Boiler pressure: too high/too low – here’s what to

If you’re fortunate enough, you won’t have had to think about your boiler pressure before. However, many boiler systems do experience issues with pressure at some point.

Thermatic Homes are experienced M&E contractors working with housing associations across the UK. As part of our mechanical services, we carry out boiler replacements for our customers – so we know a thing or two about boilers and are well placed to offer advice on how to make sure they’re running properly.

In this blog we’ll look at:

What your boiler pressure should be
How to spot if you have a problem with your boiler
How to fix your boiler pressure if it’s too low or too high

Why is boiler pressure important?

Central heating systems keep your home warm by heating cold water and pumping it around your pipes and radiators.
Most central heating systems are sealed, which means there are no vents to manage the water expanding and contracting as it cools and heats. Instead, the system is pressurised and must maintain a stable pressure to move the water around your home efficiently.

Beyond your boilers annual service, most boilers tend to need a little attention and the odd glance at the boiler pressure gauge. The boiler pressure gauge tells you what the pressure of the water is that’s circulating in the heating system.
If your boiler pressure is too low, your heating system won’t work as well – but if it’s too high there’s a strong possibility of leaks developing, so it’s so important to ensure your boiler is spot on to make sure that your system heats your home efficiently.

If you’re worried about fixing your boiler pressure yourself or fear that you may have a more serious problem, call out a professional to look at your boiler.

How to check your boiler pressure

On combi boilers, the pressure gauge is located at the front of your boiler or can be found under the control panel. If you have an old boiler, the pressure gauge might be underneath, amongst the pipework. If you’re struggling to locate it, have a look int the instruction manual.

What pressure should my boiler be?

Boiler pressure should generally be between 1 and 1.5 bars when central heating is turned off. The needle of the pressure gauge should remain above the lower limit set by your boiler manufacturer at all times. Always check your boiler manual to see what the specific recommendations are for your boiler model, as this varies depending on the manufacturer.
Even when your boiler’s working well, it’s a good idea to check the pressure occasionally. And to know how to release boiler pressure when it’s too high. And how to top up boiler pressure – when it’s too low.

What should boiler pressure be when heating is on?

A boiler’s pressure gauge will usually sit between 1 and 2 bars when it is turned on. The exact recommended pressure level will vary depending on the boiler manufacturer, but generally, anywhere between 1.0 and 2.0 bar is ideal.
The boiler pressure level will rise when heating a home or providing hot water. However, it should still stay below the upper-pressure limit of 3 bar. Always check your boiler manual to see what the recommended boiler pressure level is for your boiler.

You’ll know the boiler pressure is too low in your system, if:

There’s no heating or hot water
Your radiators aren’t warming up properly
The dial on the pressure gauge has fallen

If you’re wondering why your boiler pressure keeps dropping, the most common causes are a leak or bleeding of the radiators.

Why has my boiler pressure dropped?

Most people don’t tend to check the pressure gauge on their boiler, so they don’t tend to notice that the pressure is dropping. A sure-fire sign that your boiler has stopped working is if you don’t have any hot water or your radiators fail to heat up – this is usually a sign that the pressure in your boiler is too low.

Why is my boiler pressure low?

There are two main reasons for low boiler pressure. The first reason could be a water leak – a water leak means you’re losing water in your system = lowering the boiler pressure. It could be a small amount and unnoticeable, but if this continues long enough, your water pressure will drop slowly. Top tip – check around your radiators and pipes and your boiler for damp patches/water. Never attempt to look inside your boiler! You’ll need a Gas Registered engineer to do that.

Bleeding radiators

Bleeding your radiators takes water out of your central heating system, lowering the boiler pressure.

What can I do about low boiler pressure?

It’s possible to fix low pressure yourself, however, you should check your boiler’s manual before you start.
To re-pressurise your boiler, you’ll need to let more water into the system using something called a filling loop. These can be built into your boiler or come as an attachment. The filling loops design and the filling process can vary, but here’s a basic guide on what to do:

Turn off your boiler and allow it to cool.
Find the filling loop, or attach it to your boiler.
Make sure you can see the pressure gauge while using the filling loop.
Open the valves on both sides to let water into your system.
Wait until the pressure gauge reaches 1.5 bar then close each valve, one after the other.
Switch your boiler back on and, if you need to, press the reset button.
Don’t forget to remove the filling loop if it’s an attachment. Be careful in case there’s any water left in it.
If your pressure drops again, or you’re unsure about trying to re-pressurise your boiler, it’s best to get a professional to have a look at it.

What can I do about high boiler pressure?

If your boiler is too high your heating system will turn off. The main reasons that your boiler pressure is too high are:

Too much water in the heating system
If you’ve recently added water to your system due to low pressure this may be the culprit.
A simple fix is to bleed your radiator. This lets some of the water out and so should reduce the pressure.
Faulty boiler

If bleeding your radiators doesn’t work, one of the boiler parts could be faulty. Contact a Gas Safe Registered engineer to investigate.