radiators. Or if in doubt, call a professional as the water might be very hot.

If you have a digital gauge, this makes it even easier to see the water pressure. The numbers will be flashing if the pressure is too low or too high.

What causes low boiler pressure?

There are usually 2 reasons your boiler loses pressure. Either you’ve just bled your radiators, or there’s a water leak somewhere in the system.

1. You’ve recently bled your radiators

A gas boiler relies on a balance of water and air in its system. When you bleed a radiator, you’re letting the air out – which can change that balance. This is the most common cause of a sudden drop in pressure.

The good news is it’s usually a quick fix. Check the ‘How to increase boiler pressure’ section below.

2. You’ve got a water leak

If there’s been a slow drop in pressure, or if it keeps dropping even after you’ve added more, you might have a leak.

Most of the time, it’s fairly easy to spot. Take a look around your home for damp patches near radiators, towel rails, or even under your boiler. If you do find a leak in your boiler, don’t try to fix it. Only a registered heating engineer should open your boiler.

If you can’t see any leaks, there might be one under your floorboards. In this case, you’ll also need to call in a professional.

How to increase boiler pressure

The easiest way is to add pressure to the system. It’s simple to do yourself, but it’s always best to check your boiler manual first. Or sometimes, there are instructions on the inside of your boiler panel. And remember, never open up a boiler – you should only be using the buttons and dials on the control panel.

To repressurise, boilers need more water added to the system – depending on which type of boiler you have. There are 2 main differences to look out for:

  • External filling loop boilers. Most boilers have this – it looks like a short braided metal hose, with connectors on either end. It connects the central heating system to the mains water supply.
  • Internal filling loop (keyed) boilers. Worcester boilers usually have these – look for a white plastic key, next to a small cube underneath your boiler.

How to repressurise a boiler with a filling loop

  1. Firstly, check the pressure gauge on the control panel. If the needle is in the red (below 1), you need to add pressure. 
  2. Always switch the boiler off when repressurising, so it can cool down first if it’s hot. 
  3. Find the filling loop hose below or next to your boiler. 
  4. Double check that both ends of the filling loop are securely attached. One end is connected to the central heating system and the other to the mains water supply.
  5. The filling loop handles should be at a 90° angle to the pipe.
  6. Turn both handles slowly. You’ll start to hear water flowing into the boiler. 
  7. While the boiler is filling with water, watch the pressure dial. When the needle is just under 1.5 bar, turn the handles so they’re closed again. 
  8. Switch the boiler back on, and press the “Reset" button if you need to. The boiler manual will tell you if you need to do this, or it might happen automatically.

And that’s it! You’ve repressurised your boiler – nice work.

If you need more help, check out our step-by-step video with OVO heating engineer Rich Laniyan.

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If your boiler’s pressure stays steady – between 1 and 1.5 bar – then it’s in good working order. But if it loses pressure again, read our FAQ ‘Why does my boiler keep losing pressure’ at the bottom of this page.

How to increase pressure on Worcester combi boiler with an internal filling key

  1. Firstly, check the pressure gauge on the control panel. If the needle is in the red (below 1 bar), you need to add pressure. 
  2. Always switch off the boiler completely before repressurising, so it can cool down if it’s hot. 
  3. For Worcester boilers, you use a special key to repressurise. It’s usually underneath the boiler – sometimes in a little tray that you take out. 
  4. Remove the key. It should have two small lugs on the top, and the same on the side. They fit into slots in the filling link, which is next to a small white cube under the boiler. Push the key until the point of the arrow is almost in line with the black plastic.
  5. Turn the key, so it’s facing you. 
  6. Then very gently turn the white cube. You’ll start to hear water flowing into the boiler. 
  7. While the boiler is filling with water, watch the pressure dial. When the needle is just under 1.5 bar, turn the white cube back to its starting position.  
  8. Your boiler should now be at the correct pressure. Turn the key back around and take it out.  
  9. Dry off the key and pop it back in the tray.

Need some expert guidance? Check out our step-by-step video, starring our very own heating engineer Rich Laniyan.

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Check for signs of a carbon monoxide leak

If there’s a problem with your boiler, check for a carbon monoxide leak before you do anything else. Does your pilot light frequently blow out, or are there sooty stains around any of your gas appliances? Read our guide to find out how to spot a leak, recognise symptoms of carbon monoxide poisoning, and what to do.

Boiler pressure FAQs

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2 OVO Energy and CORGI HomePlan are part of the OVO family, and as a group serviced 215,927 boilers in 2022.