What to do if your boiler pressure is too low? Follow our easy step-by-step guide

18 November 2020 | Celia Topping

Hot water and heating – they’re the two must-haves in every home, especially as we move towards winter. After the warmer months, it’s possible your gas boiler’s pressure might have dropped – causing that most dreaded of all cries on a cold winter morning, “There’s no hot water

what to do if your boiler loses pressure

Luckily, most of the time it’s a quick fix. So save yourself the hassle and the cost of calling in a heating engineer by solving the problem yourself, with our easy-to-follow guide. 

What is boiler water pressure? 

A gas boiler’s efficiency relies on a perfect balance of water and air in its system. Steady, stable pressure means there’s hot water and heat when you need it. But if the pressure (or water level) drops too low, the system has to work harder to keep things moving, meaning higher bills for you. 

What are the risks of low boiler pressure?

Thankfully, low water pressure is nothing much to worry about – it’s not dangerous.  The only concern is that you’ll end up paying more in heating bills, and no one wants that! So let’s find out how to fix it. 

How do I know if my boiler pressure is too low?

A tell-tale sign of low pressure (apart from having no hot water) is if your radiators aren’t heating up properly. To check your boiler and take a pressure reading, open up its front panel to reveal the buttons and dials on the control board – and then:

  1. Find the circular pressure gauge, likely marked with red and green zones.
  2. Pressure is measured in ‘bar’, so the needle should be resting in the green zone, between 1 and 1.5 bar when the boiler is off (but boilers do vary, so check your manual). 
  3. If the needle is below 1, in the red zone, your boiler may have lost water, causing low pressure. If this is the case, your central heating system isn’t working efficiently. 
  4. If the pressure gauge reads above 2.75 bar when the boiler is off, you need to bring it down to around 1.5 bar. This can be done by bleeding the radiators – so check out our guide explaining how to do that. However, if in doubt, call a professional, as the water will be very hot.
repressurising your boiler

You may have a digital gauge – in which case, it’s even easier to see the water pressure. The numbers will be flashing if the pressure is too low or too high. 

Why is my boiler pressure too low? 

There are usually two reasons for a pressure drop in a boiler – either you’ve just bled your radiators, or there’s a water leak somewhere in the system. 

Boiler pressure too high? We've got a guide for that, too.

Bleeding radiators

This is the most common cause for a sudden drop in pressure in a boiler system. When you bleed a radiator, you’re letting air out. This can lower the pressure in the system and upset the fine balance needed to keep water flowing smoothly. The good news is, this is a quick fix. Find out how by reading on below. 

To find out how to bleed your radiators in 7 easy steps, check our handy how-to guide. 

A water leak

If there’s been a gradual decrease in pressure, or the pressure just keeps dropping, you may have a leak. Often, it’s easy to spot – take a look around your home for signs of damp patches near radiators, towel rails or even under your boiler. If you can’t see any leaks, it’s possible that there’s one under your floorboards. In this case, you’d need to call in a professional. Also, if you detect a leak in your boiler, don’t attempt to fix it. Only a registered heating engineer can safely open your boiler. 

How to fix low boiler pressure

In general, the easiest way to remedy the problem is to repressurise the system. This is quick and simple to do yourself, but it’s always best to check your boiler manual first to see if there are any specific instructions. These might also be found on the inside flap of the boiler panel. And remember, never use tools to open up a boiler. You should only be touching the buttons and dials on the control panel. 

To repressurise, boilers just need more water (depending which type of boiler you have). There are 2 main differences to look out for:

  • Internal filling loop (keyed) boilers - this is mainly found on Worcester boilers. It’s a white plastic key located next to a small white plastic cube, underneath your boiler. 
  • 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.

Step-by-step guide to repressurising a 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 turn off the electrics connecting the boiler before repressurising, so the boiler can cool down first if it’s hot. 
  3. For Worcester boilers, repressurising is done with a special key. You can usually find this underneath the boiler. It might be under a little tray, so you’ll need to remove this tray to find the key. 
  4. Take out the key. You can see it’s got two little lugs on the top, and the same on the side. They fit into slots in the filling link. This is found next to a little white cube, which is also under the boiler. Give the key a good push 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 little 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 little square off. 
  8. Your boiler is now 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.

Still not sure? Check out our step-by-step video showing you how to use the Worcester boiler's internal filling key, starring one of our very own heating engineers, Rich Laniyan. 

Step-by-step guide to repressurising a boiler with a filling loop (on most models of boiler)

  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 turn off the electrics connecting the boiler when repressurising, so the boiler can cool down first if it’s hot. 
  3. Find the filling loop hose below/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 to 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). 

That’s it! You’ve re-pressurised your boiler – so well done.

If you're still not sure how to do it, check our our step-by-step video with OVO heating engineer Rich Laniyan. 

If your boiler’s pressure stays steady – between 1 and 1.5 bar – then relax, your boiler is in good working order. But if it loses pressure again, read on...

Why does my boiler keep losing pressure?

If your boiler keeps losing pressure, there’s most probably a leak somewhere in your central heating system, or in the pressure relief valve. 

Loss of pressure could also be caused by a faulty expansion vessel, an airlock in the system, or corroded pipes. For any of these issues, you’ll need to call a heating engineer to fix the problem. 

Remember, it’s only safe to perform very minor boiler repairs yourself. See our guide to common boiler problems and their fixes here. And if you think it might be better to replace your boiler altogether, see our guides on how much boilers cost to repair or replace, and how long it takes to fit a new boiler.

And if in doubt about any of the above, leave the job to a professional.

Once your boiler is up and running, a regular check-up should keep everything in order. From just £13 a month, OVO Homeplan will make sure your boiler and central heating system carries on working well, so you don’t need to tinker yourself, or worry in case something goes wrong. From boilers and plumbing to electrics and drains, you’ll find a range of plans to keep you covered.

We have over 4,500 approved engineers nationwide to put things right. And our plans include:

  • Unlimited callouts
  • Parts and labour included
  • Emergency hotline

Get OVO HomePlan and be covered from as little as £13 a month

Does your pilot light frequently blow out, or are there sooty stains around any of your gas appliances? This could indicate a carbon monoxide leak. Carbon monoxide poisoning is lethal. Read our guides to find out where to place your carbon monoxide detector. Plus how to spot a leak, recognise symptoms of carbon monoxide poisoning and what to do. This guide could save your life

Over time, a new, more energy-efficient boiler could save you money. Find out about boiler replacement with OVO.

You can get an A-rated, award-winning boiler from CORGI HomeHeat (which is part of the OVO family). Want to know more? Visit MyOVO and check out the  offers from CORGI.  

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