Fairly trivial relay based HVAC priority circuit. Supports three AC units and three thermostats. The first and second control their respective AC units directly - their signals are passed through. The third only runs if 0 or 1 of the first two units are running. If both of the first two units re running the third's signal is disabled, preventing more than two systems from running at the same time.
Ordering PCB and assembly from JLCPCB should result in $2 for 5 PCBs, and $22 for assembly and parts for 5 units, so $24 for 5 assembled units not including shipping and handling.
Eventually the design will be available here once the approval process has completed. It should give you access to editing/altering it to fit your needs, like mounting holes (doh!): https://oshwlab.com/stienman/priority-hvac-cooling/
Note that I haven't made or ordered this design. While the relays are rated for 24VDC they should be fine with 24VAC, but I provide no guarantees.
Ordering PCB and assembly from JLCPCB should result in $2 for 5 PCBs, and $22 for assembly and parts for 5 units, so $24 for 5 assembled units not including shipping and handling.
Eventually the design will be available here once the approval process has completed. It should give you access to editing/altering it to fit your needs, like mounting holes (doh!): https://oshwlab.com/stienman/priority-hvac-cooling/
Note that I haven't made or ordered this design. While the relays are rated for 24VDC they should be fine with 24VAC, but I provide no guarantees.