Controlling mini-split A/C with external thermostat

[grow nerd]

Is there an easy way to connect an external, 3rd-party thermostat or controlling device to a typical mini-split system with a built-in thermostat? The way I understand the electrical system on these, it goes (on a 1-phase 230/240v installation w/o neutral):

Power Box (2 hot + ground) -> Outside Unit (2 hot + ground + signal) -> Inside Unit

I'm assuming that the third "signal" wire is the thermostat wire that supplies a 24VAC current to the outside unit's contactor coil.

So I guess the problem is, how do I control the indoor unit (air handler) with this thermostat wire, since the "signal" terminal is apparently for OUTPUT (not input) to the outdoor unit? Assuming it's a 24VAC to begin with, as in a traditional non-mini system.

Any help / info appreciated.

 

[weimer222]

Good question. I'd like to know too. I was thinking of getting a mini-split heat pump model and keeping it on automatic mode, where it would heat or cool to get at the set temperature point. However, it would be nice if I could have a lower temp set at night than during the day. If an external temperature sensor could be attached, then I could use an environmental controller with day/night settings to accommodate the differing temps.

 

[Lazyman]

Ask a sparky about a 120V-triggered 24vdc relay, there are such animals, I've seen them on the web but can't remember where. It should be the only piece you would need. Me I would probably use a power supply for a sprinkler system, it's a 120V in, 24vdc out transformer just like you need and $20 at home depot.

 

[weimer222]

Ask a sparky about a 120V-triggered 24vdc relay, there are such animals, I've seen them on the web but can't remember where. It should be the only piece you would need. Me I would probably use a power supply for a sprinkler system, it's a 120V in, 24vdc out transformer just like you need and $20 at home depot.

Except, that really doesn't fix the problem. Although you can have an external controller activate a relay when a environmental condition is met, it only provides power to the mini-split. At that point the temperature set on the mini-split controls the actual temperature. I am not sure what the original poster wants to do, but in order for me to have separate day/night temps, I would need an external environmental controller become the thermostat for the mini-split.

 

[Lazyman]

I think GN uses a Sentinel CHHC-1 controller, so he can plug a transformer into the temp control port so it triggers the AC if it gets too hot.

 

[grow nerd]

I tried a 24VAC transformer on the "S" terminal (both indoor and outdoor units) and no results.

 

[Lazyman]

Interesting, does it generate enough milliamps to trip your AC? Sometimes the polarity needs to be reversed on the output line. I use one of those to control an outdoor mister system with an electric sprinkler valve.

Oh yeah, and they burn out super easy. I went through 3 before I was careful enough with the wires. If they touch even for an instant the transformer is usually fried. Check em with a voltmeter for 24vdc output.

 

[coolx]

I don't know which brand you have. Fujitsu's mini splits come with with an optional wired remote control ($150 or so - google UTB-UUB) that you can program directly to different night and daytime 'fallback' temp settings so you don't need an external controller. Works great for me! I'd imagine other brands would come that way too as if you're paying thousands of dollars for one unit, usually for offices, they don't need to be cooled as much at night so they should be programmable to different temps.

 

[grow nerd]

Lazyman, I think it is 24VAC and not 24VDC.

I have personally not yet had any problems with the multi-tap transformer ($16 & change, or under $20 out the door) from Grainger that I got to power motorized dampers and extra thermostats.

coolx, thanks for sure. I'll keep that in mind for future use. This one came with a wireless, but I'd actually prefer wired in this situation.

 

[Lazyman]

Ah that's probably what went wrong, the transformers all put out DC.

 

[grow nerd]

I'm not so sure about that. Unless I'm mistaken, HVAC controls are 24VAC, not DC.

The transformer I have, and typically used, is to step down to 24VAC from a higher voltage AC source (such as 120, 208, 240).

On all the "normal" split / ductless systems I've seen, the 24VAC transformer is mounted in the system (in the indoor unit IIRC). The transformer should always be live, and the thermostat relays this power signal on / off to the indoor unit (which then in turn relays some of the wires to the outdoor unit). Seems usually 3 to 5 wires (depending on whether heating is used or cooling only) to indoor from thermostat, and 2-3 wires to the outdoor from the indoor unit.

 

[cocktail frank]

yep, its quite feasible.

you could even get a DPDT stat and wire it into the line voltage going to the blower.

i see no reason why that wouldnt work.

