Air in central heating - where from?

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kevin_robson

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We have had problem with air in the central heating system since we moved in. The pipework has been looked at and nothing fundamentally wrong with the setup has been found.

Have had two plumbers in but still have problems. We need to bleed regularly and have gurgling from the radiators.

So far we have extended the overflow pipework in the loft to give a bigger drop (I think thats the termanology) and played about with the pump speeds.

I've also had all the radiators off the wall and cleaned all the rubbish out of them.

Next stage is to try replacing the pump in case it is sucking air in, but is there anything else it can be?

I seem to remember reading about something in the boiler itself that coukld be causing air to enter, and seem to remember something about it being limescale buildup - does that mean anything to anyone.

I'm just trying to eliminate everything before I shell out yet more money on something that might not work.

Thanks
 
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Usually, it isn't air, it's gas produced as a result of the corrosion of your steel radiators. This also produces black sediment which will eventually cause blockages.

If the system is properly designed and not faulty, it should be enough to give it a chemical clean (to get out some of the old sludge) and refill with chemical inhibitor (to prevent future corrosion). An old trick is to drain out a little of the system water into a jamjar and drop a steel nail in, to see if it goes rusty. If not, the water is adequately inhibited.

The problem will be made much worse if you have fresh water going into the system (e.g. to make up for a leak, or if the water level in the F&E is too high) or the water being aerated (e.g. by "pumping over" from the Vent pipe into the Feed and Expansion pipe. You may be able to see signs of this in the loft.

If there is a leak on the suction side of the pump it can draw in air, though this is not the most likely cause.
 
u can also get air drawn in on rad valves that have a small leak on the gland nut hen the system is running. if it is a build up of gas from corrosion u can smell it & even light it with a match to determine if its air being sucked in or gas from corroision. unlikley 2 b the pump. most common vent & feed configuration . what type of system is it. the easiest garanteed way 2 cure it is to convert it to a sealed system if possible.
 
Thanks for the responses.
The radiators were drained, cleaned and inhibited about a year ago, and nearly all had new thermostatic rad valves fitted.

Its also been very recently drained and refilled (with inhibitor) so I dont thing that is it.

The pipe in the loft that goes into the feed and expansion tank was re-laid to give it extra height but that never made any odds either - I'm assuming that is what you are getting at?

Its an open system, but cant rememebr the name of it.

I know you both think it aint the pump, but I'm struggling to think of anything else it can be.
 
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"the water level in the F&E is too high"

How do I know if this is too high? What am I lookig for?

Cheers
 
The water level (cold) only needs to be a couple of inches deep. This is so that when the system is hot and the water expands (and any air or gas will expand a lot more) the level does not rise enough to reach the overflow. If it does, then when the system cools down again that night, the float valve will top it up with more fresh water. This water contains dissolved air, which encourages corrosion; and also it will be diluting the inhibitor.

Try tying a jar under the vent pipe (the one bent over the F&E tank) so that, if any water comes out of it, the jar will be filled. Water should never come out of the Vent pipe unless something is wrong. You can also sit up there with a torch watching the F&E, with the CH pump running at full speed and the boiler turned up hot. This is when interesting things are most likely to happen.

Also, please post a pic of the pipes showing the circulating pump, the feed pipe and the vent which should be close to it, and the electric valve.

Look round all of these for any signs of leakage or corrosion at the joints. If the pipes are very dirty, give then a clean ( kitchen scourer and fairy liquid will do) and polish up with an old rag, this will make any new leakage more obvious.

p.s. presumably there is no drowned wildlife rotting in the tank?

p.p.s next time you vent a radiator, turn the lights down and see if the escaping gas burns with a blue flame (obiously move any flammable materials like curtains well away and have your SO stand by with the extinguisher).

 
install an airseparator on the expansion/feed pipes/flow depending on layout. This will cure your problem.
 
It won't cure corroding radiators; it won't cure pumping over; it won't cure topped-up leaks, and it won't cure an air leak on the suction side of the pump.
 
if you've diagnosed all these problems the heating should be put down, or maybe buy an airseparator for £15 that prevents what is clealy a massive unwanted introduction of air into the system, the most common coming from the header tank due to poor plumbing design or loss of efficiency in the system due to ageing.
 
I haven't diagnosed any problem yet, and I don't think you have either.

I have pointed out several possible causes that your proposal will not cure.

I think you are unwise to suggest a cure when you don't yet know what the problem is that you hope to address.
 
it will. I have encountered this problem many times. :eek:

maybe you should go out more:cool:
 
unless the system was hot when u took the pictures you could loose a bit off water out of the f/e tank lower the balvalve so to have about an inch of water above the outlet when cold is the pump on speed 1. if not try it on 1 & only increase if you dont get all the rads hot. the valve on the bottom of the coil return should be only a 1/4 open.
 
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