I need some help and advise, where connect from secondary return circuit to unvented water heater only over 210 litre and why use secondary return circuit? But less than 200 litre no secondary return circuit.
Like you I am installing an unvented tank for my hot water which has a secondary return inlet.
These are designed for an installation where there is a long pipe run to the hot tap, whereby you are having to run quite alot of water before it gets hot.
By making the pipe run a circuit, and pumping hot water around it, the water in the circuit remains hot and is available instantly at the hot taps.
While it is quite a good idea, there's additional plumbing, pump installation and heat loss in the circuit to consider against the "inconvenience" issue.
I would assume that some smaller tanks do not have it because it's only likely to be relevant to larger installations.
If you don't need it - just close it off.