Thermostats come in different types. There are non-programmable digital thermostats, programmable digital thermostats and non-programmable analogue thermostats. Thermostats are connected to the furnace through a series of wires. Depending on the type of HVAC system there are between two and six thermostat wires. Two wires are used for heating-only systems, four wires for systems with heating and air conditioning, and six wires for systems which have multiple stages of heating. It is moderately easy to install a programmable thermostat.
Turn off the power to the HVAC unit. The power can be turned off by turning the corresponding circuit breaker off on the circuit breaker panel.
Determine the type of HVAC system you own. If you have a heating only system, you'll be using the red and white wires. If you have an air conditioning system, you'll also connect a green and yellow wire. If you have a system which uses two stages of heating devices, you'll be using the brown wire. If your thermostat uses power from the HVAC unit, you'll also need to connect a black wire.
Open the HVAC unit's access panel and connect the red wire to the "R" terminal and the white wire to the "W" terminal. Connect them in the same manner to the programmable thermostat.
if you have air conditioning, connect the green wire to the "G" terminal, the yellow wire to the "Y" terminal in the HVAC unit. Once again, connect them to the thermostat as well.
Connect the black wire to the "C" terminal. This is necessary if your thermostat is powered by your HVAC unit. If the HVAC unit uses second-stage heating, connect the brown wire to the "W2" terminal. Attach these wires to the corresponding connectors in your thermostat. Place the cover back on the HVAC unit.
Place your thermostat on the wall mount plate and turn the power back on to your HVAC unit. Set your thermostat to the desired level and it should maintain the correct temperature.
You may need to refer to your HVAC unit operation manual to see how many stages of heat it has.
All power must be turned off to the HVAC unit when attaching the wiring to the thermostat. If the power is not off, serious damage to the HVAC unit may occur along with possible electrical shock to you.