Central Heating

[beermonster7]

Does your central heating need to be off to bleed the air from it

[BillyBB]

No.

If the system is warm you can feel the heat reaching the top of the rad and get ready to stop.

[stanleyman]

Yes, depending on where the pump is situated it could quite easily draw air in instead of venting air out on an open vented system and make it worse. A common and confusing problem for some people.

[amaltman47]

I was always advised to bleed radiators with the central heating pump off and had been off a wee while, air in the system naturally rises. If you bleed when the heating is on, the system is under pressure.

[buildersmate]

No

I can't think of any circumstances under which the pressure in the radiator would be less than air pressure - so answer 2 does not apply. If the CH is gravity fed from the top tank, the pressure will be equivalent to the hot tap downstairs and i've never experienced air sucking into a tap when I turn it on. The effect of the CH pump is irrelevant.

Answer 3 does not apply because it generally isn't air that is coming out of the bleed - it is hydrogen created from the small reaction between the steel radiator and hot water, the other byproduct of which is black sludge. Any air present comes from the initial fill of the system. It collects in the top of one or more of the rads and isn't going to re-dissolve into the water.

[stanleyman]

Sorry buildersmate but the pump is very relevant because it has a pressure side and a vacuum side and on some systems, they are never all the same, the pump inlet can be close enough to a radiator to cause a depression and to suck in air, this will overcome the head. I've seen it many times. This also happens with poorly designed systems that can also draw air from the vent pipe or the other way pumping up the vent to the tank. If a system constantly needs bleeding it is more likely to be air drawn in rather than caused by any internal reaction.

[grahamfreer]

yes pump must be turned off as stanleyman says the neutral point on a system is determined to be where the cold feed joins the system. also if it is hydrogen comong out & not air i would suggest getting in a pro to check (another sign is black water which means serious corrosion)