Boiler pressure too high
Above about two and a half bar cold, either something is adding water or something has stopped absorbing it. One test tells you which, and only one of them costs money.
What it usually means
A sealed heating system is designed around one number: how much the water expands when you heat it. Take a system from ten degrees to eighty and its volume grows by roughly four per cent, and since the pipework cannot stretch, that extra volume has to go somewhere or the pressure goes up instead.
Where it goes is the expansion vessel. Inside it is a rubber diaphragm with pressurised air on the far side, and that cushion of air compresses as the water pushes into it. When the air escapes past a failed diaphragm or leaks out through the valve, the cushion is gone, and there is nothing left to absorb the expansion. The signature is unmistakable once you know to look for it: the gauge sits correctly when the system is cold and climbs steeply — one and a bit bar to nearly three — within twenty minutes of the burner lighting.
The other cause has the opposite signature. If pressure is high even after the system has stood cold overnight, nothing is failing to absorb anything; water is being let in. Almost always that is the filling loop, left connected with a valve not quite shut, or a built-in filling device that has begun to pass.
Both end at the same place. At three bar the pressure relief valve opens and dumps the excess down a pipe through the outside wall, which is exactly what it is for. The catch is that a safety valve which has lifted often does not reseat cleanly afterwards, so a great many boilers that were too high in November are losing pressure by January — one fault presenting as its own opposite.
How to narrow it down
Work through these in order.
What does the gauge read cold, and what does it read at full temperature?
Note it before the heating comes on and again once the radiators are properly hot. A rise from 1.2 to somewhere under 2 bar is exactly what should happen. A rise from 1.2 to 2.8 or beyond means the expansion vessel has lost its charge, and that is the finding an engineer most wants to arrive already knowing.
Is it still high after the system has been off all night?
Then the problem is not expansion, because there is none to absorb — the system is cold. Something is feeding water in, and the two candidates are the filling loop and any built-in filling valve. This is the single most useful question on the page because it splits the free fix from the paid one.
Is the filling loop still connected?
Look for a braided silver hose under or beside the appliance with a small valve at each end, or a key-operated valve built into the pipework. Both valves need to be fully closed, and a loop left attached with one of them barely open will lift the pressure a little every day until the safety valve opens.
Has the pipe coming through the outside wall been dripping?
A small copper or plastic pipe emerging near the boiler, often ending in a downward bend, is the safety valve discharge. Water there, or a pale limescale trail on the brickwork under it, is proof the valve has already opened at some point, which matters because it explains a later loss of pressure.
Did it go up right after somebody topped it up or bled a radiator?
Then it is probably nothing more interesting than an overfill, and bleeding a radiator briefly into a jug brings it back down. Filling a system while it is hot has the same result a few hours later, because the water you added shrinks as it cools and you compensate by adding more.
Is the boiler still running?
Most will keep going up to about three bar and simply lose water out of the safety valve. Some report it: Ariston show 109 above three bar, Potterton and Baxi show E117 above roughly 2.9. A gauge pinned at three bar with nothing at all coming out of the discharge pipe is the one combination to take seriously, because it suggests the valve is not opening.
Codes commonly shown with this
- F.73VaillantThe pressure sensor signal is out of range at the low end. Usually the sensor or its wiring, rather than the pressure itself.Get it checked
- F.74ViessmannSystem pressure too low on a 2020-on Vitodens with the numeric display. The burner and pump stop until the pressure is back. Viessmann tell the householder to top up below 1.0 bar.Get it checked
- E117BaxiPrimary water pressure too high, on the Duo-tec 2 and EcoBlue Advance. Usually an overfilled system or a flat expansion vessel. Let some water out and it clears.Get it checked
- 109AristonSystem pressure has gone above 3 bar. Usually this means somebody overfilled it, or the expansion vessel has failed.Get it checked
- F37FerroliSystem water pressure is outside the correct range. Because this is an F code it clears by itself once you top the system back up.Usually simple to fix
When it is urgent
Take care
What it costs
Typical repair cost
£0–£320
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Get an engineer to look at this
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.