Hacking a Nest Thermostat to control 10,000 Watts of Workshop Heaters!

//Hacking a Nest Thermostat to control 10,000 Watts of Workshop Heaters!

Nest Thermostat 240V

Control 240V line voltage heaters with this solution
and get control over your heating bill this winter!


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Garage Heater 5000W

With the winter just around the corner, heating your home will be something that will start eating up your budget.

Now there’s many different ways to heat your space: Oil, gas, and electric are the most common.

If you have electric heating then having an accurate thermostat (programmable is even better) is something you might want to look into.

That way you won’t be running the heat as much when it’s not needed, like when you’re away or at night.

One of the best one on the market today is the Nest Thermostat.  Not only is it very accurate, but offers programming, reports and is even controllable over the internet.

But…  It’s only able to control 24VAC systems, such as central heating, so if you’re like me and your electric heating system is 240V baseboards and heaters, then you can’t use the Nest Thermostat…  At least not out of the box…

In my workshop I have two 5000W overhead heaters, these have fans in them and are able to heat up the workshop fairly quickly.  They have integrated mechanical thermostat in them, but those are very inaccurate and not easy to adjust since you have to reach to the ceiling where the heaters are installed to adjust them.

Finding a programmable thermostat that supports 5000W is impossible, and even if I could find one, it would take two of them to connect each of the heaters.

In this tutorial we will see how to setup the Nest Thermostat with some Contactors and a Relay to control both forced air 5000W heaters, but you could even control more if you needed.

So let’s get started!

The NEST E Thermostat

Nest E Thermostat

Since this will be installed in the workshop, I don’t need the fancy metal enclosure of the flagship NEST thermostat, I decided to go with the cheaper NEST E Thermostat.

For my needs it has everything I need and nothing I don’t.   It still provides full reports, internet access etc…


Nest 240V Connections

Ok first thing first!


This project is dealing with very HIGH VOLTAGE (240V)!!!!

So as a disclaimer, I’m only providing this information as educational, if you decide to try this, be aware that messing around with HIGH VOLTAGE can cause major injuries and even DEATH!!!  So if you are uncomfortable, not sure, or just plain scared around HIGH VOLTAGE… PLEASE DO NOT TRY THIS!!!


Now with that out of the way let’s continue….

Click on the picture of the diagram above to see the connections enlarged.

We have two VAC transformers, one to power the NEST thermostat and a bigger one to power the contactors.

*note:  These are VAC transformers and not the familiar DC transformers that we used when creating electronic projects.  VAC is what is used in these systems (like your home doorbell).

The smaller transformer used to power the Nest thermostat has multiple options for VAC: 10, 16 and 24VAC.

The NEST E thermostat voltage input range is between 20-30VAC.

**In my testing I found out that these doorbell transformers voltage is not very accurate.  When testing the 24VAC output of the transformer, it was putting out around 29VAC, which is near the upper range of the NEST thermostat.

I chose to use the 16VAC output of the transformer which was around 21VAC and seemed safer.

So don’t forget to test your transformer output before connecting it to the NEST thermostat to make sure you stay on the lower side of the voltage requirement.  These NEST thermostat are not cheap so better be safe than sorry…

The other transformer is bigger, since it will provide power to the contactors and we want to make sure that they have maximum power when switching the 240V to the heaters.

*note:  You can find a list of all the components used in this tutorial at the bottom of this page with links to amazon.

The contactors are 24VAC and rated at 40Amps when switching 240V.  Each of my heaters (5000W) are rated at around 22.5Amps.  So well below the max of the contactors.

The Relay is a regular 24VAC fan relay that gets activated by the NEST thermostat to turn on the contactors and provide power to the heaters.

Everything is housed inside a plastic enclosure with a lockable door for safety.

Contactor Enclosure


The NEST E thermostat in this project basically acts as a simple relay.

When it detect that the temperature is too low, it will activate the Fan relay, which will turn on the Contactors.

The Contactors then connect the 240V to the heaters.

When the set temperature on the NEST is reached, the opposite happens.


When dealing with HIGH VOLTAGE and heating elements of course safety is at a maximum.  We don’t want something to go wrong and burn down our house or workshop…

In this project the main thing that could go wrong is that the Contactors for some reason get stuck on the ‘ON’ position and provide power to the heaters indefinitely, which would mean a very bad day.

My fail-safe is that in each one of my heaters has a redundant mechanical thermostat installed inide, and each one is set to around 18 degrees Celsius (65F).

5000W Heater Thermostat

So in the remote case that the contactors don’t shut off, the mechanical thermostat installed in the heaters would shut down the power, even if the contactors are still providing current.

Even if I set my NEST thermostat by accident to a ‘billion’ degrees, the thermostats inside each heaters would only allow up to 18C degrees.



I’ve been using this for about 2 months now, and I’m loving the fact that I can turn on and off the heat in my workshop whenever I want.

If I’m inside the house and know I’m gonna go in the workshop later, I can from the Nest website login and set the temperature.  I can even do it from my iPhone or even my Apple Watch if I want.

Sometime I leave the workshop and forget to turn down the heat, no problem, just do it from inside the house!

I very happy knowing that I won’t be wasting energy and money, heating my workshop in the winter when I’m not in there, and not coming into a freezing workshop when I go in.

Don’t forget to check out the YouTube video below if you want more information.

Hope you enjoyed this project and see you at the next one!

By | 2018-11-05T17:12:00-05:00 November 5th, 2018|Tutorials|


  1. Gurinder singh September 20, 2020 at 10:38 pm - Reply

    I was able to use the idea, on a 5000watt garage heater. It works smoothly. Many thanks bro..!

