Boiler Pressure Problems: Why It Keeps Dropping (Or Won't Stop Rising)

This points to a leak that only shows itself under the higher pressure and temperature of an active system, or a faulty pressure relief valve opening slightly during heating cycles.

Occasionally, yes — but repeatedly topping up a system that keeps losing pressure means water is going somewhere, and doing it for months can hide a leak that gets worse. Get it looked at if you’re topping up more than once every couple of months.

A sudden rise usually means the filling loop wasn’t fully closed after last use, or a failed expansion vessel that’s no longer absorbing the normal expansion of heated water.

Yes — water contracts slightly as it cools, so pressure often reads a touch lower on very cold mornings. A drop of 0.1–0.2 bar with temperature is normal.

A well-sealed system shouldn’t need topping up more than once or twice a year at most. More frequent than that suggests a leak somewhere, even a very small one.

The expansion vessel absorbs the natural expansion of water as it heats, keeping pressure steady. The pressure relief valve is a safety device that releases water externally if pressure gets dangerously high. A failing expansion vessel is often what causes the relief valve to open.

Not dangerous in itself, but it will slowly over-pressurise the system if left unnoticed, and it wastes water. Worth checking it’s fully disconnected or closed after every use.

If it drops straight back to red within minutes or hours of topping up, that’s a strong sign of an active leak rather than a one-off low reading — worth booking an engineer rather than continuing to top up.

Most boilers simply lock out and refuse to fire below a certain pressure as a protective measure, so the immediate risk is losing heating and hot water rather than damage. But the underlying cause can worsen if left unaddressed.

Sealed combi and system boilers need occasional manual repressurising via a filling loop, whereas older gravity-fed systems with a loft header tank usually top themselves up automatically. If you have a sealed system and top up unusually often, that’s a stronger signal of a leak.

Some newer smart thermostats and connected boilers can alert you to a pressure drop before it causes a lockout, which is genuinely useful for catching a slow leak early. It’s not a substitute for diagnosing the cause, but an early alert means you’re likely to book while it’s still a simple fix.