Three AC units in one house - How to stop them from turning on all at once

[Jason Lane]

If you don't mind delving into a bit of home automation, what I would do is to use something like this:

1. Some kind of smart AC control unit for each AC. Whether it's a smart thermostat, or something like a Sensibo that uses infrared signals to control the AC.
2. A smart home hub. As Madcodger mentioned, Hubitat is one option. I personally prefer Home Assistant (I'm experienced in both).

With those in place, and a bit of learning, you can program just about anything you want. It would be fairly simple to prevent more than 2 ACs to run at the same time. Or, if they're inverter types ACs, instead of outright disallowing 3 units to run simultaneously, you could say that if a 3rd unit is switched on, you can still allow all 3 ACs to run, but override their set points automatically so that the power draw from each AC will be lower, and therefor lowering the combined power draw.

 

[wpns]

If you don't mind delving into a bit of home automation, what I would do is to use something like this:

1. Some kind of smart AC control unit for each AC. Whether it's a smart thermostat, or something like a Sensibo that uses infrared signals to control the AC.
2. A smart home hub. As Madcodger mentioned, Hubitat is one option. I personally prefer Home Assistant (I'm experienced in both).

With those in place, and a bit of learning, you can program just about anything you want. It would be fairly simple to prevent more than 2 ACs to run at the same time. Or, if they're inverter types ACs, instead of outright disallowing 3 units to run simultaneously, you could say that if a 3rd unit is switched on, you can still allow all 3 ACs to run, but override their set points automatically so that the power draw from each AC will be lower, and therefor lowering the combined power draw.

I have a Sensibo but haven't set it up yet (in the queue), can you query the Sensibo for whether a particular unit is actually running (and drawing power)?

 

[codfish]

I use Hubitat and Honeywell T6 Pro Thermostat to control our heating (and many other things). The T6 has the advantage of high WAF and reports instantly to the Hubitat hub all state and environment changes. Thus wife can just touch thermostat and I can write rule in Hubitat to do local, hands off automation of it or other things. Allowing the family to interact in traditional ways makes everything Much better. It would be quite easy to control 3 of them with most any algorithm (down to a few seconds latency).

 

[Jason Lane]

I have a Sensibo but haven't set it up yet (in the queue), can you query the Sensibo for whether a particular unit is actually running (and drawing power)?

No, unfortunately not. That's the main drawback of the IR blaster type of controller. The best it can do is register signals that have been sent from the real IR remote, so if it picks up that signal, the state will be reflect in the app. But if the Sensibo misses a signal, then its state in the app will be out of sync.

 

[Ai4px]

I have a 4000 sq ft home, and I diy self installed 9.8kw panels on the roof (Solaredge7600) and 5.6kw panels on the ground (Solark 15k) as well as 20kwh Pytes batteries. I have three 2 ton AC units in different zones. I want to install three Microair soft starts, but I also want to know if there is a way for three thermostats to talk to each other, so that they don't turn on at the same time.

Does anybody know if there is a smart thermostat system that can do this?

YOu can interlock the thermostats? it's low power, so small relays could be used. I imagine you would have a priority order so that if the main house called for heat, the bedroom would be shut off. I would do it with 24vac DPDT relays. Run the common for the thermostat thru the contacts of the relays.

When heat pump one calls for power, it would open the contacts of a relay which contained the commons for units 2 and 3. Likewise for unit 2 having a relay who's contacts opened to interlock unit 3. Could be done outside the house where the heat pump units are located.

 

[Ai4px]

Assuming you have standard 24 VAC discrete thermostat controls (not RS-485 communicating ones), then:

Divide your units into #1, #2, and #3 priority. You need a relay that closes when #1 turns on, another relay that closes when #2 turns on. Then run #3 turn on signal in parallel through the NC contacts of both relays. If at least one of them is NOT active, then #3 unit can run. As soon as both #1 and #2 are active, the signal has no path to the #3 unit.

This simple system has some gotchas, however. Most modern systems don't stop the compressor immediately when the signal goes away. It could be a few seconds lag for #3 to shutoff. That is exactly when you have 2 units on and the third starting, worst case scenario.

Also, compressors don't like to be quickly restarted. If #1 is running, #3 is running, the #2 starts, #3 drops, then #1 stops, #3 starts again too quickly. Most modern system have timer interlocks to prevent this, however.

The real way to solve this is to run all three units to some intelligent controller, like a PLC, and program it with any desired method to keep things working. This can also prevent the starting of two units at the same time, something you would like to avoid.

A better choice might be this:

https://www.amazon.com/Relays-Inputs-8-Layer-Stackable-Raspberry/dp/B099KW57JN

View attachment 200888
Key feature, optoisolated inputs since you can't be sure your units all share the same ground/neutral for the low volts thermostat wires. You run the controls into the optoisolated inputs, then the relays decide to pass them on or not.

Bonus point, make the relays pass the signals on the NC terminals (energize relay to NOT pass controls). That way, is the system loses power, it defaults to passing controls to the units so you have some HVAC workable. Or you can make some units default on during outage, and one not, just to handle the failure case. You choose by using NC or NO terminals and whether the relay energizes to be on or off.

In normal operation, relays are in the "off" state (which means some may be energized). You watch the inputs every 5 seconds. If a unit wants service, switch its relay and the unit turns on. Next 5 seconds another unit might service, you can switch its relay. When the third shows up, do nothing, you are maxed out. When a unit doesn't want power, switch its relay to shut it off. Next 5 seconds you can start the third one. In this way, only one thing can happen every 5 seconds which avoids two starting at once, and you can control how many and in what priority they start, and even control minimum off times for compressor health.

The Pi could also do Solar Assistant at the same time, of you want, or anything else it can do since monitoring HVAC inputs is rather trivial effort. You can also log your AC run times and patterns if you show choose.

If you need more relays or inputs, this board can be stacked to get you more of them. This may be an issue if you have two stage cooling, which then means you could run all three in low stage if desired and only have to lock one out in high stage.

You do need to get down and dirty with coding it, however. Python is probably the best way.

Mike C.

I've not been impressed with RPI.... I tried hosting Home Assistant on one and it kept not booting. If the OP needs a microprocessor based solution, I would use a ESP8266 or ESP32. but I think what he wants can be done with simple 24vac relays.