Temperature Troubles: How to Install a Thermostat

There's nothing worse than a thermostat that’s not working properly. In most instances, a faulty thermostat won't work at all, or if it does, it turns your heating or air conditioning to the maximum. While thermostat installation isn't expensive, you must know how to select the correct type. It's easy to DIY install a thermostat if you have a basic understanding of wiring and control circuits.

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Types of Thermostats

There are four main types of thermostats:

• Standard non-programmable thermostats

  that provide off-and-on control

• Two-stage thermostats

  that adjust the HVAC output according to the temperature of the room

• Programmable thermostats

  you can program to come on at different times of the day

• Smart thermostats

  that can automatically adjust the temperature

In this article, we only consider thermostat replacement for standard, two-stage and programmable thermostats. It's best to let a professional install smart thermostats because of the increased complexity of the installation.

Installing a Thermostat: Step-by-Step

Things You’ll Need

• Flathead and star screwdrivers

• Voltage tester

• Wire stripper

• Masking tape (to label wires)

• Wire stripping tool

• Pencil

• Level

• Cordless drill and drill bit set

Step 1: Turn Off the Power

Go to your breaker box and switch off all breakers that are marked as HVAC, thermostat, furnace or air conditioner. Remove the thermostat cover and use the voltage tester to make certain there's no power on any of the leads. You may have to unclip the cover, but some types are held on with screws.

Step 2: Remove the Old Thermostat

Remove the thermostat from its base plate on the wall. Disconnect each wire, fasten a piece of masking tape to the wire and write the terminal letter on the tape. Take care not to let the wires fall back into the wall by using a clothespin or taping the wires to a pencil to hold the wires in place.

Step 3: Buy a New Thermostat

Make certain the new thermostat is compatible with your HVAC system and uses the same number of wires as the old one. Upgrading to a more complex type isn't feasible unless you run additional wires and have checked that the HVAC control board will work with a more sophisticated thermostat.

You can usually upgrade from a standard thermostat to a programmable type. But to do this, you'll probably need to add a wire for the C (common) terminal and attach it to the appropriate terminal on the HVAC unit's control board.

Step 4: Install the New Thermostat

Using a level, position the baseplate of the new thermostat on the wall. Mark the position of the securing holes and drill holes for the wall screws. Fasten the baseplate to the wall. Hook up the thermostat wires to the same terminal numbers on the new thermostat. If the wire ends are corroded or broken, strip the wire insulation back until you have enough bare wire to fit the terminal.

Step 5: Reconnect the Power and Test the Thermostat

Once you've installed the thermostat, reconnect the power by closing the breakers. Then, adjust the thermostat and check that it switches the HVAC on and off correctly. If the thermostat is programmable, you'll need to install the batteries and program it according to the manual provided.

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Understanding Thermostat Terminal Numbers

When you look at the terminal board of the thermostat, you'll see each terminal has a letter next to it. The most common letters and the wire colors associated with these terminals are:

• R: The power connection that's usually supplied by a red wire.

• C: A common connection that's required by programmable and smart thermostats, usually using a black or blue wire.

• W: This terminal controls the furnace heating and is normally a white wire.

• W1 and W2: These are for first- and second-stage heating when applicable.

• Y: This terminal controls the cooling circuits and usually has a yellow or blue wire.

• Y1 and Y2: These terminals are for the first- and second-stage cooling of the air conditioner.

• G: This terminal switches the air-circulating fan on and off, and it usually has a green wire.

• O/B: This terminal may have an O, O/B or a B and is used to switch heat pumps from cooling to heating and vice versa.

Common Types of Thermostats

Use this guide to find the correct type of thermostat for your HVAC system:

Heating/Furnace

Number of wires: 2

Terminals: R, W

Cooling

Number of wires: 2

Terminals: R, Y

Heating and Cooling

Number of wires: 4

Terminals: R, W, Y, G

Heat Pump

Number of wires: 5

Terminals: R, Y, G, O/B, C

Two-Stage Furnace

Number of wires: 5

Terminals: R, W1, W2, G, C

Two-Stage Air Conditioner

Number of wires: 5

Terminals: R, Y1, Y2, G, C

Using a Professional HVAC Technician

If you're replacing a thermostat with an identical unit, it's something you can do yourself, provided you understand the risks of electricity and take adequate precautions.

But as you can see from the table and list above, there are many different types of thermostats, and it's easy to make a mistake. That's why it's best to work with a professional HVAC installer who can determine the best thermostat for your application and has the knowledge and skills to install new thermostat wiring or even relocate the thermostat if necessary.