No filling loop and red expansion vessel installed. How to represurrize heating?

[dilalio]

You don't use drain cock to get air out, that is done by rad vents, expansion pipe to F&E tank and any aav's on the system... Check to see if you also have any manual bleed valves located anywhere.
As long as the F&E tank has water in it and the ball valve is working so that it can 'top up', then the 'head' of water should force air to gather at high points and inverted loops ie top of rads.
If all is working except 2 rads then check the pins on the TRVs for these rads - they may be stuck and need manipulating with pilers/ small tap with hammer.
Check pump works correctly.
And you may have blockage to those defective rads.

 

[dilalio]

The main benefit of leaving the drain cock open whilst filling is to use it as a flushing point for dirt to escape whilst adding fresh water to the system at the top via the F&E tank.

 

[Samsg30]

Thanks everybody for all the replies. This thread has cleared my mis understanding about the CH system. So I have got open vented CH along with unvented indirect hot water. Yesterday I got my system drained partially again to change one of the lockshield valve and refilled it.

Boiler reading seems odd initially but then after cleaning megnaclean filter, all worked well. There were few rads which were not getting hot at all even after bleeding, so I turned the TRV off on all other hot rads which pushed the water down to cold rads and made them hot. Hope this has sorted the issue as such and the main issue discussed on this thread is resolved.. i.e. why there's no filling loop on my system.. and i.e. because it is open vented CH fed by F&E tank in loft. Don't know how to close this thread and any other issues if I face, I'll start a new thread instead. Thanks .