Fault finding: reasoning to the likely cause

Diagnostic questions are not asking you to know one fact. They give you a symptom and ask which cause best explains it, so the skill being tested is reasoning, not recall.

A method that works on any of them

1. State the symptom precisely, including when it happens. "Cold radiator" and "cold radiator only

when the neighbouring valves are open" are different faults.

1. Decide which system it belongs to — supply, hot water, heating circuit, or above-ground drainage.

Half the wrong answers in a scenario question come from another system entirely.

1. Check the simple, local, accessible things first. Many faults have simple causes, and a check

costs minutes while a strip-down costs a day. This is not because complex faults are rare; it is because the cheap checks either find it or narrow it.

1. **Prefer the cause that explains *all* the evidence**, including the parts that seem incidental.
2. Prove it before you replace anything. Swapping parts until the symptom goes is not diagnosis.

One useful rule: a fault affecting one outlet is usually local to that outlet, and a fault affecting several is usually upstream of them all.

The patterns that keep coming up

Trap seals and drainage. A basin trap that loses its seal after a long waste run was added points to self-siphonage — the waste arrangement itself is pulling the seal out. A trap that *gurgles* when another appliance discharges points instead to drainage ventilation and seal disturbance.

Flow balance in heating. A pump that becomes noisy as most thermostatic valves close is seeing excess differential pressure with no bypass route left open. A radiator that heats only while nearby valves are open is a flow-balance or control-interaction problem, not a failed radiator.

Sizing against duty. A room that stays cold after a radiator was swapped for a smaller decorative model has an emitter whose output no longer matches the room heat loss. Nothing is broken; the design is.

Pressure loss. A sealed system losing pressure overnight with no visibly wet valve is a leak, a relief valve passing, or an expansion-vessel fault — and a tundish discharging intermittently on an unvented cylinder is the same family: a control, expansion or safety-valve fault needing competent assessment, never something to leave running.

Local restriction. One new tap running weakly while its neighbours are fine is a local isolation valve, aerator or tap connector, not the street main.

Stagnation and hygiene. Warm, stale water at first draw-off from a little-used outlet means low turnover or heat gain in that branch. A storage cistern with slime, a missing lid and warm water is a water-quality defect and is treated as one.

*Original revision guidance; confirm design and hygiene detail against the cited public sources.*