DIY Doctor

Air in Central Heating system

Postby lee » Mon Oct 12, 2009 12:01 pm


We have an open vented central heating system. I regularly have to bleed the radiators in order to remove air bubbles that make quite a noise as they travel round the system. Just as I think I have got it all out the noise starts again and I have to bleed again. The header tank in the loft is approx half full and the open vent is clear of the surface. It almost sounds as though the pump is drawing in the air, the only place I can think it comes from is the open vent. I have read on a couple of sites that the pump/vent/boiler order is important but the correct order is not always clear. How can I diagnose if the air is coming in through the open vent and does anyone have any suggestions about how to cure it? Would some kind of additive help? (Also what is the function of the open vent as the system is open to the header tank anyway?)

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Simply Build It

Postby DEEARR2 » Thu Oct 29, 2009 8:59 pm

Hi Kieth

Long wait for a reply. Firstly, the open vent is there to enable expanded water pressure to be released from the system in the event of an over heat for what ever reason. Should, in the unlikely event, the gas valve stick in the open position or the control of the gas valve short circuit, then the boiler would run on and boil the water which would expand up via the open vent into the feader tank. As the expandad water leaves the system into the header tank the water in the tank makes up the difference via, the now hot, cold feed pipe and so maintains a water supply to the system. This prevents extreme damage to the boiled. (a bit long winded)

It sound as though the entry of the cold feed and the expansion pipes into your heating system may be incorrectly positioned. If they are connected correctly then a form of hydraulc lock is formed and prevents air entering the system while maintaining the expanssion facilty. Worth having this checked.
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Postby rosebery » Thu Oct 29, 2009 11:31 pm

It sounds as though the pump is sucking in.

Check the pipe work in the airing cupboard, the cold fill and vent arrangement. Check the sucking in by going into the loft with a jam jar of water, put this over the vent and get someone else to turn the heating on whilst you watch. If the pump is sucking in then the water will get sucked out of the jam jar.

The alternative could be that it is pumping over and sucking fresh water in from the cold fill on the f&e tank. Again you will see this whilst you are up there.

Have a look and post back.

Oh BTW - what speed is the pump set on? If its at its highest try turning it down to the middle setting. That might cure it.

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