Boiler losing pressure: what the gauge means

Most modern boilers have a pressure gauge on the front, and most of the panicked calls we get about it aren't emergencies. Here's how to read it like an engineer does.

What the gauge should read

Cold, your system should sit at roughly 1 to 1.5 bar — most gauges mark this zone in green. When the heating is running, it's normal for the needle to rise a little (up to around 2 bar). Below about 0.5 bar, most boilers lock out and show a low-pressure fault code.

Topping it up

Under or near the boiler you'll find the filling loop — usually a braided silver hose with one or two small valves. With the heating off and cool: open the valve(s) slowly, watch the needle climb to about 1.2 bar, close them again firmly. That's it. Your boiler manual (or the maker's app) shows the exact loop for your model.

The bit that matters: how often?

Topping up once or twice a year is normal — water expands, systems breathe, tiny amounts escape. Topping up every week is not normal: the water is going somewhere. Usual suspects are a weeping radiator valve, a failing expansion vessel, or the boiler's pressure relief valve letting by — look for a drip from a copper pipe that exits through your outside wall.