Boiler losing pressure — where's the water going?
If you top the pressure up and it drops again within days, water is leaving the system somewhere. It's rarely evaporation — usually a weeping radiator valve, a failing pressure-relief valve dumping outside, or a slow leak into a floor.
Make it safe — before anything else
Only Gas Safe registered engineers work on gas
Anything behind the boiler casing — expansion vessel, PRV, internal joints — is Gas Safe. External radiator valve tightening is safe DIY.
Don't over-pressurise
Never take the boiler above 1.5 bar cold. Over-pressurising pushes water out of the PRV and can burst the expansion vessel diaphragm.
What Kevin (our AI) will check first
Message Kevin and he'll ask exactly these questions before we send an engineer, so we bring the right parts on the first van.
- Boiler make, model, age.
- Current pressure gauge reading, and reading after 24 hours.
- Is the outside PRV pipe dripping?
- Any damp patches on ceilings, walls, or around radiators?
- Recent work — new radiator, powerflush, filling loop left open?
Common causes of a boiler losing pressure
Failed expansion vessel
The vessel inside the boiler cushions pressure changes. When its diaphragm fails, pressure swings wildly and dumps through the PRV. £180 – £280 to re-charge or replace.
Weeping radiator valve or TRV
Slow drip from a compression joint under a radiator. Look for rust marks or damp carpet. £70 – £110 to replace the valve.
Pressure-relief valve stuck open
Once a PRV has released, the seat often weeps thereafter. Continuous slow drip outside = replace the PRV. £120 – £180.
Hidden leak into a floor or wall
Under-floor pipe, buried joint or a slow drip in a wall. Thermal-imaging or acoustic detection finds it — see our leak-detection page.
Failed auto air-vent
AAV weeps a tiny amount continuously, draining pressure over a week. Small fix, easily missed without inspection.
Honest cost band — why we won't quote blind
Typical
£90 – £180 for PRV, filling-loop or radiator-valve replacement
Complex
Expansion vessel re-charge or replace £180 – £280. Hidden-leak detection £220 – £360.
£50 diagnostic includes pressure-loss testing and a written repair plan — no guesswork.
Fixed £50 diagnostic
Flat fee. No hourly meter running.
Fair quote or refund
Transparent pricing — refunded if we don't fix it.
Vetted UK engineers
DBS-checked, insured, real reviews.
AI-triaged in seconds
KJ chats first so the right engineer arrives.
Related emergencies
See all in the emergency plumbing hub — or the leaks & water damage hub.
Areas we cover for boiler losing pressure
Same-day across Greater Manchester and Rochdale.
Boiler losing pressure — FAQs
Dealing with a boiler losing pressure? Chat with Kevin now
Real triage in seconds. £50 diagnostic visit — refunded if we don't fix it.
Related hubs: emergency plumbing, leaks & water damage, all services.
Chat with Kevin about Boiler losing pressure