Boiler losing pressure: how to find the leak that is not obvious

Practical notes on a system losing pressure

• Gauge swinging more than 1 bar between cold and hot suggests the vessel
• A dripping outside overflow pipe means the relief valve is passing
• Radiator valve glands weep slowly and dry out before you see water
• Recharging a vessel is a routine job, not a boiler replacement
• Repeated top-ups add fresh oxygenated water and accelerate corrosion

How we read a system losing pressure on site

There are only three routes out: a physical leak, the pressure relief valve discharging, or an apparent loss caused by a flat expansion vessel making the gauge swing wildly between hot and cold. We start with the vessel because it is quick to test, then check the relief pipe outside for evidence of discharge, then pressure-test the pipework. Concealed leaks under solid floors are the last suspect, not the first.

A system losing pressure in Somerset and North Somerset homes

In stone-floored cottages around Chilcompton and Kilmersdon we often trace losses to pipework buried in screed with no sleeving. Thermal imaging plus a pressure test isolates the run before anyone lifts a flagstone, which matters when the floor cannot be replaced like for like.

Where we deal with this

• Temple Cloud (BS39) · 5 mi
• Stanton Wick (BS39) · 8 mi
• Redhill (BS40) · 11 mi
• Congresbury (BS49) · 15 mi
• Sutton Veny (BA12) · 15 mi

Questions we get asked

What pressure should my boiler sit at?

Most sealed systems run around 1 to 1.5 bar cold. Check your manufacturer's plate — some vary with system height.

Is topping up harmful?

Occasionally, no. Weekly top-ups introduce oxygen and dissolved minerals that corrode radiators from the inside.