Hot Water only luke warm

[baglartimus]

I have recently replaced my old hot water cylinder as the old one was leaking.
The system is an indirect type, using gravity feed to heat the hot water and a pump for the 7 radiators.
The original hot water tank was very old and allowed the boiler hot water circuit (i.e. the water circulating around boiler heat exchanger, hot water cylinder heat exchanger and radiators) to be topped up with water from within the hot water cylinder via a vertical pipe from the heat exchanger within the hot water cylinder. There was no feed and expansion tank.
Since this old type of hot water cylinder no longer seems to be available, I replaced it with a modern indirect type which does not allow the hot water to top up the boiler circuit. Thus I had to fit a feed and expansion tank in the loft to allow top up of the boiler circuit.
I connected the hot water feed from the boiler into the top of the heat exchanger on the hot water cylinder, and the lower end of this heat exchanger back to the boiler. The feed from the feed and expansion tank tees into the pipe going to the lower heat exchanger connection near the hot water cylinder. The expansion pipe tees into the pipe going to the top of the heat exchanger on the hot water cylinder.
When I turn the hot water and heating on at the controller, the radiators all get very hot, but the water in the hot water cylinder never seems to get more than luke warm. Any pointer as to why would be welcome, and suggestions how to fix. I have since flushed the system with system cleaner, and flushed again, then filled with corrosion inhibitor, but the hot water still is only luke warm with very hot radiators.

[Puddle]

The problem you have could be an air lock,incorrect sized pipes or runs,incorrect coil inside cylinder,without see is very hard to say
However it is against regulations now to fit gravity systems on hot water from a gas boiler ,they require modern controls to allow boiler to be turned of when desired temp reached
Therefore you should make system fully pumped with motorised valve and cylinder thermostat fitted,thus meeting all current regulations
It will pay for itself with gas prices now so high