Why does my boiler keep losing pressure?

Low boiler pressure is one of the most common reasons a boiler stops working. The good news is that topping it up is usually something you can do yourself. The bad news is that if it keeps happening, there's a fault that needs fixing.

What should my boiler pressure be?

On most sealed systems (combi and system boilers), the gauge should read around 1 to 1.5 bar when the heating is cold. It rises when the system heats up, often to around 2 bar. Your boiler manual gives the exact figures for your model.

How to top up boiler pressure

1. Turn the boiler off and let it cool
2. Find the filling loop, usually a braided hose or a built-in key under the boiler
3. Open the valves slowly and watch the pressure gauge
4. Close them when it reads around 1.2 to 1.5 bar
5. Turn the boiler back on and reset it if needed

Every boiler is a little different, so check your manual. If you're unsure, call us and we'll talk you through it. Never take the boiler casing off: only a Gas Safe registered engineer should work inside a gas boiler.

Why does the pressure keep dropping?

A sealed heating system shouldn't lose water. If you're topping up every few days or weeks, something is wrong. The usual causes are:

• A small leak at a radiator valve, pipe joint or towel rail
• A leak under the floor (look for damp patches or warm spots)
• A faulty pressure relief valve letting water out through the pipe outside
• A failed expansion vessel, often shown by pressure that shoots up when hot and drops when cold
• Recently bled radiators (a one-off drop after bleeding is normal)

What about pressure that's too high?

If the gauge goes above about 3 bar, the pressure relief valve will open to protect the boiler, and you may see water dripping from a small pipe outside. That usually means the filling loop has been left open or is letting by, or the expansion vessel has failed. Turn the boiler off and call an engineer.