Boiler pressure: reading the gauge and using an existing filling loop

Many UK homes have a sealed central heating system. There is no tank in the loft topping the heating up. The water in the radiators is held under a little pressure, and a gauge on or near the boiler shows it. If the pressure drops too low, many boilers stop working and show a fault until it is put right.

This guide covers what a householder can safely do: read the gauge, and use a filling loop that is already fitted, following the boiler maker's instructions. It does not cover fitting, moving or repairing a filling loop, anything inside the boiler casing, or any gas work. For those, see when to call a Gas Safe engineer.

Do you have a sealed system?

You probably do if you have a combi boiler, or a system boiler with a hot water cylinder but no small heating tank in the loft. Older regular (heat-only) boilers are often open-vented, with a feed and expansion tank in the loft and no pressure gauge to worry about. Our explainer on combi, system and regular boilers shows how to tell them apart.

Finding and reading the gauge

The pressure gauge is usually on the front of the boiler or on its control panel. It may be a dial with a needle, often with a green zone and a red zone, or a digital readout. On some boilers it is behind a drop-down flap on the front. On some system boilers it is on the pipework near the boiler or near the hot water cylinder.

Read it when the heating has been off for a while and the system is cold. Pressure rises as the water heats up and expands, so a reading taken with the heating on will be higher.

What a normal cold reading looks like

The right figure is the one in your boiler's user manual. As a general guide, two boiler makers' homeowner pages we read on 13 November 2026 say much the same thing:

• Vaillant says boiler pressure "should generally be between 1 and 1.5 bars when central heating is turned off", and that it should stay below an upper limit of 3 bar when the heating is on.
• Ideal Heating says "the normal cold operating pressure is between 1.0 and 1.5 bar", and that on most gauges this range is shown by green markings.

Your boiler may differ. If the manual gives a different figure, use the manual.

Using an existing filling loop: the general idea

A filling loop is a connection between the mains cold water pipe and the heating system, with one or two small valves. On many combis it is a flexible silver hose or a ridged pipe under the boiler, connecting two pipes in a horseshoe shape. Some boilers have a built-in filling key or lever instead. System and heat-only boilers may have their loop elsewhere, for example near the hot water cylinder or in an airing cupboard.

Only go ahead if all of these are true:

• the loop is already fitted and you can see it clearly, without removing any cover or casing;
• you have the boiler's user manual, or the maker's instructions for your model, and they describe topping up by the householder;
• the hose, if there is one, is already connected at both ends;
• you are sure which valves belong to the filling loop. Ideal's guidance is plain: "Do not operate unidentified valves underneath the boiler."

In general terms, and following your maker's instructions for the details:

1. Turn the heating off and let the system cool.
2. Check the gauge so you know the starting reading.
3. Open the filling valve slowly. If there are two, the instructions will say whether to open one then the other. You may hear water flowing.
4. Watch the gauge the whole time. It can rise quickly.
5. Close the valve or valves firmly when the needle reaches the cold pressure your manual gives.
6. Check for drips at the loop and its valves, and look at the gauge again after a few minutes to check it is not still creeping up.
7. Restart the boiler as the user manual says, if it has locked out.

If the pressure goes too high, do not leave it. Vaillant's page says bleeding a radiator will reduce the pressure, or to ask your installer or a local plumber. Our guide to bleeding radiators explains how.

When to stop and call someone

Stop and call a Gas Safe registered engineer or a heating plumber if:

• you cannot find the filling loop, or you are not sure which valves are part of it;
• the loop is hidden behind a boiler casing or panel you would have to remove;
• there is no hose, or it has been disconnected or removed;
• the pressure will not rise when the valve is open, or the gauge reads zero;
• the pressure keeps rising after you have closed the valves. A filling valve that lets water past when closed needs repair;
• water is leaking from the boiler, a pipe, a radiator or the loop;
• water drips from a small pipe outside, near the boiler. That can be the pressure relief valve discharging;
• the pressure drops again within days or weeks. Ideal says needing to top up once or twice a year can be normal, but more often than that suggests a leak or a fault;
• the boiler shows a fault code that the user manual does not tell you to reset, or it will not restart.

Falling pressure is often a small leak at a radiator valve or a joint. Rising pressure when the heating is on can be a sign of a problem with the expansion vessel, which is a job for an engineer.

Why you should not keep topping up

Every top-up brings in fresh mains water. That dilutes the inhibitor that protects the system from corrosion and sludge. If you keep topping up, the leak or fault is still there and the system is slowly losing its protection. See inhibitor and sludge.

After draining a radiator, for example during a radiator replacement, a drop in pressure is expected. Restore it as above, or ask whoever did the work.

What should my boiler pressure be?

Use the figure in your boiler's manual. Vaillant and Ideal both describe a normal reading of about 1 to 1.5 bar when the system is cold.

Why does my boiler keep losing pressure?

Usually a leak somewhere in the heating system, or a fault with a valve or the expansion vessel. Topping up does not fix it. Call a Gas Safe registered engineer or a heating plumber to find the cause.

Is it safe to top up boiler pressure myself?

Many boiler makers expect householders to use an existing filling loop, following the user manual. Never open the boiler casing, and do not touch valves you cannot identify. If the loop is not obvious, or anything on the stop list applies, call an engineer.