The most basic of systems (such as an older ‘heat only’ forced air, oil burner, or gas furnace only need 2 wires for control. They connect to a 2-wire thermostat (generally mechanical with no power requirements).
Terminals on the stat in a 2 wire system are usually marked ‘R’ and ‘W’. They usually operate at 24VAC, and the source of this control power comes from a control transformer mounted on the furnace or boiler. The house voltage serving the furnace is transformed down to a safer level of 24 volts which goes up to the thermostat and back down. When the room temperature falls below the set-point, the contacts close completing the circuit to begin the heating cycle.
In this basic of heating systems, as the temperature of the heat exchanger rises, another contact is closed on the line voltage side of the equation, and the fan blower motor starts moving air through the heat exchanger and out through the ducting in the home. If the fan fails to run for any reason, the heat exchanger will get too hot and the hi-limit temperature device will open the circuit to the gas valve, closing the valve and stopping the heat cycle.
If your home’s system has air-conditioning (cooling), then the thermostat wiring will have at least 3 wires (some will require a separate ‘R’ terminal for heating and cooling and will be labeled ‘Rh’ and ‘Rc’ and will need 4 wires), ‘2 R’s’, a ‘W’, and a ‘Y’ terminal.
When the mode is selected for cooling (basic heat / cool thermostats have a mode selector for either ‘heat’, ‘cool’, or ‘auto’) when the room temperature rises above the set-point, the thermostat will close the connection between the ‘R’ and the ‘Y’ terminals completing the circuit to the compressor and condenser unit that provides the cooling for the evaporator coils mounted in the discharge ducting of the system. The next step up from the older home systems was the introduction of the ‘fan on / auto’ switch. This function allows you to circulate the air in the house by using the fan in the furnace without heating or cooling the air moving through the system.
In the most basic system, this functionality is provided by use of a fan center relay, and the low voltage wiring to the thermostat now will require a minimum of 3 wires (for heat only units) and 4 wires (for heat / cool / fan) for control. This additional terminal is labeled ‘G’ in the thermostat.
When the ‘fan on’ setting is selected, the contacts between ‘R’ and ‘G’ are closed turning on the relay that powers the blower motor directly regardless of a call for heating or cooling. If you are replacing an old thermostat with a new digital thermostat, the electronics in these units may need yet another wire for a terminal labeled ‘C’. This terminal is for a common connection from the control transformer that will provide steady 24 volt power between terminal ‘R’ and ‘C’ to power the thermostat itself.
If adding wires to your thermostat location is not possible, consider a thermostat that is battery powered which won’t require the terminal ‘C’ for operation.
Climate control systems have evolved away from the DIY guy, with the evolution of electronic control. However, the terminals of the thermostats of today are still labeled the same, and provide the same functions. They just get a little more complex, so check with a contractor providing the HVAC advice/installation as to what thermostat wiring is required the next time you get your system serviced. Tell him you want to go Z-wave and provide the product info from our website. He may be able to help you.
Are you going to a new home? A new system may need as many as 10 wires (like a 2-stage heat, 2-stage cooling system, heat pump reversing valves, fan control, etc.)
If the system had 2-stage heating, and/or cooling, the 2nd stage terminals would be labeled W1 and W2 for heating, Y1 and Y2 for cooling.
These descriptions are very basic, but should give you a good guide to exactly what is demonstrated in the manual of that shiny new thermostat you just purchased. Our website features the installation manuals of all the devices we sell. Take a look at the wiring diagrams compared to your own before you buy.