It may sound ridiculous, but some people are afraid that smart thermostats are bad for heating and air conditioning systems. Here’s the good news: you have nothing to worry about, but there are a few important things to keep in mind.

A relatively recent post on Reddit talks about an HVAC technician’s warning about smart thermostats:

“He said he never recommends them, and that their company REGULARLY replaces systems that fail or fail prematurely due to Nest overloading the compute systems/motherboards built into most HVAC units – even in completely new models».

The technician is not quite full of it. If your smart thermostat is connected incorrectly, it can cause problems in your HVAC system. However, his explanation is mostly full of big FUD (Fear, Uncertainty and Doubt).

It’s all about the C wire, baby

Each thermostat has a number of wires (four or five) that lead back to the electrical control panel of your HVAC system. Each wire is usually responsible for something specific, for example, one wire is for cooling and the other is for heating. Many systems have a «C-wire» (or «common wire») that is designed to provide electrical power to the thermostat to power the screen and other electronic functions.


Older systems usually don’t have a C wire because then the thermostats didn’t need electricity due to the fully mechanical operation of mercury-commutated thermostats. But today, most modern thermostats have more bells and whistles that require electricity to operate. So the target is C wire.

AC wire is not necessarily required for smart thermostats, but it is highly recommended. And if your thermostat’s wiring includes wire C, you should connect it to your smart thermostat — most of the problems that come with a smart thermostat come from not having wire C connected to it.

However, it’s not the end of the world if your HVAC system doesn’t provide a C-wire for your thermostat, as a smart thermostat might work just fine with your particular system. My system does not have a C wire, but I have used Nest and Ecobee thermostats with no problems. However, I installed the Power Extender Kit that came with my Ecobee3, which provided a solution for the missing C wire (more on that later).

What happens when your system does not have the C wire


If you want to install a smart thermostat but your system does not provide C-wire, you are out of luck. Like me, you can have an HVAC system that will still work well with a smart thermostat without the C-wire, thanks to so-called «power theft».

This sounds bad (and maybe), but may work fine on some systems. Power stealing works by drawing a small amount of power from other wires when the heating or air conditioning is running. If you don’t use your system often enough, your smart thermostat will turn on the HVAC for a short period of time to absorb some energy for a short time.

However, this same method may not work properly on your particular system and may result in a short circuit or possibly component damage over a long period of time. If this is the case (or you’re not sure if a smart thermostat will work with your system), I’d recommend calling an HVAC specialist to check your settings and see if it’s possible to install a smart thermostat without the C wire.

What if you don’t have the C wire?


If your HVAC system doesn’t have the C wire for the thermostat as mentioned above, you don’t need to worry about this. However, if you want to be careful, there are a few things you can do.

First of all, your best bet is to go to an HVAC specialist and install the C wire so that you can use the smart thermostat without problems. This usually involves completely deleting the existing wiring and starting all new wiring that includes the C wire. This is a fairly simple job for an electrician or HVAC technician.

Some smart thermostats also offer adapter kits (such as the Power Extender Kit for Ecobee thermostats. You can also buy universal kits) that can add solid-state C wire to your HVAC system via the control board. It’s not as nice as just having a real C-wire, but it’s certainly better than nothing and will get the job done.

Finally, be careful and make sure your thermostat wiring comes with a C-wire. If not, you might still be fine, but it might be a good idea to install a C-wire or adapter kit anyway.

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