  2. Josh Taylor June 6, 2020 at 7:34 am - Reply

    Love the idea and the tutorial!! I would love to adapt this to my home theater. I currently have (2) 2′ x 2′ drop in radiant heat panels installed via a wifi outlet switch. So just on or off no control. I don’t think I need the system to be as robust since each pabel is only (120v 375w 1280 BTU/Hr). I have a Nest E laying around, so this would be a great upgrade. Any guidance on components and wiring would be greatly appreciated. Thanks love the channel!

  3. Josh Taylor June 4, 2020 at 11:35 am - Reply

    Great idea!!! Video is very helpful. Right now I have two Radiant Ceiling Panels (120V 375W 1280 BTU EACH) wired with a wifi switch on the outlet. I would love to do this with my spare NEST E. I assume, I won’t need as many components and they won’t have to be high voltage but would appreciate some guidance on the proper components. If you have any ideas, Thank you in advance.

  4. Avi Naimark May 22, 2020 at 3:24 pm - Reply

    Hi Yvan

    I enjoyed your video and the clear schematic diagram of how you wired this nest to control your 2 overhead forced air heaters. 2 questions though: 1) Why do you need to install the redundant mechanical thermostat inside your heaters? Isn’t that a safety function that’s assumed to be built into the heaters; not something left up to the consumer to install if they want to. 2) why do you need the double row screw terminal strip? Can’t you just splice wires from Power coming into the box to go to both transformers?

  5. Gary March 27, 2020 at 12:59 pm - Reply

    Would I use the exact same relay, even if I was just trying to control one contactor, for one heating unit?
    The only difference between your setup and mine would be a single 5,000W heater.

  6. Nick December 7, 2019 at 9:20 pm - Reply

    Hi. Thanks for the great workaround. I want to preface this with the fact that I have zero experience with electric. I bought two nest thermostats to replace my old ones. I have a high voltage electric system with baseboard heating. In my wall where the thermostat connects are only two wires. Google says my system is not compatible, but it looks like you e found a way around this. Any idea on what I should do to connect my nest? Thanks in advance! Again, great job!

  7. justin October 27, 2019 at 1:30 pm - Reply

    Awesome! Great solution to a problem I was having as well. Could I use a 50VA transformer instead of 75?

    • brainy-bits October 27, 2019 at 5:17 pm - Reply

      The Nest thermostat, E model and others generations all require between 20-30VA so a 50VA shouldn’t be a problem. I used a 75VA since that was all I could get my hands on 🙂 Hope that helps.

      • Gronk January 30, 2020 at 10:29 am - Reply

        Great info!

        You’re almost correct, but the units are tricky. VA (Volt-Amps) is the power rating of the transformer. VAC (Volts Alternating Current) is the electric potential. Most thermostats are designed to use 24 VAC power but can work over a small range since there’s lots of variability in the actual voltage supplied from the power company. In this case, the Nest E can run safely on 20-30 VAC.

        VA ratings tell you how many amps the transformer can ideally supply–in the case of a 75 VA it’s about 3 (75 VA / 24 Volts = 3.125 Amps). That’s also because a transformer is just a voltage multiplier/divider. If your house voltage is higher or lower than 120 Volts, the transformer output will be proportionally higher or lower. As long as the total current drawn by the Nest and the relay coils isn’t over 3 A continuous, you’re fine to use just the 75 VA transformer. The 50 VA can do just over 2 A.

        I’m researching using a Nest E with my 240 VAC 7500 W electric heater in my garage and was planning on using an Aube relay with built in transformer (RC840T-240) to accomplish the same thing you’ve done with the separate components. It’s a little more complicated to wire since I’m getting power from the heater instead of a wall outlet and using the internal thermostat relay on the heater instead of switching the whole power supply to the unit.

  8. Jon Bloomfield January 8, 2019 at 4:24 pm - Reply

    Thanks for this article it is a really useful baseline.I’d prefer to simplify the design slightly, but you have real experience, so want to confirm your thinking before I start ripping parts out 🙂

    What is the reason for using a separate transformer for the nest? Did you try connecting both the contactor and nest control circuits to the TR75VA001 and have issues or were you just being extra cautious. Also, any reason for not just using one of the heater supply circuits for the primary into the transformer(s), so you just have 2 supplies in, and 2 out?

    • brainy-bits January 8, 2019 at 4:50 pm - Reply

      Hi Jon, about the 2 seperate transformers I was just being extra cautious 🙂 but it shouldn’t be a problem using only the bigger one. About using one of the heater power supply to drive the transformer, it could be done as well, make sure to use only one leg (110V) and not 240V. I did the tutorial this way to make it simpler to understand all the connections but if you are comfortable with electricity then your idea of removing components is ok. Thank you for your comment and good luck with your project.

      • Jon Bloomfield January 8, 2019 at 6:33 pm - Reply

        Thanks for the quick response.
        Yes, i have a neutral available in the supply cables so was going to steal one hot and one neutral to feed the transformer.
        I think it’s time to order some bits 😊

  9. Ray December 1, 2018 at 9:23 am - Reply

    Where did you get the Plastic enclosure?

    • brainy-bits December 3, 2018 at 10:25 pm - Reply

      Found it at an electronic surplus store, but since it’s made for low voltage stuff, it’s not up to code. Technically you should be using a metal box to make it up to code, so any electrical box should work as long as it’s big enough to hold all the components. Hope that helps and thanks for dropping by.

    • Rico October 31, 2019 at 4:40 am - Reply

      Hi the 75 v transformer I got only have 4 wires. Where would the black dashed wire go ?

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