How to Replace your Thermostat

Old Mechanical Thermostat

Many people are moving toward making their homes “smarter”. One of the easier ways to start this is to upgrade to a “connected” thermostat. However, there are a few things that need to be done before you buy your new thermostat. If you are unsure if the new thermostat is compatible, you might want to avoid some potentially very expensive damage by doing some research before you start.

Is your HVAC (Heating, Ventilating, Air-Conditioning) system “low voltage” (24v)?

Most “smart thermostats”, such as thermostats from Nest, Ecobee, Honeywell, etc., are only compatible with “low voltage” or 24 volts HVAC systems.  They can be connected to just about any HVAC system, but things like line voltage baseboard heaters, and other devices that do not have “low voltage” control require various types of interfaces between the thermostat and the heater or AC unit. I have done this in my home with a Nest thermostat, but I am a licenced electrician with a lot of experience with controls. I may write another article on how this was done if there is interest shown.

Once you have confirmed that your HVAC is compatible, you will find that changing your thermostat is quite a simple task. One way to confirm if it is a “low voltage” HVAC, is if you have one single thermostat in your house (or multiples if you’re multi-zone) that controls a large furnace in your basement or garage (there might be additional accessories such as a heat pump, too). The wires are typically 18-22 gauge or thin and multi-colored.

Some systems are not compatible at all (unless you have the proper interface). If you have a thermostat and heater pair in every room of your home, you probably have line voltage systems (which are not directly compatible with the popular smart thermostats available right now).

Also, major manufacturers such as ecobee and Nest also offer compatibility checking tools that are easy to use and will help you determine if you can have a smart thermostat with your existing HVAC system. To begin, remove the cover from your thermostat off the wall so you can see the wiring, and visit either of these links:

You might like to take a picture of the wires for future reference, in case they are different colours than the standard, or just to be sure they are being connected to the correct terminals in the new thermostat.

Do you have enough wires?

Most older thermostats worked fine for many years with just 2 wires. But for most smart thermostats, including the Nest, you’ll likely need an additional wire to plug into the C-terminal to provide continuous power to the thermostat. Maintaining a WiFi connection requires power that your old mechanical thermostat likely did not need. This may not be present on the thermostat you’re replacing. The next thing you should do, once you determine your HVAC system is compatible in the first place, is to determine if you have a sufficient number of wires.

You’ll need a C-wire for most of the popular smart thermostats available today, but there are many ways to deal with the absence of a C-wire. Personally, I used the old 2-wire to pull a new cable through the wall so I had the correct number of wires for my new thermostat.

You might want to look inside your furnace or in the wall behind your current thermostat to see if you do actually have enough wires and they’re just not in use and stuffed away, or you might have to order an adapter or new wire, or consider hiring a pro.

Take good photos of your existing thermostat wiring

Yes, this step is included in every smart thermostat installation manual but you don’t want to end up with just one or two blurry photos (or skip this step entirely).

Your photos should be clear enough to help you put your old thermostat back into service if things don’t work out. Hopefully you have done the research before you started, but if something goes awry during the smart thermostat installation you’ll be glad you can quickly “undo” and go back to what you had before. Also, if you end up having to call support they’ll likely want to know how the old one was wired.

Before I disassemble anything, I like to take some good, clear photos of existing wiring before I take anything apart.

Consider what will happen to the wall behind your existing thermostat

In some homes, swapping thermostats is no big deal – the wall is painted white and always has been, the old thermostat is small and your new one is bigger – and so on.

But in other homes, you might have to take on an additional “wall repair” project in addition to the thermostat replacement. The most common problems revealed when upgrading to a smart thermostat are wallpaper damage and mismatched paint that won’t be fully covered by the new smart thermostat.

Many smart thermostats today ship with a “wall plate” for covering up the mismatched paint or damage revealed by swapping your thermostat. Chances are, the wall plate has you covered.

The wall plate that ships with Nest thermostats is easy to install – just sandwich it between the wall and the thermostat to hide wall damage and mismatched paint that was previously hidden by your old thermostat.

But if you don’t want to use the wall plate, you might want to think (ahead of time) about whether you plan to repaint or repair the wall before or after you’ve put up the new smart thermostat.

Have a wire cutting/stripping tool on hand

Thermostat wires are thin and break easily, and removing your current thermostat might snap a few of the tips off. If this happens, you’ll need to make new tips suitable for inserting into the new thermostat. These little tips are easy to break off – save yourself some hassle by having a wire cutting and stripping tool on hand.

Make certain to have a decent wire stripper/cutter with the right gauge holes. (You can also cut and strip thermostat wires with a sharp pair of scissors, but the process is much easier with the right tool.)

A wire cutting/stripping tool will make a DIY smart thermostat installation go a bit smoother.

Should you should hire a pro?

A heating contractor will do more than just swap an old thermostat for a new one – you could also bundle in any of the following work:

  • Service your HVAC system
  • Run new wires, including a C-wire if you need one
  • Move your thermostat to a new location
  • Other electrical work around your house

Sometimes having your thermostat moved to a different location can get you better control of the temperature in your home.

If your thermostat is in front of a sunny window, the thermostat might always think the house was warm, so why run the heat? Meanwhile, every other room would be cold.

A pro is also a good choice if you have any doubts about your ability to do the job (the manufacturers say it’s DIY, but it’s not always that simple in every house!) or just want it done quickly and painlessly by someone else. The peace of mind can be worth the extra expense.

Timing is everything: Don’t do the upgrade right before the “storm of the century” is expected to hit

Sometimes it’s just a good idea to wait until the weather is stable before you tear your current thermostat off the wall.

I hope this has helped. If you noticed any problems with this article or if you have any questions, please let me know.

Thank you for reading.

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