Boiler Losing Pressure? Causes and How to Fix It
A boiler that keeps losing pressure has one of three problems: a leak somewhere in the system, a faulty pressure relief valve, or a failed expansion vessel. Repressurising to 1.5 bar restores heating temporarily, but a drop that repeats within days or weeks needs the cause found and fixed.
What is normal boiler pressure, and what counts as losing it?
A sealed heating system should sit between 1 and 1.5 bar when cold, rising slightly as it heats. Pressure drifting down over many months is ordinary life; tiny amounts of air escape through automatic vents and the occasional radiator bleed, and a top-up once or twice a year is nothing to investigate. Losing pressure means the gauge drops noticeably within days or weeks of a top-up, or the boiler locks out on a low-pressure fault (Vaillant F22, Baxi E119, Ideal F1, Worcester's low-pressure warnings all describe the same event). That pattern always has a cause, and the cause is always one of three things.
The three causes, in order of likelihood
A leak in the system
The most common by far, and the best at hiding. It does not need to be dramatic: a weep at a radiator valve, a pinhole in pipework under a floor, a poorly seated joint behind a kitchen unit. A leak too small to leave a visible puddle can still empty half a bar in a fortnight, because a heating system holds a surprisingly small volume of water. Clues: check radiator valves for crusty white or green deposits, skirting and ceilings below pipe runs for staining, and the boiler's own casing for dampness.
A weeping pressure relief valve
Every sealed system has a safety valve that dumps water outside if pressure climbs too high, through a small copper pipe on your outside wall. Once a PRV has lifted, it often never reseals perfectly, and it drips away your pressure a little at a time. Clue: go outside and look at that pipe. Staining, dripping or a green tail on the wall below it is your answer.
A failed expansion vessel
The vessel is an air cushion that absorbs the water's expansion as it heats. When its diaphragm fails or its charge leaks away, pressure swings wildly: it climbs high when the heating runs, the relief valve vents the excess, and the system reads low again when cool. Clue: pressure that rises hard when hot and falls when cold, on repeat. A vessel recharge is quick; a replacement is routine.
How to repressurise your boiler
The short version, which suits most combi boilers: with the boiler off and cool, open the two valves on the filling loop beneath the boiler until the gauge reads 1.5 bar, close them, and restart the boiler. Our Vaillant F22 guide walks through it step by step with photos, and the process is near-identical on Worcester, Baxi and Ideal models; only the location of the loop and the reset button differ. If your gauge will not hold after a top-up even for a day, stop topping up and get the cause traced; you are watering a leak.
Topping up every week? Our engineers trace pressure leaks across London, usually within the hour, with a fixed quote before any repair.
Why constant topping up makes things worse
Fresh mains water carries dissolved oxygen, and oxygen is what rusts a heating system from the inside. Every top-up feeds a little more of it to your radiators, accelerating the corrosion that becomes black magnetite sludge: the substance behind cold radiator bottoms and, eventually, power flushes. A system that holds its water stays cleaner for years longer than one being constantly refreshed. It is the quiet reason a £150 leak repair is cheaper than it looks.
What will an engineer do, and what does it cost?
A pressure-loss visit starts with the evidence you can describe on the phone: how fast it drops, whether the outside discharge pipe is wet, how the gauge behaves hot versus cold. On site, the engineer pressure-tests the system, inspects the usual suspects and traces the loss to its source. London costs run approximately: leak trace and repair £95 to £250 depending on access, pressure relief valve replacement £110 to £190, expansion vessel recharge £80 to £140 or replacement £180 to £380. Every repair carries a 12-month guarantee, and the fault is explained before it is fixed, with a fixed quote in between.
Boiler pressure, quick answers
No. It is a shutdown condition, not a safety hazard; the risk is to your comfort and, if a leak runs unfound, to whatever it drips on.
Once or twice a year is normal. Monthly is a fault. Weekly is a fault with a deadline.
Yes; much above 2.5 bar cold and the relief valve will vent. Persistent high pressure usually points at the expansion vessel or a passing filling loop valve.
Very rarely. All three common causes are repairs, not replacements.
Gas Safe registered engineer, [X] years in London heating. Registration checkable at gassaferegister.co.uk.