Boiler Pressure Too High: What It Means and What to Do

If your boiler pressure gauge is reading well above 2 bar while the system is cold, or climbing to 3 bar once the heating has been on, the pressure is too high. On most homes the right figure is about 1 to 1.5 bar cold. The usual culprits are a filling loop that has been left open or is letting water through, a system that was overfilled when someone topped it up, or an expansion vessel that has lost its charge. You can lower the pressure yourself by bleeding a radiator on a cold system, but if it keeps coming back, the cause needs fixing by an engineer.

What pressure should a boiler be?

Combi and system boilers run on a sealed heating circuit that is pressurised when it is filled. Worcester Bosch's user instructions for the Greenstar i Junior, for example, tell owners to top the system up to between 1 and 1.5 bar on the gauge and to contact their installer if they see "a permanent significant decrease or increase in pressure". Other manufacturers give similar figures, but your installer may have set a slightly different cold pressure for your property, especially in a tall house where the top radiators sit well above the boiler.

The key detail is when you read the gauge. Water expands as it heats, so pressure on a working system is always higher than on a cold one. A rise of around half a bar during a heating cycle is normal. What you are looking for is either a high cold reading or a hot reading that gets close to 3 bar, because 3 bar is where the pressure relief valve is designed to open.

If your gauge is the other way round and keeps falling, our guides on how to repressurise a boiler and why a boiler keeps losing pressure cover that side of the problem.

Why boiler pressure goes too high

1. The filling loop is open or letting by

The filling loop connects the heating circuit to the mains cold supply. It is only meant to be open while you top the system up. If one of its valves has been left partly open, or the valve is worn and lets a trickle through even when closed, mains water keeps pushing into the heating circuit and the pressure creeps up day by day, even when the boiler is off. This is the most common cause we see, particularly after a system has been topped up by someone unfamiliar with it.

Check the loop under the boiler. On an external braided hose there are usually two valves, and both should be in the closed position. On boilers with a built-in filling key or lever, make sure the lever is fully back and the key has been removed where the manual says to remove it. If the pressure still rises with everything closed, the valve is probably passing and needs replacing.

2. The system was overfilled

It is easy to overshoot when repressurising, especially if you fill a hot system. The gauge reads 1.5 bar while everything is warm, then the pressure falls as the system cools and you add more, and by the next heating cycle it has climbed well past 2 bar. If the pressure has been high ever since a top-up and does not keep rising, overfilling is the likely explanation, and bleeding off a little water on a cold system usually solves it.

3. The expansion vessel has lost its charge

The expansion vessel is a small tank, usually inside the boiler casing, with a rubber diaphragm and an air cushion on one side. It gives the heated water somewhere to expand into. Over time the air charge leaks away or the diaphragm fails, and the vessel can no longer absorb the expansion. The tell-tale pattern is a normal cold reading that shoots up towards 3 bar every time the heating runs, then falls back as the system cools. Often the relief valve opens during the hot peak, water is lost, and a few days later the pressure is too low instead. Our guide to expansion vessel problems explains the symptoms in more detail.

4. Water crossing over inside a combi boiler

If the pressure rises mainly when someone runs a hot tap or takes a shower, the cause can be inside the boiler itself. A combi heats your tap water through a plate heat exchanger, with mains water on one side and heating water on the other. If that internal barrier fails, mains pressure can push into the heating circuit. This is a boiler repair, not a homeowner job.

How to reduce boiler pressure safely

If the gauge is high but nothing is leaking and the boiler is not showing a fault, you can bring it down by releasing a little water from a radiator. Do this only on a cold system, because the water in a hot radiator can scald.

If the pressure stays where you left it, the system was simply overfilled and you are done. If it rises again over the following days, water is still getting in, most likely through the filling loop. If it is fine cold but jumps high every time the heating runs, the expansion vessel is the prime suspect. In both cases bleeding radiators every few days is only hiding the fault.

When high pressure is urgent

High pressure on its own is not usually an emergency, because the relief valve is there to stop it reaching a dangerous level. It becomes more pressing when:

• Water is running steadily from the copper relief pipe outside, rather than the odd drip.
• The boiler has locked out with a fault code and will not restart.
• Water is leaking from the boiler casing or from pipework nearby.
• The gauge is sitting at or above 3 bar and still rising.

In those situations turn the boiler off at its controls, close the filling loop if it is open, keep electrics dry and arrange a visit. If the relief pipe has been dripping for a while, read our guide to a dripping pressure relief valve, because the valve may need replacing once the pressure fault is fixed. If you ever smell gas, leave the boiler alone and call the National Gas Emergency Service on 0800 111 999.

What an engineer will check

A proper diagnosis starts with the same readings you have taken: cold pressure, hot pressure and whether it rises with the boiler off. From there the engineer will usually:

• Isolate the filling loop and watch whether the pressure still climbs, to confirm or rule out a passing valve.
• Test the expansion vessel's air charge with a gauge on its valve, with the system pressure released, and recharge it or replace it if the diaphragm has failed. See our expansion vessel replacement service for how that visit works.
• Check the pressure relief valve for wear if it has been discharging.
• On a combi, look for signs of mains water crossing into the heating side through the plate heat exchanger.

Anything inside the boiler casing on a gas appliance should be done by a Gas Safe registered engineer. It is also worth checking the warranty terms before booking, because manufacturers generally expect their own engineers or your installer to handle repairs during the guarantee period.

Frequently Asked Questions