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Boiler Pressure gauge: What It Does and Signs It's Failing

The pressure gauge is the dial (or the bar reading on a digital screen) on the front or underside of a sealed-system boiler. It shows how much pressure the water in your heating circuit is under, measured in bar. It's the one reading on the boiler you're expected to check yourself.

How it works

A dial gauge is linked to the heating water through a thin capillary tube, and the needle moves as pressure rises and falls. Many newer boilers skip the dial and show a figure from the electronic pressure sensor on the display instead. Pressure climbs a little as the water heats and expands, so read it with the system cold.

Signs it is failing

• Needle sitting at zero while the radiators still have water in them
• Reading that never moves between cold and hot
• Dial reading that doesn't match the figure on the boiler display
• Cracked or fogged glass on the dial

Low-pressure lockouts are the codes people see most on combi and system boilers, and the gauge is how you confirm them. A reading well above 2.5 bar when hot points at the expansion vessel or a filling loop left open.

Can you replace it yourself?

Reading the gauge is an owner job. Vaillant and Viessmann put a cold system at around 1 to 1.5 bar, Vaillant's user manual asks for at least 1 bar and says to top up below 0.8 bar, and the reading shouldn't pass 2.5 bar when hot. Replacing a faulty gauge means draining down part of the system inside the casing, so leave that to a Gas Safe registered engineer or, in Ireland, a Registered Gas Installer (RGI).

Buying the right part

Gauges differ in dial size, capillary length and fitting, so the engineer will order by the boiler's model name and GC number from the data badge.

Last reviewed 2026-10-21.