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controling 2 units (one at a time) with switch

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GaryR

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I could use some help in designing a switch that will control two thermostats with a switch. The thermostat's send a 24v signal to an air conditioning unit to turn it on and off. I want to put a relay between the two thermostats controlling wire so that the air conditioners CANNOT run at the same time. The circuit should let the first thermostat that wants to run send its signal to its air conditioner while breaking the circuit from the second thermostat to the second AC until the first one calls for stopping the AC (i.e., the thermostat will break the 24 volt lead when it reaches the desired temp). When this happens the other unit could then send its signal to the ac if necessary. The problem is that only one should be allowed to run at a time but once it gives up control either unit could take control. Originally I thought of just wiring the 2nd thermostat lead to the relay and if it wanted to operate it would energize the relay and take control but that would place it in a master position and I don't want to stop the working a/c until it gives up control. Again I want either t/s to be able to request control, and the first one to request control keeps it until it gives up control. Any ideas?
 
You can use a simple relay lockout configuration, as shown in the attached sketch.

When one of the thermostats activate, the associated relay is activated (which disables the other relay and air conditioner).
 

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Dougy,
I think I understand your drawing... see attached... assuming I have it correct would you happen to know of a relay module that could do this or would I need to purchase two separate relays?
thanks Gary
 

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No, that's not correct. Have another look at my drawing. The rectangle with a line through it is the relay coil, everything else should be pretty obvious - K1 is the coil of relay 1, K1.1 is the contacts of relay 1. Relay 2 has labels of K2 and K2.1 in a similar manner.

You will need two relays.
 
One point that needs to be checked is whether the 24Volts is AC or DC.

All of the HVAC thermostat systems that I have ever seen have been AC, but I'm willing to accept the possibility of a DC system.

But if it truly IS AC, then the relays either need to have AC coils, or will need a bridge rectifier for each coil.
 
Chris you were right the thermo wire is 24vac. I have modified my drawing to what I believe will work. If either thermostat calls for cooling the first one will activate the relay which will cause the second thermostat wire to open and disable it from running at the same time. When the initial thermostat releases it will allow the other to activate and disable the first one. Please let me know if I have it right... I am thinking of using this relay "< https://www.electro-tech-online.com/custompdfs/2013/04/090377a0160.pdf >". Since this is an ac relay I was hoping I could put it in series like it is shown in my drawing. Thanks for your help....
 

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Gary, you need check what the thermostat port of the AC unit is electrically. It may look like 24VAC when open, but the thermostat probably shorts across this in order to turn on the AC. As such, there may not be 24 volts with any current behind it to use to power an external relay. At least, probably not a 24 Volt one.
 
I looked at the Air Conditioning diagram and it does send 24vac to the thermostat but it most likely as you pointed out does not have much current. I guess I will need to find a very low current perhaps even a solid state relay for this. Would you happen to know of a very small low current relay that would work?
 
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