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

[Sennen]

Why has nobody asked what the LRA, RLA, and MCA of the compressor units are? With a proper soft start (Micro-air), the LRA can be significantly reduced. The RLA will help determine how many can run at a time based on the inverter specs already provided.

@BlueMarblePA , can you pull these specs from your compressor units?

 

[BlueMarblePA]

Why has nobody asked what the LRA, RLA, and MCA of the compressor units are? With a proper soft start (Micro-air), the LRA can be significantly reduced. The RLA will help determine how many can run at a time based on the inverter specs already provided.

@BlueMarblePA , can you pull these specs from your compressor units?

Excellent idea. I have to do this anyway, because I have to buy the MicroAir soft starts and get ready to install them. Of course, we did have some snow squalls today

 

[KenmattVentures]

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?

There are smart thermostat control devices, check online store or Alibaba

 

[Mason Parsons]

For your setup, installing Microair soft starts is a great idea to prevent all three AC units from kicking in simultaneously. As for smart thermostats, you might want to check out brands like Nest, Ecobee, or Honeywell.

They offer smart features that can potentially coordinate the AC units to avoid them all running at once. Just make sure to look for models that specifically mention multi-zone or multi-thermostat control. It could save you some energy and make your home more comfortable!

 

[BlueMarblePA]

For your setup, installing Microair soft starts is a great idea to prevent all three AC units from kicking in simultaneously. As for smart thermostats, you might want to check out brands like Nest, Ecobee, or Honeywell.

They offer smart features that can potentially coordinate the AC units to avoid them all running at once. Just make sure to look for models that specifically mention multi-zone or multi-thermostat control. It could save you some energy and make your home more comfortable!

I contacted Honeywell, and this is what they said, "Unfortunately we dont have devices that communicate with each other, we only have zoning panels for those applications and even then they cannot be setup to limit the amount of zones asking for cooling or heating."

 

[JayArr]

Just use relays on the signal lines from the thermostats to the compressors.

You will need Qty:3, three pole, double throw relays.

Each relay will require at least one other relay to be off before it can close and start it's respective compressor. If 2 and 3 are on 1 can't start. If 1 and 3 are on 2 can't start. If 1 and 2 are on 3 can't start.

The thermostats will continue to call for their respective compressors to start and when one shuts off they will be able to command it. There is no need to stop a thermostat from 'requesting' so regular thermostats can be used.

 

[wpns]

Just use relays on the signal lines from the thermostats to the compressors.

You will need Qty:3, three pole, double throw relays.

Each relay will require at least one other relay to be off before it can close and start it's respective compressor. If 2 and 3 are on 1 can't start. If 1 and 3 are on 2 can't start. If 1 and 2 are on 3 can't start.

The thermostats will continue to call for their respective compressors to start and when one shuts off they will be able to command it. There is no need to stop a thermostat from 'requesting' so regular thermostats can be used.

But I don't think the OP wants 'only one to run at a time', he wants 'only one can start at a time', so there's a lot more logic here. Lock out the ones that are not running for 5 seconds after any one starts?

 

[JayArr]

Look at the diagram again.

Two can run at the same time.

If any two are running the third will not start.

If two are running and third calls for action nothing happens until one of the others stops.

Eg, If the livingroom and the kitchen are both running and the bedroom gets warm the bedroom thermostat will call for cold air but the relay for the bedroom compressor will not start. The bedroom thermostat will continue to call for air but it's dumb, it won't fault or throw a code if it doesn't get it, it will just keep calling for it. When one of the others stops, let's say the kitchen, then the bedroom will turn on.

This is a whole series of relays, small 5VDC ones in the thermostats will activate the 24V three pole relays which control the contactor relays in the air conditioner that control the 240VAC to the compressor. No overlap and no need for delays or fancy logic.

Sometimes the best answer is the simplest, this requires no software, no CPU, no programming.

 

[wpns]

Look at the diagram again.

Two can run at the same time.

If any two are running the third will not start.

If two are running and third calls for action nothing happens until one of the others stops.

Eg, If the livingroom and the kitchen are both running and the bedroom gets warm the bedroom thermostat will call for cold air but the relay for the bedroom compressor will not start. The bedroom thermostat will continue to call for air but it's dumb, it won't fault or throw a code if it doesn't get it, it will just keep calling for it. When one of the others stops, let's say the kitchen, then the bedroom will turn on.

This is a whole series of relays, small 5VDC ones in the thermostats will activate the 24V three pole relays which control the contactor relays in the air conditioner that control the 240VAC to the compressor. No overlap and no need for delays or fancy logic.

Sometimes the best answer is the simplest, this requires no software, no CPU, no programming.

So you can’t run all three at the same time? and you could start two of them simultaneously?

 

[JayArr]

Yes. In actual life the chances of two of them starting at the very same instant are ridiculously long odds. Even so, if they have soft start caps it shouldn't be a problem.

I don't know where this idea that the compressors continue to run came from. I've been servicing my own AC systems for years and years and the compressors have always shut down immediately. The cooling fans on the evaporator units outside will sometimes continue to run and sometimes the blower fans inside will continue to run (depending on make/model) but I've never seen the compressor continue to run once the thermostat removes the call for action. For this reason I see no need to delay start of one as another shuts off. The OP can check his service literature and be sure, maybe he has a system I haven't seen where this compressor run-on happens.

 

[ksmithaz1]

OP states only two of three to run at the same time, not start. To be absolutely certain. . . maybe a couple of Dryer/EV priority switches (~$150 each on amazon) feeding a transfer switch ($60) for the red-headed step-child unit. Set primary on the two EV switches to the priority units, and feed the secondaries (EV outlet) to a simple transfer switch, common to the lowest prio unit.

