What the gauge should read
Most domestic boilers sit between 1.0 and 1.5 bar when cold. If yours keeps sinking below 1.0 and the boiler locks out, water is escaping somewhere. Pressure loss is a symptom, not a fault in itself.
The three common causes
A leak on the system. Often a radiator valve, a towel rail joint, or pipework under floorboards. Small leaks can evaporate before you ever see a puddle.
A failed expansion vessel. The vessel absorbs expansion as water heats. When its charge is lost, pressure swings wildly between hot and cold, and the pressure relief valve discharges outside.
A passing pressure relief valve. If there is a steady drip from the small pipe outside near the boiler, the valve is letting by and needs replacing.
What is safe to do yourself
Topping up via the filling loop once is fine, and your manual shows how. If you are topping up more than once a month, stop and call an engineer. Repeated topping up introduces fresh oxygenated water, which accelerates corrosion and sludge.
When to call us
Call if pressure drops within days of filling, if you can see damp patches near pipework or radiators, or if water discharges from the outside overflow pipe. We trace the actual cause rather than topping it up and leaving.
