How to Repressurise a Boiler Safely

The safe procedure for topping up boiler pressure, how much is normal, and the point at which you should stop and call an engineer.

Last updated: 2026-10-19

If your boiler has locked out or the heating has stopped, the pressure gauge is the first safe thing to check. Low pressure is one of the most common reasons a boiler refuses to run, and topping it up is usually something a homeowner can do safely.

What the pressure should be

With the system cold, most boilers should read between 1 and 1.5 bar. When the heating is running the reading rises — sometimes to around 2 bar — which is normal expansion, not a fault.

If the gauge reads below about 0.8 bar, the boiler’s low-pressure switch may prevent it firing.

Before you start

• Let the system cool down — never top up a hot system.
• Find your filling loop. On most combi boilers it is a flexible braided hose under the boiler with one or two small taps or a key.
• Check for obvious leaks under the boiler and around radiators. Topping up a system that is actively leaking just adds more water to the problem.

The procedure

1. Note the current pressure on the gauge.
2. Open the filling loop valve(s) slowly — a quarter turn is often enough.
3. Watch the gauge. You will hear water entering the system.
4. Stop at 1.2 bar while cold. Do not overfill.
5. Close the valve(s) firmly and disconnect the loop if it is a removable type.
6. Bleed any radiators that need it, then re-check the pressure afterwards, as bleeding lowers it.
7. Reset the boiler if the manual says to, and see whether it fires.

If pressure keeps dropping

A single top-up is normal — systems lose a little water over a year. Pressure that falls again within days is not normal. It usually points to a small leak, a faulty expansion vessel, or a pressure relief valve that is passing water outside.

Read our guide to why boilers lose pressure for the likely causes, then book a boiler repair visit.

When to stop and call an engineer

Stop and call if:

• pressure will not hold and drops again within days;
• water is dripping from the copper pipe outside (the pressure relief pipe);
• you can see water leaking from the boiler casing;
• the gauge sits very high (above 2.5 bar cold) or swings wildly;
• the boiler still will not fire after correct pressure is restored.

Never open the boiler casing, and never adjust gas pipework. If you smell gas, call the National Gas Emergency Service on 0800 111 999 immediately.