Boiler losing pressure?

What the pressure should be

Most combi and system boilers run at about 1 to 1.5 bar when the heating is cold. It rises a little when the heating is on, often to around 2 bar. Below 1 bar, many boilers lock out and show a low pressure code.

How to top it up

1. Turn the heating off and let the boiler cool.
2. Find the filling loop. It is usually a silver braided hose or a built-in lever under the boiler. Your manual shows where.
3. Open the valves slowly and watch the gauge.
4. Close them fully when it reads about 1 to 1.5 bar.
5. Reset the boiler if it shows a fault code.

Why it keeps dropping

• A leak. Check under radiators, around valves and below the boiler. Small leaks often dry before you see them, so look for staining.
• Bleeding radiators. Letting air out lowers the pressure, so top up afterwards.
• The pressure relief valve. If it is letting water out, you may see drips from a copper pipe outside.
• The expansion vessel. If it loses its air charge, the pressure swings high when hot and low when cold.

When to call us

If you need to top up more than once a month, or the pressure rises above 3 bar, book a repair. Topping up repeatedly brings fresh water and oxygen into the system, which speeds up corrosion.

Questions