Boiler Keeps Losing Pressure? Here's Why.
Quick answer: If your boiler pressure drops repeatedly, the most common cause is a small leak somewhere in the system — usually at a radiator valve, pipe joint, or the boiler's pressure relief valve. Needing to repressurise once or twice a year is normal. Needing to do it more than once a month means something needs fixing.
On site, "my boiler keeps losing pressure" is one of the most common things customers tell us. The good news: the first step (finding where the water is going) is something you can do yourself with a visual check. The fix might be as simple as tightening a valve. Here's how to diagnose it.
The 5 Most Common Causes
1. Leak in the System
Water is escaping somewhere — even a tiny drip loses enough pressure over days to trigger your boiler's low-pressure lockout. The most common leak points are radiator valves (especially the joints where the valve meets the pipe), pipe connections under floors or behind walls, and the boiler itself.
Most common — ~50% of cases
DIY check: Inspect every radiator valve for dampness. Check visible pipe joints in the airing cupboard and under the boiler. Look for water stains on walls and ceilings below radiators. A leaking valve can often be fixed by gently tightening the packing nut with a spanner.
2. Faulty Pressure Relief Valve (PRV)
The PRV is a safety device that releases water if the pressure gets too high. If it's faulty, it can release water constantly at a slow drip — even when the pressure is normal. You'll usually find a copper pipe running from the boiler to an outside wall.
Common — ~20% of cases
DIY check: Go outside and find where the copper overflow pipe exits your house (usually near the boiler's location). If water is dripping from the end of this pipe, your PRV is discharging. This is a professional fix — the valve needs replacing by a Gas Safe engineer.
3. Expansion Vessel Needs Re-Charging
The expansion vessel absorbs the pressure increase when water heats up. Over time, it loses its air charge and can no longer do its job. This causes pressure to spike when the heating is on (triggering the PRV to release water) and then drop when it cools.
Fairly common — ~15% of cases
Sign to look for: Pressure rises above 3 bar when heating is on, then drops well below 1 bar when off. This is a professional fix — re-charging requires a pump and should be done during a boiler service.
4. Recently Bled Radiators
If you've just bled your radiators, you've released water along with the trapped air. This naturally reduces the system pressure. It's completely normal and not a fault.
Not a fault — just needs topping up
Fix: Simply repressurise your boiler back to 1.2–1.5 bar. If the pressure stays stable after that, there's no problem. This is a 2-minute job.
5. Filling Loop Not Fully Closed
If the filling loop valves aren't fully closed after repressurising, mains water can slowly enter the system and then escape through the PRV. This creates a cycle of pressure rising then falling that looks like a leak but isn't.
Occasionally — ~5% of cases
Fix: Check both filling loop valves are firmly closed (turned fully clockwise). On some boilers, the filling loop should be physically disconnected after repressurising. Check your boiler manual.
Step-by-Step: How to Find the Leak
1
Repressurise to exactly 1.5 bar
Top up the pressure so you have a clear starting point. Note the exact reading. See our repressurising guide if you need help.
2
Check the pressure after 24 hours (heating off)
Leave the heating off for a full day and check the gauge. If it's dropped, water is leaving the system somewhere. If it's stable, the issue may only occur when the heating is running (see expansion vessel).
3
Inspect every radiator valve
Run your finger along each valve connection. Even the slightest dampness means a leak. Pay extra attention to the nut where the valve connects to the radiator — this is the most common leak point in the entire system. A damp towel placed under a suspect valve overnight will confirm it.
4
Check under and around the boiler
Look for drip marks, water stains, or damp patches on the wall beneath the boiler. Internal boiler leaks often show up as moisture on the underside of the casing.
5
Check the PRV overflow pipe outside
Go outside and find the copper pipe that exits the wall near your boiler. If it's dripping or there are water stains below it, the PRV is releasing water. This needs a professional fix.
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Save Money vs Call a Professional
| Fix | DIY Cost | Plumber Cost |
|---|---|---|
| Tighten leaking radiator valve | £0 (free) | £80 – £150 |
| Repressurise after bleeding | £0 (free) | £80 – £150 |
| Close filling loop properly | £0 (free) | £80 – £120 |
| Replace pressure relief valve | Professional only | £150 – £300 |
| Re-charge expansion vessel | Professional only | £80 – £200 |
| Leak diagnostic visit | — | £60 – £120 |
When to call a professional: If you can't find a visible leak, if the PRV is discharging, if pressure swings wildly between heating on and off (expansion vessel), or if you see water coming from inside the boiler casing. Never open the boiler casing yourself — only Gas Safe registered engineers should work inside the boiler.
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Frequently Asked Questions
The five most common causes are: a small leak (radiator valves, pipe joints, or the boiler itself), a faulty pressure relief valve slowly discharging water, an expansion vessel that needs re-charging, recently bled radiators that reduced water volume, or a filling loop that isn't fully closed. Leaks account for roughly half of all cases.
Once or twice a year is perfectly normal, especially after bleeding radiators or during seasonal temperature changes. If you're topping up more than once a month, there's almost certainly a leak or component fault that needs investigating by a heating engineer.
No. Low pressure is not dangerous. Your boiler will simply lock out and refuse to fire until the pressure is restored — this is a built-in safety feature. You won't have heating or hot water, but there's no risk of explosion or gas leak from low pressure alone.
Depends on the cause. Tightening a leaking radiator valve yourself costs nothing. A plumber diagnostic visit costs £60–£120. Replacing a pressure relief valve costs £150–£300 fitted. Re-charging an expansion vessel costs £80–£200. Finding and fixing a hidden leak can cost £100–£300 depending on accessibility.