Heating wont work without DHW on

[tapsworcester]

I have been to a house where the heating won't run unless the DHW is also selected. Can somebody please diagnose this for me?

The system is unvented, the heating is pumped by a Grundfoss, the DHW is gravity fed 28mm pipework. The boiler is an old Mistral oil boiler. The control is a Honeywell 24 hour controller. There are TRV on the radiators.

Thanks for your help.

 

[TVP&H Ltd]

If on gravity dhw and pumped heating then this is correct, as the dhw calls for boiler to fire and the ch calls for pump to operate. You stated that the system was unvented...

 

[Chalked]

Unvented on gravity hot water! Coil drops inside so can't work very well. Hope there is sufficient control.
Aside that, the clock for Gravity hot water will always bring the rads in too.