Boiler Losing Pressure? The Real Causes (And When It Means a Hidden Leak)

Every combi boiler owner knows the ritual: pressure gauge drifting down, a fiddle with the filling loop, back to 1.5 bar, repeat next week. The ritual is so common that people forget what it means — sealed heating systems are sealed. If pressure keeps dropping, water is leaving the system. Every time.

Where the water goes: four suspects

1. Radiator valves and joints

The most common and cheapest culprit. Check every radiator: valve bodies, gland nuts, and the connections into the radiator itself. Look for weeping — a slight wetness or crust of scale rather than a drip. Bleed points can also seep. A towel run around each joint on a warm system finds most of these.

2. The pressure relief valve (PRV)

The PRV discharges water outside through a copper pipe on your external wall if system pressure climbs too high — and once a PRV has operated, it often never reseals perfectly. Check the discharge pipe outside: dripping or staining means the valve is passing water continuously. The gauge behaviour that goes with this is pressure that drops and a valve that dumps water when the system heats up.

3. The expansion vessel

The expansion vessel absorbs the volume change as heating water warms and cools. When its internal air charge is lost, pressure swings wildly — high when hot, low when cold — and the PRV starts discharging the excess. The classic signature: you top up to 1.2 bar cold, the gauge hits 2.5+ bar hot, and by next morning it is below 1 again. This is a component fix, not a leak — but it causes real water loss through the PRV.

4. A hidden leak on the heating pipework

When the boiler components check out and the radiators are dry, what remains is the pipework you cannot see — under floors, in walls, buried in screed. A pinhole in a buried heating pipe can pass litres a day when the system is hot and pressurised, and the first visible symptom is often nothing more than the dropping gauge. By the time a damp patch appears, the leak has usually been running for months.

Diagnosing in the right order

1. Cold overnight test. Pressurise to 1.5 bar, heating off, check in the morning. A drop with the system cold and PRV dry points firmly at a pipework leak.
2. Check the PRV discharge pipe outside for drips and staining.
3. Inspect every radiator joint with the system warm.
4. Test the expansion vessel — a job for an engineer with a gauge, and often just a recharge.
5. If all clear: trace the pipework. This is where central heating leak detection comes in — thermal imaging with the system hot paints the buried pipe runs through the floor and shows the leak as a spreading warm bloom.

Why "just keep topping it up" is expensive advice

Fresh water brings fresh oxygen and minerals into a system designed to run on the same deoxygenated water for years. Constant topping up accelerates internal corrosion, breeds sludge, shortens boiler life — and if the cause is a buried leak, it finances the slow soaking of your floor structure. The top-up is not the fix; it is the meter running.

If your gauge will not hold for a week, book a diagnosis. We check the boiler components first and only go looking under floors when the evidence says so.