Radiators and boiler pressure: what to check and how often

The short answer

On a sealed central-heating system with a pressure gauge, the reading to look for is between 1 and 1.5 bar when the system is cold. Both Worcester Bosch and Vaillant give that range. Worcester Bosch says a system will usually need topping up once or twice a year, so an occasional small top-up is normal. A system that keeps losing pressure is not.

What to check, and when

• The gauge, cold. Look at it with the heating off and the radiators cool. This is the reading the manufacturers' range refers to.
• The gauge, hot. Worcester Bosch says that if the pressure rises by more than 1 bar once the system is hot, the expansion vessel may need repressurising, and that this needs a service engineer.
• Radiators that stay cool at the top. That usually points to trapped air rather than a pressure problem, and bleeding the radiator is the usual remedy.

After bleeding a radiator

Bleeding lets air, and some water, out of the system, so look at the gauge afterwards. If it has fallen below the range above, the system needs topping up through the filling loop, using the procedure in your own boiler's manual.

Bleeding can also work the other way round. Vaillant says that if pressure is too high, bleeding a radiator releases water and brings it back down. It also says to avoid pressure above 2.5 bar, to prevent damage.

When to stop and call someone

• The pressure keeps dropping after you top it up. Vaillant recommends a professional inspection in that case, and Worcester Bosch says to contact your installer if you need to repressurise much more often than once or twice a year.
• The pressure climbs well above the cold reading when the heating is on.
• The pressure will not rise when you follow the manual's filling procedure.

What this guide does not cover

Manufacturers' manuals differ on filling-loop design and on the exact procedure, and we have not found a manufacturer source that sets a fixed interval for flushing a heating system. So this guide gives none. Check your own boiler's manual, and ask your technician at the yearly service.

Turn it into a reminder

A pressure check costs a minute and is easy to forget. In Epimeleia, add a recurring task to your boiler record, such as a monthly pressure check through the heating season, and log each reading. A slow downward trend is much easier to spot in your notes than from memory.