Mini Split AC limiting usage
 

Hey all
I am about to start renting out my house as a vacation rental.

I have 4 minisplit AC units that we don't use much, but renters tend to set them at 16*C (60*F) and try to make a meat locker out of their bedroom... even if the doors are open or they aren't home.

So instead of spending $500/month on electricity (can be done, I assure you), I'd like to spend some time/money up front to see if I can limit their AC abuse.

First some assumptions:
A) If you travel to an exotic tropical location (costa rica) you should be ready to experience some warm temperatures- I'd like to limit the AC units to where they can't cool past about 70 degrees (21*C)

B) if you aren't in the house, it is okay for the temps to climb a little, maybe as high as 80*F (28*C), especially if they fall again when you return

C) If there is nobody home for more than 8 hours, I'd like the AC to just plain turn itself off. So if a maid or tenant forgets the AC completely, it won't try to keep the house cool for a week of emptiness.


My idea is pretty simple, but I'm not sure if it will work and/or if it will cause harm to the AC units.

I'd like to get a motion-sensor controlled thermostat that I can set to simply cut power to the compressor of the mini split system. If the interior temp of a given room drops below 70* degrees, the fan inside will continue to operate, but the compressor will shut down and refuse to make the room colder. When it warms up (and it will warm up, our outside temps are usually in the 90s) then the compressor will kick in again.

I'd also like for the motion sensor to kick up the temperature setting whenever it goes for over X hours without motion. Maybe set the lowest temp at 78* F when nobody is present. At night that feature would have to be overridden so that people who are sleeping don't awaken to a hot room.

Personally, I don't run the AC much and when I do, I run it at 78. But vacationers are a testy bunch, as a rule, and will almost certainly complain if they can't make the place cold. I just want to prevent them from sleeping under a comforter or sitting around in a sweatshirt on a 90* day.

ideally, the motion sensor or the thermostat could be set such that if it gets no input after 24 hours, it'll just shut down. I'll leave a note for the renters telling them that the AC shuts down every 24 hours unless they adjust the temp at some point, or something like that.

Thoughts?

Mostly, I seek input on the technical aspect: Can I wire into the compressor power source without frying something? that sort of thing.

Thanks!

Are the thermostats wired? (My Sanyo has the thermostat in the remote control).

If they are wired, then you may be able to solve part of the problem by installing a second (hidden)
programmable thermostat in series with a wall thermostat. It could be set to limit the low end temperature.


Or, you could install some power monitors,

http://i46.photobucket.com/albums/f1...r/NCL/F160.jpg
(This one will display actual $ or Pounds, in real-time).

And tell the renters keep track of their energy use, so they can pay you
for any excessive use. Allow them x number of kWh a day (on average), for 'free'..

At a bare minimum, get the digital thermostats that you can program to have a high and low limit. (Use an excuse. Tell them you limit it to 65 because otherwise the A/C freezes up.) Also, put in a rotary 10-hour timer next to the thermostat with instructions on how to turn it on. That way, you don't have to show up immediately to turn off the A/C after they leave.

Or just go with the 65 degree limit and jack up the price $10 a night to make up for any abuse.

Direct Compressor Control is a Bad Idea...
 

Wiring directly to the compressor could have undesirable consequences. Most all A/C systems have a built-in delay circuit to force a delay period between power-ups. If you took control of the compressor away from the existing circuit board you'd likely shorten the life of the compressor.

Your idea is a good one and the decisions that you want a device to make are a bit complex. A microprocessor like an Arduino should be able to do it. It's not a trivial task you are attempting, but sounds like it might be a good product. You'll need someone who is fluent in microprocessor development, hardware and software, and also someone with the savvy to read the schematics of the controller board of your mini-split(s). It may be the same person.

When you toggle the on/off button on your mini-split remote controller, the IR receiver on your wall unit receives the command, and sends it to some leg on some chip that begins the turn-on or turn-off sequence (but always inserts a delay before turn-on). That is the place you want your controller to intercede, not at the compressor level. That way, rapid changes of state will be buffered and will not harm your compressor.

But you have identified a worthy problem, and there are oodles of rentals and motels and they are shifting to mini-splits, due to lower cost of operation.

Good luck with this one, it could prove ecologically advantageous to us all, and economically advantageous to the successful designer of such a device.

Best Regards,

-AC_Hacker

still noodling this idea
I've seen some interesting door-sensors, would love to incorporate the ability to kill the AC when the door is open. not the whole unit... just tell the indoor portion to turn off the compressor when the door is opened.
c'mon free market! invent something! :)

Sandcruiser

I believe you should call Hugues at Log One. He has developed exactly what your looking for. All proven no need to reinvent the wheel. He has developed set back thermostats with motion sensors and algorithms for apartments and offices and they are beautiful. Hugues great guy and even a better engineer.

Randen