St6400c thermostat wont run the central heating independantly

our honeywell st6400c thermostat worked fine when installed but now will not run the central heating system on its own - the hot water system works fine - the central heating system only seems to work when the hot water tank is empty and when the hot water function is running- we can't run the central heating independently could this be a problem with the control panel? is this a regular occurence with the ST6400c?