A boiler loses pressure when water escapes from the heating system or a component can’t hold pressure. The usual culprits are a small leak on a radiator, valve or pipe joint, a pressure relief valve letting water through, an expansion vessel that’s lost its charge, or air released after bleeding radiators.
On most sealed systems, the gauge should read around 1 to 1.5 bar when cold. If you’re topping up more than once or twice a year, there’s an underlying fault worth fixing. Our heating team can track it down.
Quick diagnosis guide
| What you notice | Likely cause |
| Pressure dropped after bleeding radiators | Normal; top up |
| Slow drop over days or weeks | Small leak somewhere in the system |
| Pipe outside dripping | Pressure relief valve discharging |
| Pressure shoots up when heating on, then drops | Expansion vessel needs attention |
| Water under the boiler | Internal leak; engineer needed |
| Damp patch on a ceiling | Leak on pipework or a radiator above |
Checks you can make safely
- Look around radiators for drips, damp or corrosion at valves and joints
- Check under the boiler for water or staining
- Look outside for the copper discharge pipe from the relief valve and see if it’s dripping
- Watch the gauge with the heating off and on, noting how far it rises
Don’t remove the boiler casing or adjust internal parts.
Topping up pressure
Use the boiler’s filling loop or filling key, following the manual:
- Heating off and cooled
- Open the valves slowly
- Close them at the recommended pressure
- Reset the boiler if it has locked out
If you can’t find the filling loop, ask an engineer.
Why it matters to fix leaks
- Water damage to floors and ceilings
- Fresh water entering the system brings oxygen, which speeds up corrosion inside radiators
- The boiler may lock out when you need it most
Our post on why some radiators don’t heat up properly covers related heating problems.
When to call an engineer
- Topping up more than once or twice a year
- Pressure rising close to 3 bar when the heating runs
- The relief valve pipe keeps dripping
- A leak you can’t find or reach
- Any water inside the boiler
Pressure problems in older Oxfordshire homes
In period homes around Abingdon, Wallingford and Oxford, older radiators and pipework can start leaking after a switch to a sealed system with a combi boiler, because they’re now running under pressure. If pressure problems started after a boiler change, have the whole system checked. If sludge is part of the problem, power flushing may help.
Frequently asked questions
Why does my boiler lose pressure overnight?
Usually a slow leak or the relief valve letting water past. An engineer can pressure-test the system to find it.
Can low pressure stop my hot water?
Yes. Many combi boilers won’t run heating or hot water if pressure is too low.
Is it safe to keep topping up?
Occasionally, yes. Doing it often just masks a leak and adds fresh water that encourages corrosion.
Get it sorted
If your boiler keeps losing pressure, give us a ring on 01865 817 611 or send us a message and we’ll come and find out why.
