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 =96 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