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What's the best way to keep both these furnaces from running at the same time when the generator is running
 

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I own stock in FotoMat!
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Wire just one of them from the panel that is fed by the genny.
 

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Easy to keep both off but to make so either can run by itself would take some relays.

Two ways come to mind. An alternating set up so each can one only one cycle before the other can run.

Or use lock out relays so when unit A runs it locks out unit B until A shuts down and the same the other way.

I think the second way would be how I would go, that would allow one to run many cycles in a row if the other is satisfied.
 

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Easy to keep both off but to make so either can run by itself would take some relays.

Two ways come to mind. An alternating set up so each can one only one cycle before the other can run.

Or use lock out relays so when unit A runs it locks out unit B until A shuts down and the same the other way.

I think the second way would be how I would go, that would allow one to run many cycles in a row if the other is satisfied.
:nerd::nerd::nerd:
 

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An adjustable timer, controling a DPDT contactor,
The time period would be adjustable,
The DPDT contactor would be wired into the two furnace control systems after the thermostat,
So as each thermostat clicks in, it then has to wait for the contactor to cycle thru to its turn.

Not difficult at all !
 

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An adjustable timer, controling a DPDT contactor,
The time period would be adjustable,
The DPDT contactor would be wired into the two furnace control systems after the thermostat,
So as each thermostat clicks in, it then has to wait for the contactor to cycle thru to its turn.

Not difficult at all !
I don't see any need to complicate things with a timer.

Two ice cube relays added to the units control circuits would do it.
 

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If it is a 2 story dual zone system just allow the 1st floor unit to heat.
The heat rising will be enough for the second floor.
 

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An adjustable timer, controling a DPDT contactor,
The time period would be adjustable,
The DPDT contactor would be wired into the two furnace control systems after the thermostat,
So as each thermostat clicks in, it then has to wait for the contactor to cycle thru to its turn.

Not difficult at all !
SPDT I believe is what you mean.
 

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I installed a Generac 25Kw water cooled last year and had an intelligent transfer switch that came with four low voltage load shedding relays. I wired the thermostat cables through the load shedding. When the generator starts it powers up the panel but holds off the low voltage relays for 2 minutes and then closes the relays at 30 second intervals while monitoring the output load. If the load is reaching maximum output it starts unloading the relays. My customer had 3 air conditioners that had to run as his house has a lot of wood and the last time we had a power failure the wood warped and he couldn't get the doors open.
 

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I would recommend starting with the low voltage leads, easier to work with. As others have said some type of relay.
 
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