installing dual outdoor condensing units to be used as primary and emer. back-up

[FEISTY]

I'm converting an existing indoor dual condenser system to outdoor rooftop side by side new units. I require operation of primary unit with auto-switch for emer. back-up. I'm confused as to what type of control system to utilize and how to pipe it in. I'm doing single pipe run from condensers to a single evap. coil inside small walk-in cooler. Two into one manifold or individual electronic pressure drop sensor?? Thanks for ANY HELP!!!:confused: jeffrey5151

[Dr._Fleck]

I've installed a system similar to this before.

It had a specially made control panel that had 2 electronic controllers in it, they automatically changed over the system in use every now and again (not sure of the timings) to even out the wear on each one. The difference was that this was 2 complete systems in one room, which i think would work better for the sake of buying another blower.

Can't remember the defrost arrangement.