How to keep both AC units from working at the same time?

Alright, so I just had a 20KW generator installed to power the whole house. I have two units, 5 Ton and 3 Ton and I don't want them ever to run at the same time, because the two would be too much for the 20Kw. The units are a year old, and I'm using the Honeywell digital thermostats How can I keep the two from ever running at the same time? Should I replace the thermostats with two that talk to each other, or can the Honeywells do that?
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Just leave one of them unplugged.
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Oscar_Lives wrote:

How creatively sarcastic.. Oscar, please go back to the alt.simpleminds group where you came from.
Can anyone really help with a solution?
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No, it is fucking impossible. You are a fool if you keep trying this.
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It would be relatively simple to rig up some logic with a few 24v relays. Trouble would be finding someone locally to do that for you. Basically you want each system when it turns on to also power an extra relay. Then put the normally closed contacts on each relay in series with the thermostat for the other system. So when one system turns one the relay opens those contacts and prevents the other system from turning on. A decent service tech should be able to do it if you can find one willing to.

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James wrote:

James,
Thank you for your help in the matter. What you're saying makes sense. I certainly want to at least give it a try. I know anything is always possible.
I'd like to think Oscar is still being sarcastic and that he's having a rough day. However after seeing some of his unwitted responses his in past replies, who can take him seriously?
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On 17 Dec 2006 18:47:49 -0800, snipped-for-privacy@gmail.com wrote:

Simple, just disconnect the Red low voltage wires. The systems will then never run at the same time......................or ever. Bubba "Didnt like that answer? You got exactly what you paid for."
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Since you weren't specific, let's say that it's just the AC function you're trying to interlock and not a heat pump and it's a 24V AC system. Again, you didn't specify what controls you already have (2 thermostats and some sort of txfer switch, I hope) so what follows is really academic.
This will get you started. Please see the "notes" section for some followup you're going to have to do yourself! If you want to hack together all these features, you really don't want to do it with relays--see again the "notes" section.
In this example, the "Y" wire of the first thermostat turns on the compressor or the 5T unit. Connect the coil of a 24VAC relay from the "Y" output of that thermostat and the opposite leg of the 24VAC transformer.
Similarly, for this example, the "Y" wire of the second thermostat is used to turn on the compressor of the 3T unit. Lift the wire from the "Y" terminal of that thermostat and land it on the N.C. contact of the relay. Connect the common terminal (not the coil common--the common for the N.C. contact you just landed!) to the "Y" terminal of the second thermostat.
Now, the signal to turn on the 3T compressor has to go through those N.C. contacts. This will give the 5T unit priority and both units will never be on together.
If you want to be able to run both units together when you're not on emergency power, just put the N.O. aux contacts of the generator transfer switch in series with the coil of the new relay.
Notes:
Fan: Typically, the fan output is turned on whenever the thermostat calls for AC. Depending on your system, you'll have to deal with this. One way would be to use a double pole relay and wire the fan signal through the other set of N.C. contacts just like the compressor signal.
Restart Lockout: You'll probably want to use a restart-lockout relay to prevent the 3T unit from starting up again too soon after it's been shut down.
Short-cycle lockout: Once the 3T starts, you'd probably want it to finish its cycle before dropping for the 5T. You can install this circuit twice, once with the 5T as primary and once with the 3T as primary. To prevent relay races, wire N.C. contacts between the thermostat output and the relay coils in both instances. You get the added benefit of sharing priority equally if you do it this way.
However, at this point, relay logic becomes silly. You should be doing all this on paper instead of just going out and buying relays, and at this point, your design is probably pretty complicated.
Instead, buy a smart relay (a little tiny PLC is what it is) and program it however you like. Some sources--www.factorymation.com, www.automationdirect.com.
Have fun, and let us know how it turns out.
snipped-for-privacy@gmail.com wrote:

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Over engineered the crap out of that one didn't you?
snipped-for-privacy@hotmail.com wrote:

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jamesgangnc wrote:

I think that was my point.
Thank you for your support.
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snipped-for-privacy@hotmail.com wrote:

I'll consider your nice and comprehensive direction. Thanks!
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On 17 Dec 2006 18:47:49 -0800, snipped-for-privacy@gmail.com wrote:

You just need a couple of 24v relays that have a normally closed set of contacts. Wire each of them in parallel with the active wire (usually y) and ground of each thermostat and then tie the active wires thru the NC contacts of the 'OTHER' relay.
The first call from either thermostat will shut off the Y line to the other thermostat.
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snipped-for-privacy@gmail.com wrote:

On 17 Dec 2006 18:47:49 -0800, snipped-for-privacy@gmail.com wrote:

I assume you dont care if they both run when on normal power and its that if you switch to emergency power when you dont want both units running.
If that is the case I would have a relay powered off the transfer switch or generator which would de energize one or the other a/cs.
The relay, when energized, would break the control wire through a set of N.C. contacts to the condensing unit (your choice) compressor 24v contactor coil.
Consult the installer of the generator for a specific wiring location to power up the relay when the generator starts. Done.
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"Power's Mechanical" wrote:

not done.
you're forcing the customer to choose 1 of 2 when he could have both.
the two 24v relays are the best choice. the ho can purchase them off the web & install them himself. once in place, operation is automatic.
customer has the option of using either ac, but never both at the same time.
now put a sticky on each t-stat that says:
for ac, turn off the other thermostat
done.
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Actually I could see working the third relay into the picture. Use a 115v relay with two nc contacts. Wire it to the generator so it is pulled when the generator is running. The nc contacts can be used to bypass the other two relays so that when the generator is not running both systems can run at the same time. If you have a problem with hi/low voltages in proximity then add a 24v transformer at the generator and use a 24v relay for the third one.
snipped-for-privacy@gonefishin.net wrote:

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snipped-for-privacy@gonefishin.net wrote:

Sorry but thats an idiotic idea imo. Now your forcing him to always use only one a/c at a time during non emergency conditions. The two relay idea is ok, but only if they are in play when the generator is running.
Probably could use a bank of relays and really get crazy with redundant switching. Every time the stat calls.... click clack click clack relays pulling in and dropping out. Make sure to mount them on the duct work. :)
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Only takes three to include the capability to activate the single system limit while the generator is on. One relay powered by the generator to enable the system, then one relay for each system to lock out the other system while running.
I agree it's starting to reach the point where a control circuit with solid state switches would be the preferred solution. But it takes less background knowledge to rig up a small logic system with three relays than it does to lay out the circuit for it using solid state stuff. For a one off requirement the relays are still the way to go.
You could argue it's a potentially common need and design a control board in a box that can do it. I'd go ahead and make it smart enough to handle three or four systems since most of the whole house generator customers tend to have bigger houses with multiple hvac systems. Then market it to the generator companies.
Power's Mechanical wrote:

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jamesgangnc wrote:

The main thing is to have the alternate a/c control circuit enabled only when the generator is running.
It could be done a hundred differant ways.
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