Pressure Problems
Boiler Losing Pressure? Causes and Fixes
Reviewed by our Gas Safe registered engineers · Last updated 21 July 2026
Pressure that keeps dropping usually means water is escaping somewhere — a small leak, a released valve, or the boiler’s own expansion vessel. Topping up helps for a while, but a persistent drop needs finding. Here’s how.
Quick answer: A boiler that keeps losing pressure is nearly always letting water out somewhere — a small leak at a radiator valve or joint, air released when bleeding, or a failing expansion vessel or pressure-relief valve. Top up to ~1.3 bar to keep going, but if it keeps dropping, the leak needs tracing.
Common Causes
Why a boiler loses pressure
These are the usual causes, roughly in order. The colour on the left shows whether it’s a safe homeowner check or a job for an engineer.
| What you see | Likely cause | What to do |
|---|---|---|
| Pressure drops slowly over weeks | Tiny leak or radiator bleeding | Top up; check for damp patches at valves and rads |
| Pressure drops within a day | A visible leak somewhere | Find and fix the leak; book an engineer |
| Drops just after bleeding radiators | Normal air release | Repressurise to ~1.3 bar |
| Water from the external overflow pipe | Faulty PRV or expansion vessel | Needs an engineer to check |
| Pressure hits zero every few days | Persistent hidden leak | Book an engineer to trace it |
| Gauge always reads low | Failed pressure sensor or gauge | An engineer can test and replace it |
Safe Checks
What you can safely check
Safe to check
- Top the pressure up to ~1.3 bar with the filling loop
- Check for damp patches or drips under radiator valves and pipe joints
- Look for water pooling under the boiler or at the external overflow pipe
- Note how quickly it drops — days, or weeks — to help us trace it
Book an engineer if…
- You’re topping up more than about once a month
- There’s a visible leak from the boiler or a radiator
- Water runs from the external overflow (PRV) pipe
Engineer Insight
What we usually find
A slow drop over weeks is usually a weeping radiator valve or a tiny joint leak — often cheap to sort once found. A fast drop, or water from the overflow pipe, points to the pressure-relief valve or a waterlogged expansion vessel. We pressure-test the system to find exactly where it’s escaping rather than just topping it up.
Costs
What repairs typically cost
Typical North London guide prices — you’ll always get an upfront quote before any work starts.
FAQs
Losing pressure — your questions answered
Why does my boiler keep losing pressure?
Because water is escaping somewhere — usually a small leak at a radiator valve or joint, air released when bleeding, or a failing expansion vessel or pressure-relief valve. Topping up is a temporary fix; the leak needs tracing.
How often should I need to top up the pressure?
Rarely — maybe once or twice a year after bleeding radiators. If you’re topping up every few weeks, there’s a leak or a faulty component worth investigating.
Is losing pressure dangerous?
Low pressure itself isn’t dangerous — the boiler locks out to protect itself. But the leak causing it can damage floors and ceilings over time, so it’s worth fixing promptly.
What is the expansion vessel?
It’s a small pressurised tank that absorbs the expansion of heated water. When it fails or loses its charge, pressure swings and can escape via the overflow pipe. An engineer can recharge or replace it.
Can you find a hidden leak?
Yes — we pressure-test the system and check radiators, valves, the boiler and the overflow to pinpoint where water is escaping, rather than just topping it up and hoping.
Related
Keep troubleshooting
What a low reading means and how to bring it back safely.
Read guidePressureHow to Repressurise a BoilerStep-by-step with the filling loop — back to 1.3 bar.
Read guidePressureBoiler Pressure Too HighThe opposite problem — and how to lower it safely.
Read guideLeaksBoiler Leaking WaterWhen a leak is the reason pressure keeps dropping.
Read guideBoiler ProblemsBoiler Not Turning OnLow pressure is a common reason a boiler won’t fire.
Read guideFault CodesBoiler Fault Code LibraryLook up a low-pressure fault code by brand.
Browse codesBrowse all Pressure Problems guides, or head back to the Help & Advice Centre.
Pressure keeps dropping? We’ll trace it.
Book a Gas Safe engineer to find the leak or check the expansion vessel, or call and tell us how fast it’s dropping — we’ll give you an honest steer first.
Gas Safe Registered 918817 | North London specialists | Upfront pricing