Renovation-Headquarters Banner

Replacing A Furnace Thermostat

Replacing an old or defective thermostat is a project easily undertaken by a DIY home handyman. With the rising cost of energy, replacing an old thermostat with a modern electronic programmable device, as shown in Figure 1, could save you hundreds of dollars in energy costs.

electronic programmable furnace thermostat
Figure 1 - Programmable electronic thermostat

The thermostat is the device that turns your heating and/or cooling system on and off relative to the temperature requirements in your home.

Older thermostats used a bi-metal coil that expanded and contracted as the temperature at the thermostat changed. As it expanded and contracted it would, through a vial of mercury at the end of the coil, tell the HVAC (heating, ventilating and air conditioning) system to turn on or off as required. Over time the bi-metal coil will deteriorate and the thermostat will have to be replaced.

Modern thermostats are electronic, eliminating the mechanical bi-metal coil. They have also expanded capabilities that allow homeowners to set temperature requirements based on specific days of the week and hours during those days and some can even control humidification equipment. A modern thermostat, properly programmed, can save the homeowner money on energy expenditures.

Selection of electronic thermostats.

With the older style bimetal thermostats it was important that the thermostat be level, as the position of the bimetal strip was reasonably critical in having an accurate temperature. The electronic thermostats do not need to be level in order to work, however they should be installed level for aesthetic reasons.

Your old thermostat will have 2 to 6 wires connecting it to the furnace controls. In many cases there will be wires that are coming from the furnace that are not connected to the old thermostat. The reason there may be unused wires is one of convenience for HVAC installers. In many instances they will purchase their control wiring so that it will handle any type of furnace and thermostat installation, hence in some installations all of the wires may not be needed.

If the wires are not connected to the old thermostat, they are not connected to the new one either.

As you disconnect the wires from the old thermostat mark them to identify the terminals that they came from, this will make installing the new thermostat much easier.

If you are relocating a furnace thermostat you may wish to consider using a wireless unit. Wireless thermostats avoid the problem of fishing wires through walls from the furnace control center to the new thermostat location.

Additional information on thermostat location within a home.