Boiler Pressure Too High

What's happening?

A sealed heating system holds a fixed amount of water. When that water heats it expands, and a vessel inside the boiler with a cushion of air takes up the extra volume so the pressure rises only a little. Pressure that is too high has two usual explanations. Either too much water has been added, most often because the filling loop was left slightly open after topping up, or the vessel can no longer absorb the expansion because its air charge has gone. In the first case the gauge is high even when the boiler is cold. In the second it looks fine cold and shoots up when the heating runs. Either way the pressure relief valve is the safety stop: at about 3 bar it opens and sends water out through a small pipe to the outside of the house.

What usually causes it?

• The filling loop valves not fully closed, so mains water keeps creeping in
• Topping up past the 1 to 1.5 bar mark and never letting the extra out
• An expansion vessel that has lost its air charge, so pressure spikes every time the system heats
• A ruptured vessel diaphragm, which lets water fill the air side
• A pressure relief valve that has lifted before and not reseated
• A failed pressure gauge or sensor that reads high
• A heating coil leaking mains water into the system on an indirect cylinder

Is it dangerous?

Not by itself. The relief valve and the boiler's overheat controls are there to deal with it, and a drip from the outside pipe means they are working. It still needs fixing, because a relief valve that keeps opening can stick partly open and the boiler may lock out. If the gauge climbs steadily with the boiler off and the loop closed, stop topping anything up and book an engineer the same day.

Can I fix it myself?

Check the filling loop first: both valves should be fully closed and the hose should be cold. With the boiler off and cool, you can lower the pressure by bleeding a little water from a radiator bleed valve into a container until the gauge reads about 1.2 bar. Do that only from the radiator, never from the boiler. If the pressure is back up within a day with the loop closed, or it only rises when the heating is on, that is the vessel or a leaking coil and it needs an engineer.

When should you call a heating engineer?

Call a heating engineer if the pressure keeps climbing after you have lowered it, if the pipe outside drips whenever the heating runs, or if the gauge goes past 3 bar. Call a Gas Safe registered engineer for anything that means opening the boiler casing: the vessel, relief valve and sensors are all inside or on it.

What will the heating engineer actually do?

• Check the filling loop and the mains isolating valve for a slow pass, and replace the loop if it will not seal
• Read the pressure cold and hot to show whether the vessel is failing
• Test the expansion vessel charge against the system and recharge or replace it
• Inspect and replace the pressure relief valve if it has been passing
• Check the indirect cylinder coil for a leak where mains water could enter the circuit
• Set the pressure and watch it through a full heat cycle before leaving

Who fixes this

• Boiler repair
• Heating not working

Questions people ask

Sources & References

1. Vaillant: what pressure should my boiler be?
2. Vaillant: what is an expansion vessel?
3. Vaillant: what does a pressure relief valve do?
4. Viessmann: what to do if your boiler pressure is too high