Low Boiler Pressure: Causes and Fixes

What is Low Boiler Pressure? Why is it important?

When your boiler is working correctly, heating and moving water around your central heating, you’ll barely notice it’s there. Until it stops. Your boiler needs a steady level of pressure within the system so that it can push the water all the way around your house – through the radiators, up to the next floor and so on. If you have low boiler pressure then you’ll find that your boiler is struggling to move the warmth all around the house – you might not have warm radiators in a room upstairs, or far away from the boiler. 

However, if the pressure is too high in the boiler then, hopefully, the boiler will switch off before any damage is done and you’ll just be left without heating or hot water. If it doesn’t though, then any weak points in your heating system, including pipes and radiators, will make themselves known via leaking.

How can I find out if I have Low boiler pressure? 

Gas Combi Boilers are the most common boiler these days, which is what we install. To find the boiler pressure on these types of boilers, there’ll be a pressure gauge within the control panel – there’ll likely be an analogue pressure gauge indicating if you have low boiler pressure or not and either an analogue or digital temperature gauge. 

If you have a ‘regular’ boiler providing just heat, then the pressure gauge will be installed as part of the pipework just underneath the boiler instead. 

If you’re using an Open Vented central heating system, which is identified by having a big tank up in the loft and a tank in an airing cupboard, then you likely won’t be suffering from low boiler pressure. These systems are pressurised by gravity – the tank(s) in the loft allow water to flow down through the system due to the syphoning effect. This means that the weight of the water on top will push the water around your system. On these systems, there is no pressure gauge.

These systems are also incredibly inefficient and outdated. You could save hundreds of pounds per year by switching to a newer model, and most of the time we can get this for you for free! Check out our application form here and you may receive full funding, or at least a very substantial discount. 

Low Boiler Pressure Gauge

How much pressure should my boiler be under? 

Most boilers will have an operating pressure between around 1 to 1.5 bar, below 1 bar is classed as ‘Low Boiler Pressure’. Usually there’s a green zone, or an indicator on the pressure gauge to show you where the needle should be idling. It’s important to note that this is where the needle should lay whilst the boiler is idle – if someone’s running a bath then it should be higher than this, but if it’s over 1 bar out of the green zone this would indicate that the pressure is too high. This could be a fault with apparatus within the boiler such as the expansion vessel or the pressure release valve. 

How do I increase my boiler pressure?

With boilers, some will need topping up every now and then. If you find that you often have low boiler pressure and you’re having to do this often then you may need to service the boiler, in which case you’ll need to hire an engineer for that. If your boiler is old though (over 7+ years old) then we may be able to replace it for you, for free! Check out our ECO Grant page here

To get to fixing that low boiler pressure and refill your boiler, start by turning the boiler off and allowing it to cool down. Next you need to identify where the filling loop is – this will either be one tap on older boilers, two taps on newer boilers or sometimes two screws which you should have a key for. Check that you can see the gauge from where you are next to the taps, sometimes a second person is required. 

If you have two taps, they’re typically blue and black, if there’s one tap it’s typically black. 

If there’s just one tap, twist that 90 degrees counter-clockwise. You’ll hear water flowing and your pressure gauge should be going up. If it doesn’t turn fully then the valve may have seized slightly – if this is the case then remove the screw on the tap (don’t worry, water isn’t going to leak out). Then use an adjustable spanner or pliers to twist the brass section you’ve just revealed. If it doesn’t turn fully then just allow the water to flow slowly. It’ll take time but your pressure should go up. You may also need to bleed your radiators upstairs. Each time you bleed the radiators, the pressure inside the boiler will drop slightly, so there’ll be a fair bit of back and forth. It’s not ideal, but it’s the only way. 

Once your radiators are fully bled and your pressure’s back to around 1.5bar you’re good to go. It’s probably time to start thinking about a new boiler though – and we can help get you one supplied and fitted, typically for free. To find out how, click here. 

If there’s two taps or screws then you’ve got the full loop and you shouldn’t have to bleed any radiators! Start with the black tap first, and turn that 90 degrees counter-clockwise and then the blue tap. You’ll hear water rushing in which is completely normal. Once your pressure gauge is showing around 1.5 bars of pressure then you’re good to turn the taps back off again.

The boiler should maintain its pressure after these steps. If it doesn’t, go around the house and check for any leaking radiators and damp patches around pipework – if you can’t see this, then you’ll be needing a gas safe engineer to inspect the system. It’s best not to wait, as if there’s a hidden pipe that’s leaking then you’re about to run into a heavy bill for replacing both the piping and any flooring or walls around it. 

What if I over pressurise my boiler? 

If you’ve fixed your low boiler pressure, but haven’t managed to turn off the taps in the correct range then you may be stuck with too much pressure now. While that can be more damaging, there’s still a simple fix to lower it back down. So if you fill the pressure up too high, don’t sweat it! Bleed it instead. Keep your boiler switched off and grab a bucket and some towels, you need to bleed those radiators. On the tops of your radiators there’ll be a nut with a line cut out through the centre. There’s special tools that fit this perfectly but if you’ve got a thin flathead screwdriver or a sturdy knife you may be able to twist it. As you twist, water will start to leak out so make sure you have a bucket underneath if possible or throw down several towels. Once the water is more than a trickle and there’s a steady flow, stop twisting the nut.  This will take a while, but it’s the simplest way to reduce boiler pressure. 

When you switch your boiler and heating back on, the pressure will go over 1.5 bar, this is normal but if it climbs to around 3 bar then this is over pressured – bleed a radiator to reduce this. 

Are you entitled to a Free Replacement Boiler?

At Borthwick Heating we work with the UK Government to install free boilers to eligible UK Homeowners. To check if you qualify click the button below.

Borthwick Heating Inuslation UK is a trading style of Borthwick Group (Energy) Limited Registered in England & Wales| Company No. 06753929 

Borthwick Heating & Insulation are regulated by the Financial Conduct Authority. We are a Credit Broker and not a lender. We offer Credit Facilities from a panel of lenders. Borthwick Group (Energy) Limited is the full legal and registered trading name of all activities that advertise themselves as Borthwick Heating & Insulation and online as www.borthwickheating.co.uk


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