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The balance of water and air in your central heating system determines boiler pressure; excessing either will lower boiler pressure. The pressure gauge must be set carefully for your boiler to operate effectively and safely. If you discover that the boiler pressure is very high, your hot water and central heating systems may have problems, which can be dangerous.

Does your boiler have high pressure? If it’s in the alarming red section on the pressure gauge on the front of your boiler, you’re probably wondering why it’s happening and how to get the needle back in the nice, safe green part of optimal pressure.

In this blog, we’ll go over the primary causes of rising boiler pressure and solutions to help you lower boiler pressure again.

What Is My Boiler Pressure Meant To Be?

The sweet spot for your boiler pressure is 1.5 bar, in the middle of the ideal 1-2 bar range. Most boiler manufacturers say that 1.3 bar is the best pressure, but don’t worry if that’s not exactly what you get. You won’t have any trouble as long as it’s in the working range.

It doesn’t matter if you have a Vaillant, Worcester, Baxi, or Ideal boiler; all of the major brands should have the same pressure.

Anything over 3.5 bar is cause for concern. At this point, we recommend getting a professional 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, your pressure release valve is jammed, or the safety measures for the boiler to lockout have malfunctioned.

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

Boiler Pressure Rises When The System Is On

Don’t worry if the pressure in your boiler is normal when there is no heating demand (you aren’t running hot water taps or turning on the heat), but goes up when you do. This is normal.

If turned off, a boiler’s pressure will increase as it heats up and drops. It’s only a tiny change in pressure, but it does fluctuate. When you turn on the heat or hot water, the pressure rises up because the water in your system expands.

If you notice a significant increase in pressure when there is a heating demand, we recommend getting a gas-safe engineer to inspect your system and ensure its safety.

Cause 1 – The System has been Overfilled with Water

Now that we know what normal pressure is let’s look at what might be the cause of your boiler pressure being too high.

The most common cause of high pressure is the overfilling of your central heating system. It happens when you turn the valves on the filling loop to repressurise the system. If you leave the filling loop valve open for too long, too much water will be drawn into the system, making it too pressured.

After emptying the radiators, you might need to repressurise the system because air can get into it. We talked about the balance between water and air earlier in this blog. Think of it like yin and yang. If there is too much of one or the other, the balance is off.

It’s easy to restore harmony to your once peaceful boiler system and won’t cost you anything (if you have a radiator bleed key). The old shoelaces and batteries are usually located close to the radiator bleed key in your odd-and-ends drawer.

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

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

To determine whether it has had the desired effect, bleed some water and then check your boiler pressure gauge. This is the cheapest and simplest way to address high boiler pressure issues. Fingers crossed it works for you.

Cause 2 – Check Your Filling Loop Valve

A full loop valve that isn’t closed properly is the second potential cause. If you’ve recently repressurised your system by topping up the water by opening the filling loop valve, the valve may not have been thoroughly closed.

This would let a constant water flow into your system, increasing the pressure. Check to make sure that both filling loop valves are turned off (clockwise).

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

Cause 3 – Pressure Release Valve

Your pressure relief valve, or PRV, releases water when the boiler’s pressure rises.

Your boiler’s pressure relief valve is a small copper pipe that runs from your boiler to the external wall behind your combi boiler. Check this pipe for leaks. If you find water on or around it, it could indicate an issue with your PRV.

You should call a gas-safe engineer to investigate. They must perform a few checks on your boiler to determine the issue.

Cause 4 – Faulty Expansion Vessel

Your combi boiler will have an expansion vessel inside. This part is responsible for keeping the pressure steady and stopping quick changes in pressure.

If there is too little air in your expansion vessel, the pressure may become too high. And it could result in water coming out of your pressure relief valve. Too much air could result in an over-pressurised system, which could also result in water coming out of the pressure relief valve.

Again, trouble with a faulty expansion vessel is not something you can fix yourself. You will need to get a gas-safe professional to diagnose the problem.


Hopefully, one of the solutions in this article will help you fix your high boiler pressure or at least reassure you that your boiler pressure is actually within the normal range.

In addition to the issues above, old age can contribute to a misbehaving boiler. If you have questions about your boiler pressure that are not answered in this article, use our live chat to speak to a Gas-Safe engineer who can advise you further.

Hope this informative guide helps you diagnose any issues you have (including high boiler pressure) with Worcester boiler pressure, Vaillant boiler pressure, Ideal boiler pressure, Viessmann boiler pressure, and Baxi boiler pressure.

The timeframe can vary depending on the workload of your chosen installer and the availability of boilers. However, once your application is approved, you can expect the installation in a few days.

Anyone who has a boiler installed before 2005 or has an inefficient boiler can qualify for a boiler grant in the UK. To meet the requirements, you must be a homeowner or private tenant of the home in the UK.

The ECO4 Scheme is a government-funded program that provides grants and funding to homeowners in Scotland, who want to install energy-saving measures in their homes. The aim of the Eco4 scheme is to help people reduce their energy consumption and save money on their energy bills.

Yes, it’s quite easy. You should regularly check that the evaporator and air intake grill are clear of leaves and other particles.

To be eligible for a first time central heating grant, you must not have prior central heating installed in your homes. Also your home’s EPC rating should be lower than D.