I've wired in a new honeywell rth6300b that's controlling gas heating with air conditioning. I'm confident that the wiring is correct as I followed the installation instructions, but when I set the system to AUTO HEAT and set a high temperature, it makes a loud click but doesn't fire up. If I do the same for AUTO COOL it works fine and the a/c comes on like it should do. Any help or advice would be most gratefully received. James

Sounds like the furnace is broken and not the thermostat.
See the FAQ at the top of the main HVAC page for a little more help on what kind of furnace you have and how it should operate.

Thanks for the suggestion Northern Heat. The thing is the furnace will fire and the heat will come on if I switch over to HEAT ON, the problem is when the unit is set to AUTO HEAT; it doesn't fire under that setting. Any further suggestions? James