Boiler pressure too high? What to do

Cold, your boiler should sit around 1 to 1.5 bar. Running, it will rise a little. If it is pushing 3 bar and the pressure relief valve is dripping outside, something needs attention.

The easy cause: overfilling

The most common reason is somebody topped it up too enthusiastically with the filling loop. Bleeding a radiator will bring it back down. Open the bleed valve with a key, let water out into a cloth or bowl, watch the gauge, close when it reaches about 1.2 bar cold.

The filling loop left open

Check the loop is fully closed at both ends. A loop left slightly open keeps pushing mains water into a sealed system and the pressure has nowhere to go.

The one that needs an engineer

If you bring the pressure down and it climbs straight back up, the expansion vessel has almost certainly lost its charge. The vessel exists to absorb the expansion when water heats up. When it fails, the pressure spikes every time the boiler fires.

It can sometimes be re-pressurised. Sometimes it needs replacing. Either way it is an engineer's job, and it is a common fault on boilers past about eight years.

Is it dangerous?

Not immediately. The pressure relief valve is there precisely to dump excess pressure rather than let anything burst, and that dripping outside pipe is the system working as designed.

But a PRV that keeps discharging will eventually fail to seal, and then you have a slow leak and a pressure loss problem instead. Do not leave it for months.