Reverse circulation new build

[Gasmk1]

should there not be a non return valve on the return as the return from cylinder where it tees in could go either way. just a thought

 

[fixitflav]

Sorry I meant to say he moved the pipe under boiler its the last t on the return now. I think its the Bypass but not sure.

The more common problem, rads getting on when HW called, due to rads returns not all being commoned before the cylinder return joins, has been avoided.
But you could still have the problem you describe if it's piped as my #36. If the zone valve feeding the top rads is closed, the other one open, the top rads will get warm, I showed the flow path. It only happens because you have zone valves. If just one auto valve fed all the rads, with TRVs to control temperature in different areas, it wouldn't happen.

 

[Best]

Just had my first look at the photo of the updated pipework. I can't see anything major now wrong with the heating system pipes.
The cylinder tee in last on return, as you would expect.
(There can be some heat gravitating on a return where it keeps rising, like in the photo, if it has a path)
Only thing I can think of is there are more than one main tee off to a zone causing a loop going to the On zone.
But I need to think on it sometime I get time

 

[fixitflav]

as the return from cylinder where it tees in could go either way.

No, it would only do that if the rads returns were not commoned before cylinder return comes in.