Gas Central Heating

[Spreeny]

When 'bleeding' the radiator does the pump need to be switched on or off?
Tia

[donny48]

https://www.uswitch.com/energy-saving/guides/how-to-bleed-a-radiator/

[Postdog]

Short answer. On. The pressure is needed to force air out until the water appears.

[bhg481]

I don't know anything about combi boilers but conventional systems aren't usually pressurised and don't have a pressure gauge on the boiler. For conventional systems, just bleed it ie, no need to run it hot first as donny's link implies. The other thing to be aware of is that it is hydrogen, not air, in the radiators, created by the steel in the radiators corroding. If you put rust inhibitor in the system, as I do, you may never need to bleed the radiators. I added extra radiators to my system in 1996 and have never had to bleed the radiators since. Be careful though, some boilers need certain types of inhibitor and adding the wrong one may cause damage. If you have a maintenance contract on you system you may not be allowed to add it.

[bhg481]

Let me just add that if you have two water tanks in your loft, the large one will be the header tank for your hot water and the small one will be the header tank for your central heating. This allows for expansion when the water heats up and supplies water to replace any lost by evaporation. This is the only pressure in the system and is sufficient to drive out any gas when you bleed. If you have only one tank in the loft you might have a system which does need to be pressurised.