Why Does My Boiler Pressure Keep Dropping?

A sealed heating system should not need constant topping up

After a system has been filled and bled, the cold pressure should normally remain reasonably stable. A small change with temperature is expected, but needing to add water every few days or weeks means pressure is being lost or the gauge is not reflecting a healthy system. Repressurising restores the reading but does not repair the cause. Keep a note of the cold pressure and how quickly it falls.

Start with visible radiator and pipe leaks

Check radiator valves, bleed screws and accessible heating pipework for dampness or staining. Tiny leaks can evaporate on hot surfaces and leave only a crust or discolouration. Towel rails and valves hidden behind furniture are easy to miss. If one joint is wet, that may be enough to explain a gradual loss. Do not repeatedly tighten heating fittings while the system is pressurised; a small seep can become a larger leak.

Look at the pressure-relief discharge outside

Most sealed boilers have a safety discharge pipe that terminates outside. If water is dripping from it, the pressure-relief valve may be passing. That can happen because the valve itself has been disturbed or because another fault is causing excessive pressure as the system heats. Note whether the outside drip appears only when the boiler is hot. Do not block or cap the discharge pipe; it is a safety outlet.

Expansion-vessel faults can create a rise-and-fall pattern

If the expansion vessel has lost its air charge or failed, pressure may climb sharply when the heating runs and then fall very low as it cools, sometimes after the relief valve has discharged water. This pattern is different from a steady slow leak. Boiler-internal work should be carried out by an appropriately qualified heating engineer. Recording the cold and hot gauge readings is very useful before the visit.

Concealed pipe leaks are possible but should not be assumed first

Heating pipes under floors can leak without obvious water at the surface, but visible valves, boiler discharge and pressure-control components are easier to check and often more likely. If no external source is found, the system can be investigated more systematically. Do not agree to lifting floors solely because the gauge falls; the pressure pattern and isolation tests should support that conclusion.

What to tell the heating engineer

Say how often you top up, what the pressure reads when cold and how high it rises when fully hot. Mention any visible radiator seepage, recent bleeding and whether the outside discharge pipe is wet. If the boiler shows fault codes when pressure drops, record them. Those details help distinguish a straightforward leak from an expansion or relief-valve problem and make the first visit far more productive.

Avoid creating a cycle of topping up and losing water

Fresh mains water contains oxygen and minerals, so constantly refilling a heating system is not ideal even if it keeps the boiler running. Once the cause is repaired, the cold pressure should settle at the level recommended for the system and stay broadly stable between heating cycles. Watch the gauge for several days and check that the outside relief discharge remains dry. If the pressure still rises sharply when hot, tell the heating engineer because that can point back to the expansion side. If it falls with no visible leak, further investigation may be needed. The useful outcome is a system that maintains pressure without regular intervention, not one that simply lasts a little longer between top-ups.

If you have recently had radiators removed, replaced or bled, tell the engineer because a short period of pressure adjustment can follow system work. A pressure problem that began months later is less likely to be simple commissioning air. The timing helps separate a new fault from the aftermath of maintenance.

If the boiler has recently been serviced, do not assume the service caused the pressure loss, but do mention the timing. A technician can then check whether any valves or components disturbed during the work are now showing signs of seepage.