How do you know if you need to replace your home thermostat?

Well, I don't mean to sound snarky, but when it doesn't work anymore! The thermostat, in its most basic form, is just a switch to turn the heater on. If the heater turns on and off like it should, it is OK. If the heater doesn't turn on, or worse yet doesn't turn off, you should look into getting a new one. Some tests you can perform: Heater doesn't turn on? Get at the wires running to the thermostat and touch the two control wires (You will have to look into the furnace documentation if you have more than two wires) together. If the furnace clicks on, it's the thermostat. If it doesn't, it's the furnace. Heater doesn't turn off? When the furnace is running disconnect one of the control wires. If the furnace shuts off it's the thermostat. If it doesn't, it's the furnace.