Why Is My Boiler Losing Pressure?
A boiler that keeps losing pressure almost always points to a leak or a failed expansion vessel. Here's how to narrow down the cause and what to do next.
What boiler pressure actually means
Your boiler's pressure gauge shows the pressure of the water inside your central-heating system. When the system is cold it should sit between 1.0 and 1.5 bar. If it falls below about 0.5–0.6 bar the boiler will lock out to protect itself from running dry — that's when most homeowners notice the problem, because the heating stops and a fault code appears.
The 5 most common reasons a boiler loses pressure
Pressure doesn't drop on its own — water is leaving the system somewhere. These are the causes we find again and again on call-outs across Glasgow:
- A slow leak on a radiator valve or joint — often so small it evaporates before dripping, so you never see water.
- A leaking pressure-relief valve (PRV) discharging water outside through the copper blow-off pipe.
- A failed or discharged expansion vessel — the most common cause of pressure that rises high then crashes to zero.
- Air recently bled from the radiators, which lowers the system volume and therefore the pressure.
- A pinhole leak inside the heat exchanger, which usually shows as pressure loss combined with a fault code like Vaillant F75.
How to tell which one you have
Repressurise the system to 1.0–1.5 bar using the filling loop, then watch what happens. If the pressure climbs above 3 bar while heating and then drops to zero, the expansion vessel has failed. If it falls slowly and steadily over a few days with no visible water, suspect a micro-leak on a valve or joint. Check outside under the PRV pipe — a damp patch or drip there means the relief valve is passing.
What you can safely check yourself
Topping the pressure back up is a homeowner job and will get your heating running again, but it is treating the symptom, not the cause. The leak or faulty vessel still needs finding and fixing, otherwise the pressure will keep falling and the boiler will keep locking out.
- Top up to 1.0–1.5 bar via the external filling loop, then close both valves fully.
- Check all radiator valves and joints for green stains or damp — a tell-tale sign of a micro-leak.
- Look outside under the PRV blow-off pipe for drips or a damp patch.
- Bleed a radiator that feels cold at the top, then re-check pressure.
When to call a Gas Safe engineer
If the pressure drops again within a week of topping up, you have a leak or a failed expansion vessel that needs professional repair. Repeatedly repressurising a leaking system wastes water, can introduce fresh oxygen that accelerates corrosion, and in some cases a passing PRV can cause the system to lose inhibitor — leaving radiators to rot from the inside. Book an engineer to find the leak, replace the vessel or valve, and re-dose the system with inhibitor.
Frequently asked questions
Why does my boiler keep losing pressure overnight?
An overnight pressure drop almost always means water is leaving the system — a micro-leak on a radiator valve, a passing pressure-relief valve, or a failed expansion vessel. Topping up the pressure gets the heating back on, but the cause needs a Gas Safe engineer to find and repair.
Is it dangerous if my boiler pressure keeps dropping?
Not immediately — the boiler locks out at low pressure to protect itself. But the underlying leak wastes water, can let fresh oxygen into the system causing corrosion, and the boiler won't run until it's fixed. It's not an emergency unless you smell gas or see a major leak.
What pressure should my boiler be at?
Between 1.0 and 1.5 bar when the system is cold. Below about 0.5–0.6 bar the boiler will lock out (commonly showing a low-pressure fault code such as Vaillant F22 or Baxi E119). Above 3 bar the pressure-relief valve will discharge water outside.
Can I fix low boiler pressure myself?
Yes — repressurising via the filling loop is a homeowner job. But if the pressure keeps dropping, you have a leak or a failed expansion vessel that needs a Gas Safe engineer to repair.