CENTRAL HEATING SYSTEM SUCKING IN AIR THROUGH OVERFLOW PIPE

I have a a domestic hotwater system with an oil fired boiler. I am a DIYer, not experienced in plumbing.
When I am using the heating, the system sucks in air through the overflow pipe which hangs above the expansion tank. Therefore you need to bleed the risers that are in the loft to release the air every few days. Note when only heating water (as it is now during the summer) it does not suck in air.
A bit of background......
The central heating system is probably 20 years old. The boiler is probably 7 years old & recently serviced. There are 9 rads in a 3 bed bungalow.
Last winter several rads were not getting hot. At this stage I do not know if the system was sucking in air.
To remedy the lack of heat in the rads I replaced a faulty actuator & the Grundfos pump. I am confident both replacements are working well & done correctly.
Whilst the system was drained down I did a diy flush of the system using full mains pressure in both directions through the system rad by rad & removed a reasonable bit of muck. Put the system back together & put Fernox inhibitor in. Then balanced all rads & successfully got heat to all of them. In truth better than for several years.
After doing all this I then realised that I was getting air sucked in via the expansion tank overflow. I did even cover this pipe off for a bit with clingfilm & the system ran for a couple of days with minimal air getting in. You could see the cling film breathing in & out. The cling film then broke, I guess due to pressure of suck.
I can hear the Grundfos pump sometimes 'free wheeling' in air that it has sucked down from the above expansion tank overflow pipe. Basically it is as if the pump cannot receive enough water to pump it pushes it fine but cannot receive it quick enough. It does this even on the slowest Grundfos pump speed of 1.
I am out of ideas for what to do next. All advice appreciated.
Many thanks
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On Sun, 17 Aug 2008 04:36:24 -0700, Neil DG wrote:

This is seriously unfortunate. The pipework is laid out is such a way that this happens.
You may get some help here if you describe the layout fairly carefully. Especially the relative positions of the pump, boiler, zone valves, header tank, feed pipe and vent pipe.
You will probably need some fairly in depth help to get this sorted.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
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On Sun, 17 Aug 2008 17:15:29 +0000 (UTC), in uk.d-i-y Ed Sirett

Quite a few people's spam filters delete posts with ALL CAPITAL subject lines, so you probably will do better to re-post it with a normal subject line.
-- Phil
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I have a a domestic hotwater system with an oil fired boiler. I am a DIYer, not experienced in plumbing.
When I am using the heating, the system sucks in air through the overflow pipe which hangs above the expansion tank. Therefore you need to bleed the risers that are in the loft to release the air every few days. Note when only heating water (as it is now during the summer) it does not suck in air.
A bit of background......
The central heating system is probably 20 years old. The boiler is probably 7 years old & recently serviced. There are 9 rads in a 3 bed bungalow.
Last winter several rads were not getting hot. At this stage I do not know if the system was sucking in air.
To remedy the lack of heat in the rads I replaced a faulty actuator & the Grundfos pump. I am confident both replacements are working well & done correctly.
Whilst the system was drained down I did a diy flush of the system using full mains pressure in both directions through the system rad by rad & removed a reasonable bit of muck. Put the system back together & put Fernox inhibitor in. Then balanced all rads & successfully got heat to all of them. In truth better than for several years.
After doing all this I then realised that I was getting air sucked in via the expansion tank overflow. I did even cover this pipe off for a bit with clingfilm & the system ran for a couple of days with minimal air getting in. You could see the cling film breathing in & out. The cling film then broke, I guess due to pressure of suck.
I can hear the Grundfos pump sometimes 'free wheeling' in air that it has sucked down from the above expansion tank overflow pipe. Basically it is as if the pump cannot receive enough water to pump it pushes it fine but cannot receive it quick enough. It does this even on the slowest Grundfos pump speed of 1.
I am out of ideas for what to do next. All advice appreciated.
Many thanks
Does it suck down even on slow speed? Did you replace a low volume pump with a bigger one? If so, perhaps you could try closing the isolating valve below the pump a tad.
Our one does the same if we have it on full power. Ideally the pump should be nearer the bottom of the system, but ours is only a foot or two lower than the boiler and header tank. I had wondered about putting an extension on the vent to take it under the header tank water level, but found in the end, that once I had attended to the real problem (knackered 'micro' switches in the three port valve head, that meant that the rads never got a full feed from the pump) I found that the pump did the job well, even on the minimum speed, at which it did not suck down the water column.
S
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