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Boiler Pressure Too High: How to Fix?


Your boiler is always treading on a thin pressure line. It’s continually adjusting pressure such that it’s not too low or too high. But since it was installed, you’ve never given this much thought. After all, what has pressure got to do with heating and hot water? Well, as you’ve recently discovered, boiler pressure plays a significant role in proper boiler function.

Think of it like blood pressure. The boiler is your body, and the pipes are your blood vessels. A spike in blood pressure has serious repercussions. A nurse measures your blood pressure using a sphygmomanometer. For your boiler, the pressure gauge is all you need.

So, what does high boiler pressure mean for your boiler and how can you fix it?

What is boiler pressure?

Boiler pressure is the balance of air and water in the boiler components and pipes. The system pressure should be enough for the boiler to pump hot water to your shower, bathroom, or heat to the radiators.

Low or high boiler pressure causes problems to the boiler. If you experience boiler pressure problems often, you should get a new boiler. Luckily, modern energy-efficient boilers are affordable.

What is normal boiler pressure?

Many boiler manufacturers recommend a 1.3 bar pressure. The usual pressure details are always outlined in the boiler manual. Moreover, the average pressure range is marked green. Anything under 1 bar means your boiler has lost pressure.

What boiler pressure is too high?

Anything above the 2 bar should be taken seriously. When your boiler hits 3 bar, it enters the red zone on its pressure gauge. At this point, the boiler will shut down as a precautionary measure. It will display a fault code. These codes vary from boiler to boiler and depend on the brand. The common error codes include:

E9, 224V, 1065B, A1, 2970B for Worcester Bosch. These error codes apply to Worcester Bosch Greenstar 25i, Worcester Bosch CDi, Worcester Bosch Greenstar 30i, Worcester Bosch CDi Compact, and Worcester Bosch CDi Highflow.

F.12, S.41, F.75, F.74 for Vaillant boilers. The errors apply to Vaillant EcoTEC Plus, Vaillant EcoTEC Pro, Vaillant EcoTEC, Vaillant EcoFIT, and Vaillant EcoMax.

F1 for Ideal. Ideal boilers display pressure issues in English. The errors apply to Ideal Logic, Ideal Vogue, Ideal Logic Plus, Ideal vogue Max, Ideal Isar.

If you see any of the above, we suggest consulting a Gas Safe boiler engineer.

What causes a spike in boiler pressure?

To fix high pressure, you need to understand the underlying problem. It could be a serious mishap in the system or merely wrong pressure settings.

Here are common causes of high Combi boiler pressure.

Overfilled System

The most common cause of high pressure is an overfilled boiler system. Adding water to the boiler system through an external filling loop causes a spike in boiler pressure. Boilers need water to function efficiently. The water helps prevent a dry fire in the system, damaging the heat exchanger, boiler pump, and many expensive parts.

But if you add too much water to the system, you cause unnecessary strain to the boiler system because the water exerts pressure to the boiler components, including copper joints. The pump joints may blow, and the heat exchanger will crack.

How to fix it

So how do you fix the high pressure in this case? Simple, drain the water from the system. You have two option points to drain the excess water:

  • Radiator bleed valve
  • Drain cock

With the radiator bleed valve, you need a specific radiator key. And because draining water through the radiator valve is messy, you should cover the floor and walls with towels. Also, have a bucket to catch the excess water. Stop bleeding the system at 1.3 bar.

If you can, locate your property’s drain cock and connect a hose partially opening the valve. This will drain the water. When you use this bleeding point, you will need help to keep an eye on the boiler pressure and shut the valve at 1.3 bar.

If you drain too much water from the system, don’t worry. You can refill it later through an external filling loop.

Normal boiler operation

Many homeowners complain and look for solutions to possible faults when heating or hot water is on, and the boiler pressure increases by 0.3 bar.

But there’s nothing wrong with this. Naturally, boiler parts expand when heated. As such, when the hot water or heating is on, the pressure will increase slightly. However, the boiler should have pressure release options to stop the pressure from rising too high.

Generally, a 0.3 bar pressure increase during hot water and heating is standard.

How to fix

Request a boiler engineer to check the pressure during maintenance and service. Any increase higher than 0.3 bar from normal pressure is a sign of an underlying problem.

Faulty pressure release

Yes, boiler pressure increases during heating. And while a 0.3 bar pressure increase is normal, the rise should be gradual and not sudden. If it increases fast and by more than 0.3 bar, the pressure release system has a problem.

Different boilers are created with different pressure release systems. These parts include pressure release valves, auto air-vents, and expansion vessels. If there is a pressure problem, these parts may be faulty. Rather than releasing pressure, they may be containing it. As such, the pressure that builds up in the boiler system has no escape.

Eventually, the boiler system pressure increases to high levels, and the system locks out.

How to fix

In this case, you should call a boiler repair engineer to assess the boiler. The engineer will be checking on any part that helps in releasing pressure. Most likely it’s a faulty component that should be replaced. The cost of a replacement pressure valve is between £100 – £350

To ensure the offer they give is reasonable, get multiple quotes from expert boiler engineers.

Partially open filling loop valve

The last possibility is that the boiler’s filling loop is not entirely turned off. The filling loop is the part you use to increase boiler pressure.

If the boiler valve is partially open, it will allow a continuous leak of water into the boiler’s system. This will result in a constant rise in boiler pressure even when heating and hot water is turned off.

How to fix

Hire a boiler engineer to disconnect the filling loop.  If the valve is stuck at partially open, a replacement for the filling loop is needed. The cost of replacing a filling loop is between £80 – £150.

Air Lock

Occasionally, the air is trapped in the heating system. Pockets of air in the boiler system cause a rise in pressure. The air pockets have to be removed to reduce stress on the system.

How to fix

To remove the air pockets, you have to bleed the system. Bleeding the system will stabilize the boiler pressure. To do this:

  • Buy a small key from a local hardware shop. Explain to the attendant that you need a key to bleed the boiler.
  • Fit the key into the radiator slot.
  • Turn the key slowly to release pressure to normal pressure.

Blocked pipes leading to the expansion tank

A boiler expansion tank is designed to accommodate expanding volumes from the boiler system. A block in the pipe leading to the expansion tank prevents the smooth flow of water into the container. The blockages are caused by accumulated dirt and debris.

How to fix

Locate the position of the block in the pipe. Remove the pipe, clear it and refit it into the system.

How often should I check for boiler pressure?

There’s no need to check on boiler pressure daily or weekly. Boiler pressure often remains stable. The expansion tank compensates for volume differences and consequently pressure caused by contraction and expansion of boiler parts.

You should check boiler pressure:

  • At the start of a heating season – August time for us Brits.
  • Two months after the beginning of the heating season.
  • After bleeding boiler radiators and the day after.

Is high boiler pressure dangerous?

High boiler pressure inhibits boiler efficiency. Depending on the underlying problem, a rise in pressure will cause the boiler to shut down to prevent over-pressurizing. If the cause is a faulty pressure valve, the pressure might get too high. And at very high levels, the boiler is at the risk of having a steam explosion.

When you notice a spike in boiler pressure, you should address it ASAP. If any of the above quick DIY fixes don’t work, or a boiler part needs replacement, call in a professional boiler engineer to handle it.


If your old boiler pressure keeps fluctuating outside the normal green range, it can be inefficient and running it can get expensive. In this case, we recommend getting a new boiler. A new and efficient boiler will save you money in the long run. Actually, with the saves, it might buy itself. For a new installation, ensure you hire a Gas Safe boiler engineer.

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