 

[wpns]

OP states only two of three to run at the same time, not start

/*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.*/

Correction: The OP has changed from the above to "no more than two running at the same time", which is different, and opens a different can of worms. It's all about the detailed specification...

 

[Sennen]

/*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.*/

I looked it up. All of the 'smart' thermostats don't talk to each other and multi-zone thermostats don't control more than one device. You'd have to custom engineer something as above. US AC Thermostat circuits are 24VAC so you'd have to likely use relays.

My thoughts are to use a circuit that has all the inputs and outputs for whatever is controlled and triggers whenever a call for cooling is initialized to add a delay to the other circuits if they get a simultaneous call. The Micro-Air does fan then compressor about 3 seconds later, so a 5 second delay. It would have to be set to not interrupt existing cooling calls, though. Surprised something like this doesn't exist in a prepackaged format.

 

[time2roll]

Yes. In actual life the chances of two of them starting at the very same instant are ridiculously long odds. Even so, if they have soft start caps it shouldn't be a problem.

This is the real answer. I see this as trying to solve an issue that does not exist.

Just don't time the programs to all drop the temperature at 4:30pm so the house is cool when you get home. Set them at 15 minute intervals. Or set the clocks 5 minutes apart if the time does not sync to the wifi or something else.

 

[BlueMarblePA]

Wow. I am amazed by the knowledge on this forum. It is below freezing here outside Philadelphia today, but I am going to start on the MicroAir and then maybe play with the thermostat timing a little bit. In the meantime, I would love to learn how to do the relays. Maybe @JayArr can provide links to the hardware needed? I am a complete newbie with respect to HVAC. I will use lots of youtube videos to install the MicroAIR. I installed my own solar (Solaredge 7600 rooftop and Solark 15k 20kwh battery), and so I love to learn!

 

[Sennen]

Microair is easy. I initially put it on with Self-Taps, but swapped it out to rivet nuts about 2 weeks after taking this pic since the self-taps were vibrating. *Very thin metal

 

[stienman]

This is a complicated problem, and there are several levels of solutions.

The problem I have is that without knowing the worst case scenario for the home in question then we're all just shooting in the dark. Assuming these only provide cooling and not heating then it isn't as bad since you won't suffer catastrophic damage if the units fail (it'll get hot, but it's not going to causing pipes to freeze like a heating system failure might.)

The demand relay system is a good, easy, and cheap way to get started, though, and you can move to a more complicated system if this doesn't meet your needs. 6 tons of cooling in a northern climate with a 4,000sqft house, though, should be fine with the simplest relay demand system.

As long as you're using good AC units, and good temperature controllers they'll have appropriate compressor protection delays built in so you shouldn't cause any issues with a simple priority circuit:


A quick route and the PCB would be something like:

 

[stienman]

Design can be found here, should be easy and cheap to produce via jlcpcb, pcbway, or any similar service. Of course it could be easily hand wired as well.

OSHW project:

Priority HVAC Cooling - EasyEDA open source hardware lab

In EasyEDA open source hardware lab, open source square pools all kinds of electrical design engineerings together to realize resource sharing easily.

oshwlab.com

Open in editor:

https://pro.easyeda.com/editor#id=639431591fc34094b3a847918ac0c330

 

[stienman]

The problem with a simple priority system as I propose above is that the 3rd priority unit will be starved under extreme conditions. If the first two units call for cooling most of the time the temperature in the third will creep up over time and the temperature difference will grow significantly. In a shared space like a home this may be ok as the air mixes everywhere, but if you have three levels then it's likely that one will always seem hotter than the other two on the hottest days.

So a smart system will take into account not just whether two are running and their priority, but also the temperature difference between setpoint and measured, and when running two at a time can't maintain the setpoint everywhere, the smart system gives time to each to keep the difference between the setpoint and the measured value in each zone as low as possible while giving cooling to each.

With 6 tons over 4,000sqft in a relatively northern climate, and assuming reasonably good insulation, though, I honestly doubt you'll ever need more than a simple prioritization circuit like the above.

Further, modern 2 ton units don't take all that much energy, you should be able to run all three for under 6kW of power, leaving 6-8kW for your other household needs. If you have modern inverter AC units then you don't need to worry about startup either.

 

[ksmithaz1]

Further, modern 2 ton units don't take all that much energy, you should be able to run all three for under 6kW of power, leaving 6-8kW for your other household needs. If you have modern inverter AC units then you don't need to worry about startup either.

To this point . . . Your efforts might be better directed at a simple control relay on the lowest priority unit. Hook it to a panel load monitor. If your total panel load exceeds (say) 95%, turn off the relay. If the purpose is to prevent overloading the panel this makes the most sense. It's OK to run at ~105% of rated capacity for very short periods (ie long enough to detect and disable some loads), and as long as it's not too often it should not degrade the breakers. I think you might find you are using significantly less demand power than you think with the splits.

I have a similar arrangement connected to my oven/range. A transfer switch PV or grid to output. I have a Pi monitoring load, it will cut the range (40A circuit) to the grid if my total power exceeds 90A. It was set to 70, and it actually tripped over once, though at the time it was not even on. Thinking it over it dawned, you generally don't turn everything on the range on all at once, so I bumped it. It has not tripped over since, to the point I turned the breaker off at the grid panel. When I get the third 18KPV, my inverter output will match my grid capacity, and my only overload worry will be charging two cars at the same time.