Vaillant ecoTEC Plus 837 filling loop problem

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Sorry to raise yet another question on this topic but have read many of the threads and searched elsewhere without getting anywhere.

I'm not sure if I've ever got the loop to work properly in the couple of years we've been here but on the last couple of occasions when I've done 'major' work like fitting a new rad/new pipework I've really struggled to get any water back in. (Had to resort to an 'external loop', aka garden hose onto a drain cock, sorry but I was desperate!).
I'm just about to do the next stage of rad/pipe renewal and want to sort out what's going on properly this time.

Initially found the left hand valve handle was rounded off so had a spanner on it to be sure. Water definitely spurts from right hand valve when opened as have disconnected the flexi to prove, but when I turn the left hand valve little or nothing flows into the system at all as far as I can tell.

A few questions:
1. Is it likely/possible that the LH valve mechanism (or non-return part, if there is one) is broken even though the stem seems to quarter turn ok?
2. In view of 1. I have been looking to replace the valve and have noticed that in the photos the slot in the head of the main body's stem (not where the plastic handle fits) points clearly towards the water inlet, as shown in photo. On my boiler the slot is at about 45 degrees. Could this be a factor in that somebody has turned it with a screwdriver in the past and effectively isolated the inlet? I'm reluctant to just turn it myself without a bit of guidance from somebody in the know as I've been unable to get any idea what this element does or how it's meant to be configured.
3. If 2 is the case is there a valid reason why they might have done it, that might mean a new valve is still the best course? Any checks I could do?

Any advice would be appreciated.
Thanks, Andrew
Screenshot 2022-01-05 174720.gif
 
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The slot should be in line with flow of water, that pic shows it to be fully open.
The valve marked in the pic is to isolate the boiler from the heating circuit for maintenance etc.
 
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Thanks - but when you say in line with the flow of water, doesn't that mean the position shown is fully closed?
I've just seen a video of a guy isolating to drain the boiler down and he turned the slot to the position shown.
In any case this suggests mine is halfway between the two - odd as the heating circuit seems fine (and it's quite big, 13 rads).
I'd better go and have another look...

Actually it's an exaggeration to say 45 degrees - the slot is pointing almost fully in line with the return pipes but not as straight as the other valve. Didn't have a key to hand to see if it would move the extra bit.
Still doesn't solve the main issue - why the filling loop doesn't seem to be getting through, unless that last little bit could be cutting it off maybe? I really don't want to drain down and find I can't fill again.
 
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