Why does my boiler lose pressure?

Bleeding, a leak or a failed expansion vessel — how fast it drops tells you which.

Difficulty: beginner
Time: 30 min
Read: 2 min
Safety: warning

Short answer

Repressurise via the filling loop to 1-1.5 bar cold. But how quickly it drops is the diagnosis: over months is normal, over days means a leak, and pressure that swings up when hot and down when cold suggests the expansion vessel.

Topping up a boiler is a normal task. Needing to do it repeatedly is not, and treating it as routine maintenance means a slow leak carries on damaging something out of sight.

Safety

Repressurising via the filling loop is a normal user task on most boilers, but do not attempt any other work on a gas appliance — it must be done by a Gas Safe registered engineer. If pressure drops repeatedly over days rather than months, there is a leak, and a leak inside a wall or floor causes serious damage.

Step by step

Find the pressure gauge and the filling loop.The gauge should read 1-1.5 bar when the system is cold. The filling loop is usually a braided hose with one or two valves beneath the boiler.
Repressurise with the heating off and cold.Open the valves slowly, watch the gauge, and close them as it reaches about 1.5 bar. Do not overfill — too high is as much a problem as too low.
Note how fast it drops.The most useful diagnostic step. Over several months: normal, top up occasionally. Over days or weeks: there is a leak. Immediately after bleeding radiators: expected, and not a fault.
Check for visible leaks.Around radiator valves, pipe joints, and under the boiler itself. Small drips evaporate and leave a faint white or greenish stain rather than a puddle.
Check the pressure relief discharge outside.A small pipe emerging through an external wall near the boiler. Water dripping from it means the system is discharging — usually a failed expansion vessel or an overfilled system.
Suspect the expansion vessel if pressure swings.Very high when hot, very low when cold. The vessel absorbs expansion, and when it fails the pressure has nowhere to go. An engineer can recharge or replace it.
Look for hidden leaks if nothing is visible.Under floors and in walls. Damp patches, unexplained warm spots on a floor, or a musty smell. This needs finding — a slow leak in a floor void does substantial damage.
Get an engineer for anything beyond topping up.Repressurising is a user task; finding and fixing a leak, or replacing an expansion vessel, is not.

Tips

Photograph the gauge weekly for a month. The rate of loss is the diagnosis and it is hard to judge from memory.
Pressure always drops slightly after bleeding radiators — that is the air being removed, not a fault. Top up afterwards.
Do not keep topping up indefinitely. Water constantly entering the system brings fresh oxygen with it, which accelerates corrosion and sludge.

If it doesn't work

Drops over months

Cause: Normal minor loss — Fix: Top up occasionally. Nothing further needed.

Drops over days

Cause: A leak somewhere in the system — Fix: Check visible joints and radiator valves, then get an engineer to trace it.

High when hot, low when cold

Cause: Failed expansion vessel — Fix: Engineer — the vessel needs recharging or replacing.

Water dripping from an outside pipe

Cause: Pressure relief valve discharging — Fix: Usually overfilling or a failed expansion vessel. Engineer.

Questions people ask

What pressure should my boiler be?

Usually 1-1.5 bar when the system is cold, though check what your model states. It rises when the heating is on, which is normal.

How often should I need to top up?

Occasionally — once or twice a year is unremarkable. Weekly means there is a leak, and that needs finding rather than topping up.