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.