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!