Air In Central Heating

[beanebabe]

I have drained the central heating system down several times but still have air trapped in the system. Turning the pump onto high does help, but this was previously set on 2. Can anyone advise on the best way to refill the system to avoid getting air locks? Does the 3 port mid position valuve have to be set to be latched open manually?

Thanks in advance.

[AlBags]

Is this a pressurised system? Or unpressurised?
opening 3-port prob wont help much.
You can try backfilling with hose if all else fails, but can be a bit messy.
Try filling and bleeding nearest rad to pump first, then continuing to fill. Run system on min temp so you can feel if rads get warm at all. Look for rads in order (nearest first) from the pump that fail to get warm. The air lock is usually between the rads that are warm and next ones that are cold. This is often a flat run underneath 1st floor floorboards. For bleeding, make sure lockshield valves are fully open. Turn up pump speed.

[AlBags]

Forgot .. open 3-port manually on latch if you cannot run boiler/pump. So long as you fill Primary (boiler) circuit, you should get circulation to 3-port and cyl hot water coil, so boiler is safe to run, and you should feel Primary circuit is warm.

[buildersmate]

Having established which rads are cold, make sure the 3-port valve is set to a position to enable CH (in other words, CH=on, water=Off).
Then use the pressure of the pump to push the airlocks along. As soon as one reaches a rad, the air rises to the top of the rad and no longer remains in the pipework. Bleed air out from the top of each rad in turn, ensuring the lockshields at both rad ends are open. (Take a note of how many turns of the fixed-position lockshield are required to open it up, so that you can rebalance afterwards back to the same position).
If that still isn't pushing the air out, turn off some of the rads that are working normally using the lockshield.
The pressure of the pump will always be powerful enough to shift air along the pipework system, especially if the circulating water has no other place to circulate other than the non-working rad circuits.

[beanebabe]

Hi There,

thanks very much for the suggestions, a bit more info for you. My system is an open vented system. All radiators do get warm, so the air is not completely blocking flow. I can hear the air when it runs through the pump, but the main impact is in the boiler area, where it sometimes knocks the boiler off and causes a loud banging in the pipes. I bled a lot of air out of the system initially, but after week there is very little now resting in the radiators. However, there is enough in the pipework to cause problems with the boiler/pipes. Can anyone also suggest a good source of reading material on this subject? I have tried various sites.

Any suggestion on what temp to set the boiler at to try this fix ? It's normally set between 2 and 3.

Thanks again for your help

[buildersmate]

The book that I use for reference a lot is called 'The Which? Book of Plumbing and Central Heating', published by Which? magazine. It's very practical without being OTT and doesn't cost a lot (mine is over 15 years old now - it may not be still available).
When one refills a system for the first system after drain-down, in the first few minutes, one can hear the pump 'hunting' as it experiences sections of pipework with no water in it. The system has to be designed such that water will naturally flow into the area of the pump - otherwise it is left spinning in fresh air. Another reason why pumps should be mounted with pipework running vertically (not horizontally) through it.
Until most of the air comes out, one nevers attempts to turn the boiler on, as one has to get water into the boiler water chamber.
After that, it doesn't take many minutes of running to extract most of the remaining air into the tops of the rads - from which it cannot escape back into the pipes - assuming your rad connections are at the bottom of the rads?
Bleeding then is effected using the little key - as you know.
Having done that ONCE, the amount of air in the system should be tiddly - certainly not enough to cause the problems you mention (banging). If a few days later you are saying banging occurs, you are going to have to work out where MORE air is getting into the system. The most likely culprit is via the expansion tank in the loft.

[beanebabe]

iI Guys,

thanks very much for your answers, more food for thought and different things to try!

[montecatini]

There is possibly a vent/bleed screw on the circ pump, possibly also a vent valve at the top of the boiler itself dependent on make/style.

[gucciman]

theres no way of filling a system without getting airlocks and getting rid of them is a real pain.

there should be air vent points in the pipework, you normally find them near to the cylinder and they look like little dead ends of pipe with a vent of some sort on the top, unscrew the vents cap until water runs out.

you can also try venting from the pump by carefully unscrewing the large cap in the centre.

Quite often we find the best way to rid all the air is to leave the system turned off overnight, let all the air rise to the top and then vent.

turning the pump speed up normally makes the air problem worse rather than better i.e. it can suck air into the system.

Finally, if all else fails and you call out a heating engineer there is a way we can remove all the air by deliberately boiling the system but this is for the expert and should not be attempted by yourself

Finally,