Hot water problem

[Willcu]

Indirect hot water cylinder is failing to heat the water via the heat exchanger. Boiler, pump and diverter valve working fine, cylinder stat is ok and central heating works fine when selected
Hot water in flow pipe but return is cold with evidence of oxidising and possible leak around connection onto cylinder. Could the coil have failed inside the cylinder or is there some other explanation? In the meantime we are heating water via the immersion heater.
Thanks for any suggestions.

Willcu.

 

[ChrisR]

Check there's water in the system, and that water comes out of the vent point by the cylinder if there is one.