Air compressor

Use some air, then shut everything else down and follow the low thumping sound. You'll find it back in the corner.

I wouldn't shift the setpoints, but if they all have magnetic starters consider wiring in a triplexer between all the pressure switches and starter coils to rotate the running order - first call starts them 1 - 2 - 3, second call goes 2 - 3 - 1...

And if they are all big piston units, when one dies look into a constant-run screw compressor with a VFD for the 'main unit' and save the piston jobs for backup. Will help your demand charge a Lot just by eliminating all the start surges.

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