guide

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

23 November 2021 | Celia Topping

Hot water and heating – they’re the 2 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!”.

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? 

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

A gas boiler’s efficiency relies on a perfect balance of water and air in its system. The right balance creates steady, stable pressure, which means there’s hot water and heat when 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?

These are some of the tell-tale signs 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 (more on this below)

Is low boiler pressure dangerous?

No. Thankfully, low water pressure is nothing much to worry about – it’s not dangerous, and shouldn’t cause damage to your boiler. The only concern is that you’ll end up paying more in heating bills, and no one wants that! 

What should boiler pressure be? 

Your boiler pressure should typically be between 1 and 1.5. But all boilers vary, so check your manufacturer’s instructions to be sure. 

To check your boiler’s pressure, open up its front panel to reveal the buttons and dials on the control board – and then:

  1. Find the circular pressure gauge – this is probably marked with red and green zones.
  2. The needle should be resting in the green zone – between 1 and 1.5. But boilers do vary, so check your manual. 
  3. If the needle is below 1 (in the red zone), your boiler may have lost 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.

Boiler pressure gauges

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. 

What causes low boiler pressure?

There are usually 2 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.

Reason 1: you’ve bled your 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 our “how to increase boiler pressure” section below. 

(And to learn how to bleed your radiators in 7 easy steps, check our handy how-to guide.) 

Reason 2: you’ve got a water leak

If there’s been a gradual decrease in pressure or the pressure just keeps dropping, you might 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. If you do detect a leak in your boiler, don’t attempt to fix it. Only a registered heating engineer can safely open your boiler. 

How to increase 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 instructions might also be found on the inside flap of the boiler panel. And remember, never open up a boiler. You should only be touching the buttons and dials on the control panel. 

To repressurise, boilers just need more water added to the system (depending 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 – this is mainly found on Worcester boilers. It’s a white plastic key located next to a small white plastic cube, underneath your boiler. 

How to repressurise a boiler with a filling loop (most models of boiler)

  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). 

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

If you're still not sure how to do it, 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 relax, your boiler is in good working order. But if it loses pressure again, read our section on what to do if your boiler keeps losing pressure.

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 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 small 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 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.

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Boiler pressure: frequently asked questions

Your boiler pressure gauge will probably use colours to show what pressure your boiler should be. Normal pressure levels will be marked in green. If your boiler pressure drops too low, you might see that the needle has dropped into the red.

For most boilers, you should expect to see your boiler pressure gauge reading between 1 and 2 when the heating is on. It will be slightly higher than it would be while the heating is off, but not so much that it goes above 3.

Every boiler is different. For specific instructions about your make and model, you should read your boiler manufacturer’s instructions.

Most boilers will show a pressure reading of between 1 and 1.5 when the heating is turned off. 

Every boiler is different. For specific instructions about your make and model, you should read your boiler manufacturer’s instructions.

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.

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

Yes. Any leaks in your system can cause your boiler pressure to drop. This includes a leak from your taps, radiators, or pressure relief valve.

Pressure relief valves stop your boiler from building up too much pressure. The valve opens to release pressure when the level gets too high. 

Sometimes, low boiler pressure can be caused by a faulty pressure relief valve, which is leaking water from your central heating system. If this is happening, you will see a drip coming from a small copper pipe that leads to outside your property. 

If you think your pressure relief valve is faulty, it’s time to call in the professionals. Fixing it is not a DIY job.

Take care of your boiler with OVO HomePlan

Once your boiler is up and running, a regular check-up helps keep everything in order. From just £13 a month, OVO Homeplan will help make sure your boiler and central heating carries on working well, so you don’t need to worry in case something goes wrong. 

And, if you’re on OVO HomePlan, you’ll have a nationwide network of Gas-safe registered engineers ready to put things right. And our plans include:

  • Unlimited callouts
  • Parts and labour
  • A 24/7 emergency helpline

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

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

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

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.

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