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Thermostat Replacement, A Step By Step Guide

This thermostat replacement guide will take you step by step through the process of selecting and installing a new unit. A call to a hvac dealer to do this would cost at least $200. With the help of an experienced hvac technician, you can do it yourself and save. You can watch a professional do it on the player below and even shop for a new thermostat.


We will only look at installing digital units here because there is really no benefit to installing a mechanical unit.

Problems with your existing thermostat?

If you are having problems with your furnace or air conditioner, you can check the thermostat for proper operation. To begin, shut the power off to the furnace and/or ac unit and switch the fan from auto to on mode. Then, if you are having furnace problems, you can disconnect the r and w terminals at the thermostat. Twist the two wires together and then turn the power back on. If the furnace comes on, the thermostat is bad and you should continue with thermostat replacement. If the furnace does not come on, the problem is not the thermostat. If you are having ac problems, disconnect the wires from the r and y terminals and twist them together. Turn the power back on to the furnace and/or ac unit. If the system runs the thermostat is bad and should be replaced. If the ac still does not run, the problem is not the thermostat.

What type of thermostat do I need?

The first step is to survey your heating and cooling systems to see what type you have (heat pump, boiler, furnace, electric baseboard) If you have a boiler you need to count the number of wires in the old thermostat. If you have three wires it requires a special unit. Another thing is to check the number of stages you have of heating and cooling. This can best be done by looking inside the old thermostat. If you have more than one stage of cooling there will be y1 and y2 terminals with wires on them. If you have a two stage furnace, there will be w1 and w2 terminals with wires on them. Next, you must know the source of power available to the unit. This can be accomplished by looking at the existing control. If you are replacing a digital unit, look for a C terminal. If there is not a C terminal or there is no wire attached to it, you will need a battery operated replacement unit. If there is a C terminal with a wire attached to it, You can use a system or battery powered unit. If you are replacing a mechanical thermostat, it is best to get a battery powered replacement.

** HOT TIP ** It is usually a good idea to look for a new unit that is dual powered. This means it can be powered by the system or battery. It will tell you on the package.

Now that you know what type of replacement you need, you can check our thermostat ratings page to find the best brands.

Thermostat Replacement Procedure

  • Shut off the power to your heating and/or cooling systems
  • This will protect your hvac equipment.

  • If you are replacing a mechanical unit, you can now remove the cover and/or unit to get to the wire connections.
  • On some units this is accomplished by loosening two or three tiny screws. Some units will have to be removed from the wall to expose the wire connections. Digital units usually just snap off the mounting base.

  • Write down the terminals and the color of wire attached to them
  • ** HOT TIP** Watch out for two or more wires that are the same color. This is rare but it is possible that you will see it. If this is the case, make sure you mark the wires in another way.

  • If you didn't have to do so to get to the terminals, now you can remove the old base from the wall.
  • This is usually done by removing two or three screws.

  • Mount the new base in its place
  • Most new units come with plastic drywall anchors to mount them. Next, reattach the wires to the terminals. Be sure to put them back exactly like they were on the old one.

    ** HOT TIP** You should seal the hole in the wall where the wires come through. This can be done with silicone, putty, etc. Make sure you do not get it on the terminals. If this is not done, drafts can come through the wall affecting the accuracy of the unit. This is rarely done even by professionals.

  • You may need to set switches or program the unit to tell it what type of system you have
  • Read the instructions provided with it to be sure.

  • Put the unit on its base
  • Most of them snap on but some are held with small screws.

  • Turn the power back on to your heating and/or cooling systems.
  • You can verify proper operation and program according to the instructions. Most units have the instructions printed on a label inside the cover.

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