 

[grow nerd]

Thanks for chiming in, cocktail frank (always nice to have your input in a thread).

Unfortunately it doesn't seem to work with at least this (inverter-based) mini split. I would have thought it'd work just as easily as it would with a traditional full-sized split by supplying 24VAC, but apparently the "signal" wire carries more than just 24VAC line voltage... carries self-diagnostic info as well as other data from sensors.

At least that's what I'm interpreting as told by the seller / manufacturer.

 

[oceans7]

IR solution

IR solution

I've achieved "scheduled" control of my aura 36k BTU minisplit by using scheduling replay of specific IR commands at predetermined times using a globalcache IR device on my network.

It was surprisingly difficult to sort out but it reliably changes the temp at 9AM and 9PM just like I want.

With VPN access, I am able to control the temp remotely via my smartphone, which is pretty slick as well.

I'd love to hear of a better solution if one is out there.

 

[tenthirty]

You could use a zwave device such as a vera to control the temps with the lights and VPD if you so desired.

 

[CaliKine]

Ah that's probably what went wrong, the transformers all put out DC.

I'm sure you meant that no transformers put out DC voltage, right?

 

[CaliKine]

I'm not so sure about that. Unless I'm mistaken, HVAC controls are 24VAC, not DC.

The transformer I have, and typically used, is to step down to 24VAC from a higher voltage AC source (such as 120, 208, 240).

On all the "normal" split / ductless systems I've seen, the 24VAC transformer is mounted in the system (in the indoor unit IIRC). The transformer should always be live, and the thermostat relays this power signal on / off to the indoor unit (which then in turn relays some of the wires to the outdoor unit). Seems usually 3 to 5 wires (depending on whether heating is used or cooling only) to indoor from thermostat, and 2-3 wires to the outdoor from the indoor unit.

HVAC conventional systems are all 24vac.. Don't even try to mess with applyimg power of any kind to any wiring on any mini split. Its not that simple.

You can however control it in an "on/off" method if you like using any conventional thermostat by one of the following methods.

1) Using a transformer that you provide, have one of the secondary wires (24vac) run to the Rh & Rc of your prefered thermostat. From the thermostat run a wire from the Y1
terminal to a 24vac relay coil terminal . Then run a wire from the other coil terminal to the
other secondary wire of your transformer to complete the circuit. So, what you have mow is a relay that will close when the thermostat calls for cooling.

Now, find L1 on the inside section of the mini split coming from the outdoor section.
remove it from the terminal where it is connected and re route it to the normally open contacts of your relay . Then run a wire from the common terminal of your relay to the terminal that the L1 wire was connected to.

So in a nutshell what you have is an external thermostat that interrupts the L1 power to the indoor section using a relay activated by your new thermostat. When the thermostat calls for cooling the mini split is allowed to run. So now just set your mini split to the lowest setting and let your new thermostat do all the controlling. Mini splits have a feature called "auto restart" which just means that when power is interrupted it will remember what it was doing and when power is put back on it will resume doing it. So just set it on cool at a low setting and you are good to go!

 

[Q7u7ybHy4J5G]

I'd love some more information on what you did to achieve this. I have two mini-splits in the same room. They don't have any programmability such as night/day temps and I'd love to be able to automate it.

 

[Q7u7ybHy4J5G]

Sorry, I should have been specific. I was referring to:

I've achieved "scheduled" control of my aura 36k BTU minisplit by using scheduling replay of specific IR commands at predetermined times using a globalcache IR device on my network.

It was surprisingly difficult to sort out but it reliably changes the temp at 9AM and 9PM just like I want.

With VPN access, I am able to control the temp remotely via my smartphone, which is pretty slick as well.

I'd love to hear of a better solution if one is out there.

 

[CaliKine]

Mini Split systems are great because they are very versatile, no duckwork and they are super efficient. I was bummed when I found out all I got was a handheld remote controller . I could barely even see the tiny numbers on the 2" square screen, no backlight display, it sucked. And then it would get lost like all remotes do once in a while. So I decided that I had to do something. I looked into some tado device but it was too complicated and just added more crap to my space that was already crowded. Then a friend told me about an adapter that you could install with a nest thermostat that would control a mini split and even give you remote control capability. I ordered one and installed it in about ten minutes and was able to throw the remote away. The device is called a nest mini split adapter. Works great!