Gauge falling only when the heating ran
The property
A detached 1990s house at the edge of town, its downstairs radiator pipes laid in a concrete floor under tiles and carpet.
The problem
The gauge dropped on days the heating ran and barely moved when it was off. No sealer had been added, and nothing looked damp.
How we found it
With the circuits isolated, only the downstairs one lost pressure. Once the floor was warm, thermal imaging showed heat spreading from one point in the hall, and tracer gas rose there.
The outcome
With the spot marked and photographed, one tile and a patch of screed were lifted. The joint underneath was repaired while we were there, quoted separately.