Boiler Pressure Too High: Causes and How to Lower It

What counts as too high

On most combi and system boilers the gauge should sit somewhere around 1 to 1.5 bar when the heating is cold. It rises a little as the water warms and expands, which is normal. Check the instructions for your own boiler, as the exact figures vary.

If the needle goes close to or above 2.5 to 3 bar, or into the red section of the gauge, the pressure is too high. At that point the pressure relief valve inside the boiler may open to protect it, and the boiler can lock out.

Common causes

High pressure usually comes down to one of these:

• The filling loop was left open after topping up (see how to repressurise a boiler), or its valve is letting water by
• The system was topped up while hot, so it reads high once fully heated
• The expansion vessel inside the boiler has lost its air charge
• On a combi, a worn internal part letting mains water into the heating side

Bringing the pressure down safely

First, check the filling loop under the boiler. Both taps on it should be fully closed. If the needle is still climbing with them shut, a valve may be passing water, and that needs a repair.

With the heating off and the system cool, you can let a little water out by bleeding a radiator. Hold a cloth under the bleed valve, open it with a radiator key and let water out until the gauge drops to the normal range, then close it firmly. Go slowly and keep an eye on the gauge so you do not drop it too far.

Water dripping from the pipe outside

A small copper pipe leaving the wall near the boiler is the pressure relief discharge. If it drips or runs after the pressure has been high, the relief valve has done its job. Once a relief valve has opened, it does not always reseal cleanly, so it can keep weeping and cause the pressure to fall instead.

That cycle of too high, then too low, is a sign that the root cause has not been fixed. Our guide to a boiler losing pressure covers the other side of the problem.

When it needs an engineer

If the pressure climbs again within a day or two of being lowered, or keeps rising every time the heating runs, the expansion vessel or a valve inside the boiler is the likely culprit. Recharging or replacing an expansion vessel and changing internal parts is work for a Gas Safe registered engineer, not a homeowner.

Do not keep releasing water by bleeding radiators as a routine fix. Every refill brings fresh oxygen into the system, which adds to corrosion and sludge.

Need it sorted? Book a boiler repair

Send a photo of the gauge and the filling loop to 07429 632804 on WhatsApp, read about our boiler repairs, or use the quote form and Chris will be in touch.