Why boilers lose pressure
A slow, occasional drop over weeks or months is often just normal system loss — small amounts of air and water escape from a sealed heating system over time, and topping up now and then is completely routine. It's a different story if the pressure drops quickly, or repeatedly, within days.
- A pinhole leak somewhere in the pipework — sometimes visible as damp patches, sometimes not visible at all if it's under a floor.
- A leaking pressure relief valve — often shows as water dripping from an external pipe outside the property.
- Bled radiators — if you've recently bled air from your radiators, that releases some pressure too.
- A fault inside the boiler itself — a failing expansion vessel or internal seal, which needs a proper inspection to confirm.
What's safe to check yourself
Most boilers have a filling loop — a small braided hose connecting the cold mains to the heating system. If your manual shows one and you're comfortable with it, slowly topping the pressure back up to around 1.2–1.5 bar is safe. If pressure drops back down again within a day or two, that points to a genuine leak rather than normal loss, and it's worth getting looked at rather than repeatedly topping up.
Pricing
Diagnosis and first hour: £99. If a part such as a pressure relief valve or expansion vessel needs replacing, that's quoted clearly before it's fitted.