Back to List of FAQs Q Can I use a GemTech thermostat in a zoned HVAC system? A Yes, The master control unit of the thermostat opens and closes baffles in the forced air system to open an area or close an area for either heating or cooling. This allows different areas of the house to be at different temperatures as a function of the areas own thermostat using a single HVAC system. The selection of HEATING or COOLING is made by the master controller. If you wish to use programmable thermostats at the remotes, you must remember to switch the remotes from HEAT to COOL, whenever you switch the main thermostat from HEAT to COOL. This is because the heating and cooling programs are different. Some forced air zoned systems master controllers use both B and O for changing from HEAT to COOL and some use just one of the letters, B or O. Be sure it is a gas forced air zoned system before connecting both O and B. Some of the names of forced air zoned heating are Durazone, Trol-a-temp.