Boiler Pressure Too Low? What To Do

2 October 2026 · 4 min read

Low pressure is one of the most common reasons a combi boiler stops working. Most systems run at around 1 to 1.5 bar when cold — check your manual for the exact figure.

Why pressure drops

Bleeding radiators, small leaks, or a faulty pressure relief valve can all reduce pressure.

How to top it up

Most combis have a filling loop underneath. With the boiler off and cool, open the valves slowly until the gauge reaches the recommended level, then close them. Follow the manufacturer's instructions for your model.

When to call an engineer

If you need to top up more than once every few weeks, there is likely a leak or a failing part. Never remove the boiler casing yourself — only a Gas Safe registered engineer should work on the gas parts of your boiler.