Why heating system pressure keeps dropping between top-ups

How we read gradual system pressure loss on site

The search starts outside: a wet patch under the external pressure relief discharge pipe means the relief valve is passing, often because system pressure is set too high or the valve seat has worn. Indoors, weeping radiator valve glands, a pinhole in old pipework, or a failing expansion vessel diaphragm are the next suspects, checked roughly in order of how quick they are to test.

Things worth knowing about gradual system pressure loss

• An outdoor discharge pipe with staining points at the relief valve
• Damp patches at radiator valve joints often dry between checks
• A vessel with a burst diaphragm makes the gauge swing sharply when heating fires
• Underfloor systems can hide leaks within screed for months
• Never ignore pressure loss — it accelerates internal corrosion

Questions we get asked

Is it safe to keep topping up in the meantime?

Occasional top-ups are fine while a cause is being found, but repeated weekly top-ups should prompt an actual diagnosis rather than a routine.

Could it just be the gauge?

Gauges do fail, but check for physical evidence of water loss first before assuming the display itself is at fault.

Extra detail on gradual system pressure loss

Older stone properties around Radstock with pipework chased into solid floors are the hardest to trace by eye alone, so we lean on pressure testing and, where needed, thermal imaging before any flooring gets disturbed.

Where we deal with this

• Freshford (BA2) · 7 mi
• Mells (BA3) · 4 mi
• Wookey Hole (BA5) · 11 mi
• Berkley (BA11) · 7 mi