Hot water on - reverse flow ? into CH

[doitall]

Can you follow the return back to the boiler from the cylinder, and post how many tee's there are.

 

[twgas]

You to to trace the HW return back to the boiler, you will more than likely have 2 tees (after the by-pass), there should be only 1.

 

[doitall]

You to to trace the HW return back to the boiler, you will more than likely have 2 tees (after the by-pass), there should be only 1.

Nothing to do with the HW return

It's the primary (common) return the OP needs to check.

And yes I probably know what you meant.