Hi Peter, I presume you are referring to your hot water cylinder. If it is not heating when the boiler is on, it can be for a number of reasons.
1. The balancing valve has been unknowing turned off.
2. There is a motorised valve on the circuit that is faulty or thermostat powering it is turned down too low.
3. There is a blockage in the pipe.
4. If there is also solid fuel on the installation, the SF pump may be dumping its heat.
5. If the system is pressurised and the pressure is low, the system may need more water and pressure to reach the cylinder coil.
If you click on our Member Search Facility button and search your area for one of our qualified members & click under General Plumbing, it sounds like you may need a plumber to visit and sort the issue.