SmartThings Community

Controlling a minisplit/wall ac directly from nest/ecobee (without flair)

(Nate) #1

After searching and searching I thought I came to the conclusion the only way to control the AC in my guesthouse (MR COOL minisplit) was to use a FLAIR device. Well it turns out the software from FLAIR is quite bad and I didn’t get a timely response from their customer services. So I was forced to devise this better and cheaper solution to get a nest to safely talk with any AC unit…

For this design the AC unit needs to stay on all the time. I thought about cutting the power to the unit for nest control but this is bad for the compressor. I measured the power usage and found my minisplit used almost no power when it was on but the room was already cool enough (around 8W). So my thinking was if I can use the nest to fake the MRCOOL minisplit into thinking it was very cold or very hot in the room it will override mrcool’s internal systems.

The thermistor on the mrcool (or another mini split) changes resistance based on the room temperature. I cut the wire running to this. I took some resistance measurements and found around 15kohms of resistance when it’s like 50deg and 8kohms when it’s like 90deg in the room. I bought a 24vac relay:
and bought a pack of resisters. Nest will supply 24vac power on the yellow pin and close the relay when it tells the AC to cool. Short the thermistor with the cold resistance level (15kohms) all the time and then put this in series with a resister that will get you to 8kohms when in parallel when the relay closes. Please lookup the parallel resister formula or use an online calculator. The mrcool also tells you the temperature it thinks and so you can use trial and error or a potentiometer. It’s somewhat similar to this medium post on how to control a millivolt heater with your nest except this guy just needs to open/close the connection and we need to change between 2 resistances:

(see see someone else who is a better writer is doing this… must be safe!)

Well that’s it, when the Nest says cool, my minisplit thinks it’s 90 degrees in the room and when the Nest says don’t cool then the minisplit thinks it’s 50 degs in the room lol. Having this hard-wired solution is so much better than a cloud-based setup using flair or something. Not to mention it costs like $20 to make. Sorry for the sloppy write up but I’m hoping this will be enough information to help some folks out!

(Edward Niedziejko) #2

It’s clever, I’ll give you that. Nice idea. I do hope you plan on putting some shrink wrap or at least electrical tape over those connections though.

(Nate) #3

There is really no other safe solution to control a proprietary air contrition system. Obviously if it was a coil electric heater you can just cut the power with a big relay. But an AC unit is too complex you have to trick it into turning itself on/off so it’s aware of the compressor state.

Yes, all taped up now. I’ll try to take some better photos after the next guests leaves. fyi it’s 24vac and nest current limits the connections so don’t be too afraid to dig into there. Just watch the thermistor side that might not be as protected.

(Augusto Geovanny Martínez Cortés) #4

Have you ever tried the “smart cuby” controllers? Cuby Smart on Amazon MX

Im in Mexico right now and im using this for controlling every minisplit unit on the house. In fact, according the support in a few days they will release ST compatibility feature


I was invited to participate in a betatest for a Honeywell device/app combo to control mini split HVAC. I didn’t do it because I’d already settled on an adapter that connected to my NEST thermostats. But it looked like a good solution that’s probably going to be on the market soon.

(Nate) #6

Working link:

The issue I have with these devices is they don’t know the original state of the AC unit (they can only send commands, not receive the state). If I have a guest it in the guest house set the AC to 68deg. There’s no way to read this temperature from the unit using a rf blaster. So when the nest goes from away back to home the nest doesn’t know what temp to go back to.

Lastly, my stupid mrcool beeps whenever it gets a IR signal. I don’t really want to hear those beeps every time the nest decides to tweak the temperature by 1 degree.

(Gary Maurizi) #7

I’m really interested in this, and have just enough electrical knowledge to maybe set this up myself.

I’m just curious, does the Mr. Cool unit you purchased do both heating AND cooling? because you don’t mention heating?

What Mr. Cool unit is it exactly that your using? what model? because some/most mini split systems have two thermistors i think? one for the heater coil and one for the AC/fan and you only mention one thermistor?

With your setup with the nest and this single thermistor Mr. cool hack, when the nest sends the resistance to the thermistor to tell the mr. cool that it’s 50 degrees, the AC stops cooling correct? wouldnt you want to set it up to say stop cooling at 70 degrees and then start heating below 60? or does your setup already do this and I’m just missing or not understanding something?

I could really use the help! love this idea in concept, but not sure if I’m fully understanding and if it would work with my small california studio apartment where I plan to use a Mr. Cool unit for both my heating AND cooling and want the nest to control both?

Thank You so much for your help!

PS. I’m basing this question on a youtube video i saw of a man repairing thermistors in a mini split system, and he says there are two thermistors one for the AIR and one for the COIL? is the Mr. cool different and only has one thermistor? and then if you tell that thermistor “50 degrees” it BOTH stops cooling and starts heating?

–Gary M

(Nate) #8

Everything has been working great with this setup. I have it in a guest house I rent and so with zero supervision it has been running no issues since my first post.

I’m actually using the Mr. Cool (DYI 12k) for just cooling and I’m using Williams gas heater for the heating. Both are rigged to the nest the same way: Nest sets a pin to 12vac to command the device on, that trips a relay which shorts the thermostat or applies a specific resistance.

If you want it use the same device for heating and cooling - since the nest has 2 output pins for both systems I think what you’re proposing is correct. You could set the mrcool to 70 and put it on heat&cool mode. Also make the default resistance make the mrcool think it’s 70 in the room. Then when the next ac pin triggers set the resistance higher so mrcool thinks it’s hot, same for cool -> set resistance lower and the mrcool will kick into heat mode.

To start, I would buy two of those relays and get them working with the nest. You should hear them click when going into heat or cool mode, respectively. Then separately find the 3 resistance values that make the mrcool either be off, cooling or heating. Then lastly connect the units together.

We could design little circuit board to do this if like enough people wanted it. It would be universal, working with any thermisister-based heater/cooler.