Why Is My Boiler Pressure Too High – Causes & Simple Fixes Re September 15, 2020

Why Is My Boiler Pressure Too High – Causes & Simple Fixes

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Does your boiler pressure keep rising? Is your boiler pressure high and your boiler is leaking water?

If your needle is in the scary red section of the pressure gauge on the front of your boiler then you’re probably wondering why it’s happening and how to get it back in that nice, safe green section of optimal pressure.

In this article, we’ll cover the main reasons that cause an increase in boiler pressure and explain what you need to do to fix it and get your blood pressure back down too.

Boiler pressure is the balance of water and air in your central heating system; too much of one or the other will affect your boiler pressure.

If you’re suffering from low boiler pressure where the needle dips below the optimal 1.5bar level, we have covered that in our low boiler pressure blog.

If your boiler is out of warranty and you would like a no-obligation quote on a new boiler, then take the WarmZilla survey. We install boilers starting from £1489 inc. VAT including wireless controls, system filter, system cleanse, and between 5 -10 year boiler guarantee. You can even add radiators and thermostatic radiator valves to your order.

Our online survey takes less than two minutes to complete and by answering some simple questions about your home we’ll be able to suggest boilers that will suit your hot water and heating requirements.

If you want to keep your current boiler (but with lower pressure) then read on for the answers you seek.

What is my boiler pressure meant to be?

The sweet spot for your boiler pressure is 1.5bar, this is in the middle of the ideal range of between 1-2 bar. Most boiler manufacturers recommend an optimal pressure of 1.3bar, but don’t worry if it’s not quite exact, as long as it’s within the working range you won’t have a problem.

It doesn’t matter whether you have a Vaillant, Worcester, Baxi, or Ideal boiler, the recommended boiler pressure is the same for all major boiler brands.

Anything over 3.5bar is cause for concern, we would recommend getting a professional in at this point to take a look.

Your pressure gauge goes up to 4 or 5 depending on the model, but it’s unlikely it will ever get this high! The only time the pressure would get that high is if there is an issue with the gauge itself or your pressure release valve is jammed and the safety measures for the boiler to lockout have malfunctioned.

If your boiler pressure becomes too high or too low your boiler will lockout and cease to function as a safety precaution. An error message will usually flash on the LCD display letting you know why your boiler refuses to do its job.

Boiler pressure rises when system is on

If your boiler pressure is in the normal range when there is no heating demand (you aren’t running hot water taps or you haven’t turned the heating on), but you’ve noticed the pressure increases when you turn your heating or hot water on, don’t worry. This is normal.

A boiler’s pressure will increase as it heats up and drops if it’s turned off. It’s only a small change in pressure but it does fluctuate. The pressure increases as you demand heating or hot water due to water expansion throughout your system.

If you’re noticing a large increase in pressure when there is a heating demand then we would recommend getting a gas safe engineer to take a look at your system and make sure it’s safe.

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Cause 1 - System has been overfilled with water

Now that we’ve established what normal pressure is, it’s time to look at what could be the cause of your boiler pressure being too high.

Overfilling your central heating system is the most common cause of high pressure. It occurs when you repressurise your system by turning the valves on the filling loop. If you leave the filling loop valve open for too long it will draw too much water into the system, creating an overly pressurised system.

You might repressurise your system after bleeding the radiators as air can get into your system. Earlier in this article, we mentioned the balance between water and air in the system, think of it as yin and yang, too much of one or the other, and the balance is out of harmony.

It’s easy to restore harmony to your once peaceful boiler system and it won’t cost you anything (if you have a radiator bleed key). The radiator bleed key can usually be found at the bottom of your odds and ends drawer, next to the batteries and old shoelaces.

If you can’t find it because your odds and ends drawer has become unworkable then you can pick one up from a hardware store for a couple of quid.

Take your radiator bleed key and insert it into the valve of your radiator and turn it anti-clockwise to release first the air, then some water from your system (have something to catch the water in to hand). As water is bled from your radiator the pressure on your boiler should come down.

Bleed some water and then check your boiler pressure gauge to see if it’s had the desired effect.

This is the easiest and cheapest fix for high boiler pressure issues. Fingers crossed it works for you.

Cause 2 - Check your filling loop valve

The second potential cause is a filling loop valve that hasn’t been closed properly. If you’ve recently repressurised your system by topping up the water by opening the filling loop valve then the valve may not have been fully closed.

This would let a constant flow of water into your system, which would increase the pressure. Check both filling loop valves are turned off (clockwise).

This can be a cause of high boiler pressure and a very easy one to fix.

Cause 3 - Pressure release valve

Your pressure relief valve or PRV as it’s known in the trade is responsible for releasing water when the pressure starts getting too high in your boiler.

Your boilers pressure relief valve is a small copper pipe that will come out of your boiler and go back out through the external wall behind your combi boiler. Check this pipe for any leaks. If you find any water on or around this pipe it could indicate an issue with your PRV.

This is something you will need to call a gas safe engineer to look at. There are a few checks they will need to do on your boiler to see what the issue is.

Cause 4 - Faulty expansion vessel

Your combi boiler will have an expansion vessel inside, this part is responsible for keeping the pressure consistent and preventing sudden changes to your boiler’s pressure.

Too little air in your expansion vessel and the pressure may become too high. resulting in the water coming out of your pressure relief valve. Too much air could result in an over pressurised system, this could also result in water coming out from the pressure relief valve.

Again, an issue with a faulty expansion vessel is not something you can fix yourself. You will need to get a gas safe professional to take a look and diagnose the problem.


Hopefully, one of the fixes in this article will have helped you to sort out your high boiler pressure, or at least reassured you that your boiler pressure is actually within the normal range.

In addition to the issues above, old age can play a part in a misbehaving boiler. If your boiler is starting to have more issues than you would expect and it’s out of guarantee it might be worth getting a quote from WarmZilla for a new boiler.

We guarantee to offer the best price on new boilers, to save you time finding the right company. We’re also rated excellent on Trustpilot, with a 4.7 out of 5 average review score.

If you have any questions about your boiler pressure not answered in this article then use our live chat and speak to a Gas Safe engineer who will be able to advise you further.

We have written blogs on boiler fault codes for different boiler manufacturers which may help you diagnose any issues you have (including high boiler pressure) with Worcester boiler pressureVaillant boiler pressureIdeal boiler pressureViessmann boiler pressure, and Baxi boiler pressure issues.

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