High boiler pressure? Here’s what to do about it
28 January 2021 | Aimee Tweedale
Just like you and I, your boiler works best when it isn’t under too much pressure! In fact, the pressure has to be just right: not too low, and not too high.
If your boiler pressure’s too high, it can wreak havoc on your central heating system. Keep reading to find out how to identify high pressure, and what you can do about it.
What is boiler pressure, and why does it matter if it’s too high?
Gas boilers keep your home toasty by pumping hot water through the house. How? With the magic ingredient: pressure.
When we talk about boiler pressure, we’re generally referring to the balance of water and air in your boiler system, including the pipes. If the pressure isn’t right, then the water won’t be able to travel around the system the way it should – and you might be left out in the cold.
What should your boiler pressure be? How to check the pressure
To check the pressure on your boiler, take a look at the pressure gauge on the front. It will look like a dial, usually starting at 0 bar and going up to 4 or 5 bar.
The ideal pressure for your boiler is usually marked on the dial in green. The pointer should be within the green zone (usually between 1 and 1.5 bar) – if it’s pointing at a red zone, your pressure is too low or too high.
Most boiler manufacturers recommend maintaining pressure between 1 and 1.5 bar when the heating’s on. When it’s off, your pressure reading might drop to between 0.5 and 1 bars – but don’t worry, that’s normal!
Check your boiler’s manual for more detailed information on what pressure your particular brand of boiler should be.
What happens if boiler pressure is too high?
If your boiler pressure gets too high, you might be heading for a boiler breakdown. Some models are designed to shut themselves off (or go into “lock-out”) if the pressure goes above a certain level. Sometimes you might also hear a juddering or banging sound coming from your boiler.
Don’t panic – it may be possible to fix the problem at home, without calling a heating engineer. Read on to find out more about possible causes of high pressure and how to relieve it.
Another classic giveaway of high pressure is a water leak. You might see this in the form of damp patches underneath joints in your pipework, or at the place where your pipes connect to your radiator. If you have a leak somewhere in the house, it’s best to call a plumber.
Is high boiler pressure dangerous?
Your gas boiler is designed to keep you safe. If the pressure gets too high, some models of boiler will shut down to protect you. Others may have a pressure relief valve that can ease the problem.
Reasons why your boiler pressure could be too high
What causes high pressure in boilers? The most common culprit is – funnily enough – trying to fix low boiler pressure. If you recently tried to repressurise your boiler, you might have gone too far the other way. The good news is, it’s easily solved.
These are the 3 most likely reasons your boiler pressure could be too high:
- You recently repressurised the boiler. If you recently topped up the water in your boiler, there might now be too much water in the system. Read more about this in our guide to low boiler pressure.
- There’s a faulty part. Various parts of your boiler – such as the pressure release valve or the diverter valve – could cause high pressure if they malfunction.
- Your boiler is getting on a bit. Is your boiler over 10 years old? If so, its pressure problems could be a sign of it struggling in old age. Old boilers can be expensive and inefficient to run – so it might be time to think about replacing it.
What to do if boiler pressure is too high: a guide to reducing boiler pressure
So: you’ve identified that your boiler has high pressure. What now?
How to bleed your radiators
If you think that it could be because there’s too much water in your system, the first thing to try is bleeding your radiators. This is usually done to release trapped air. But you can use the same technique to let out some of the water in your radiators and restore balance to your boiler pressure.
All you’ll need is a radiator key, which you can buy from most DIY shops. Make sure you also have a cloth or towel to hand – the water may not be crystal clear, so you don’t want it dripping straight onto the carpet! Then follow these simple steps:
- Turn off your heating, and wait for the system to cool down.
- Find the bleed valve on the side of your radiator.
- Put your cloth or towel underneath the valve to protect your floors.
- Fit the radiator bleed key in the screw and turn it anti-clockwise for about a quarter of a turn. You should hear a hissing noise!
- Wait until all the air has escaped; at this point, you should see some water seeping out.
- Once you’re satisfied you’ve let out enough water, tighten up the valve again, and turn the heating back on.
How to use the filling loop to reduce pressure
Another trick you can try is to check your filling loop. The filling loop is a device that connects your boiler to the mains water supply.
Your boiler will likely have 1 of 2 types of filling loop:
- Internal, or keyed: if you have this type of filling loop, you should see a white plastic key next to a small white plastic cube, underneath your boiler. These are usually found on Worcester boilers, as well as many combi boilers.
- External: most boilers have this. It looks like a short braided metal hose, and has two valves or taps. You can typically find it underneath the boiler.
Usually, you’d use a filling loop to repressurise your boiler, by hooking it up to the mains water supply. In this case, you want to do the opposite: make sure that the filling loop is not connected to the mains, as that could be causing high pressure.
If you have an internal filling loop, make sure that the key is not in the boiler, and the filling loop is closed. Check your boiler manual for more details on how to do this.
If you have an external filling loop, make sure that the taps are closed – and the filling loop should also be disconnected from the boiler when not in use.
If your boiler pressure stays high, or keeps on going up, you may have a fault with the filling valve, or another crucial boiler part. Unfortunately, if the fixes above don’t work, it’s best to get help from a professional, who can diagnose and repair the problem. Book an appointment and one of our Gas-Safe-registered engineers will come round and check it over.
Think there’s something else wrong with your boiler? Read our guide to common boiler problems and how to fix them.
Common error codes for boilers with high pressure
No matter what type of boiler you have – whether Worcester, Vaillant, Baxi or otherwise – your pressure could be too high. Here are some common error codes you might see on different brands, which tell you if the boiler has too much pressure.
|Type of boiler||High pressure error codes|
|Worcester Bosch||E9, A1, 224V, 1065B, 2970B|
|Vaillant||F.12, F.74, F.75|
Time for a new boiler?
If you’re thinking that the most logical next step is to get yourself a more efficient new boiler, check out our explainer on replacing your boiler, and our guide to low-carbon options for heating your home.
Got more boiler questions? Read our boiler guides:
- Different types of boiler, and how to know what yours is
- How to reset your boiler when it locks out
- The ultimate guide to being efficient with heating and hot water
- Boiler installation: how long does it take to fit a boiler?
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