Boiler Keeps Losing Pressure: Causes & What To Check
If your boiler pressure keeps dropping — even after you've topped it up — there's a fault that won't fix itself. The four usual causes are a system leak, a passing pressure relief valve, a failed expansion vessel, or a weeping filling loop. Here's what it means, what you can safely check, and what it costs to put right.
What does it mean when a boiler keeps losing pressure?
Your heating system is a sealed loop of water under pressure — normally between 1.0 and 1.5 bar when cold. If pressure keeps falling, water is leaving the system somewhere: through a leak, a passing safety valve, or a faulty expansion vessel. Topping up restores it temporarily, but the underlying fault needs finding before it causes damage.
Why does my boiler keep losing pressure?
The four most common causes, in order of how often we see them:
- • A leak on the system — even a tiny weep from a radiator valve, joint, or hidden pipe drops pressure slowly. If you can't see it, it may be under floors or in walls.
- • Pressure relief valve (PRV) passing — the PRV dumps water if pressure gets too high; if it starts opening at normal pressure, the system slowly loses water.
- • Failed or flat expansion vessel — the vessel absorbs pressure changes as water heats and cools; if its diaphragm fails or it loses its air charge, pressure swings and the PRV dumps water.
- • Filling loop left open or weeping — the filling loop should be closed after topping up; if it's left slightly open or its valves weep, pressure behaves erratically.
Can I just keep topping it up?
You can safely repressurise via the filling loop, and an occasional top-up is normal. But if you're repressurising every few days, you're masking a real fault — and continually adding fresh water brings in oxygen and minerals that scale up the heat exchanger and shorten the boiler's life. Find the cause instead of feeding it.
Can I fix this myself?
Safe homeowner checks
- Repressurise to 1.2 bar via the filling loop
- Look for visible leaks at radiator valves and joints
- Check the PRV discharge pipe outside for drips
- Bleed radiators and re-check the pressure gauge
- Read the pressure gauge and note the reading
Engineer required
- Replacing the pressure relief valve (PRV)
- Recharging or replacing the expansion vessel
- Tracing a hidden leak under floors or in walls
- Repairing a weeping filling loop or valve
- Any work inside the boiler casing
Gas work legally requires a Gas Safe Registered engineer. Tampering with gas appliances is dangerous and unlawful.
Safe checks you can make first
- • Repressurise to around 1.2 bar via the filling loop. If pressure holds for days, you may have just needed a top-up; if it drops again within hours or a day, there's a real fault.
- • Look for visible leaks — check radiator valves, pipework under the boiler, and around joints for damp, green scale, or drips.
- • Check the copper PRV discharge pipe outside — if it's dripping when the boiler is at normal pressure, the PRV is passing and needs replacing.
- • Bleed radiators to release trapped air, repressurise, and watch the gauge. If pressure still falls, book an engineer to trace the cause.
Need this fault diagnosed? Book a Gas Safe engineer.
What should you NOT attempt yourself?
The pressure relief valve, expansion vessel, and any internal diagnosis sit inside the boiler casing and are engineer work. If there's no visible leak and pressure still falls, the leak may be under a floor or behind a wall — that needs professional leak detection, not DIY.
What will a Gas Safe engineer check?
- • Full pressure-loss diagnosis across the system
- • Pressure relief valve (PRV) test and inspection
- • Expansion vessel pressure check and recharge
- • Filling loop and auto-air vent inspection
- • Visual leak check, plus hidden leak detection if needed
- • System repressurise and re-commissioning
How Urban Suburban can help
Our Gas Safe Registered (594547) engineers find and fix pressure-loss faults across Surrey, Berkshire, Buckinghamshire and West London — from a passing PRV to a hidden underfloor leak found with thermal imaging and pressure testing. Most diagnoses complete within the first hour, with any part quoted and agreed before fitting.
Related help guides
Frequently asked questions
Why does my boiler keep losing pressure?
The four most common causes are a leak somewhere on the system, a passing pressure relief valve, a failed or flat expansion vessel, or a filling loop left open or weeping. A slow pressure drop almost always means one of these.
Is it safe to keep repressurising my boiler?
Topping up occasionally is fine, but if you're repressurising every few days you're masking a real fault. Continually adding fresh water also brings in oxygen and minerals that scale up the heat exchanger and shorten the boiler's life.
How much does it cost to fix a boiler losing pressure?
Most pressure-loss repairs cost £90–£220 in labour plus the £150 call-out (which includes the first hour). If a part like the PRV or expansion vessel needs replacing, that's quoted and agreed before fitting.
Can a boiler lose pressure without a leak?
Yes. A passing pressure relief valve or a failed expansion vessel can dump water through the PRV discharge pipe outside — so pressure falls without any visible leak indoors. Check the outside pipe for drips.
Technical content & review
Technical content by URBAN SUBURBAN GAS
Gas Safe Registration: 594547 · NVQ Qualified engineers
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