Boiler losing pressure? Here's what's happening

Your boiler should sit between 1 and 1.5 bar when the heating is off. If you're topping it up every few weeks, water is escaping somewhere, and water doesn't escape from a sealed system without a reason.

Check these three things first

Radiator valves. Look underneath every radiator for damp patches or rust streaks. A weeping valve is a cheap fix.

The pressure relief pipe. It's a small copper pipe that comes out through an outside wall, usually near the boiler. If it's dripping, the boiler is dumping pressure, and that points at the expansion vessel.

Recently bled radiators. If you bled them last week, the drop is normal. Top it up once and see if it holds.

When it's the expansion vessel

The expansion vessel absorbs the pressure change as water heats and expands. When it fails, pressure spikes when the heating fires, the relief valve dumps it, and the gauge drops.

It's a repairable fault, and on a boiler under 8 years old it's worth repairing. On a boiler over 12, it's usually the first of several.

The one that isn't fixable

A leak in the heat exchanger. Water is getting somewhere it shouldn't inside the boiler itself. On most boilers, a new heat exchanger costs more than half the price of a new boiler.

If a Gas Safe engineer tells you the heat exchanger has gone and the boiler is over ten, get quotes for a replacement before you spend anything on it.