4 Deg*F is a design day for Mt Pleasant, MI - nearest city I could find. Lansing comes in at 1 Deg so selection of 2 Deg for a temp below which to design for aux strip is solid.

Geofan - thanks for the note on thermostats finishing a call with aux vs auto downstaging.

Interesting, the geo designer gives values for 2 deg and 7 deg and 2 deg was where the Space load first goes greater than the
Geo Btu/h so 4 deg is probabably the design day they used, good information.  thanks Geofan.

There is no hard and fast rule stating that 'design day' temps must govern temperature for designed switchover to aux heat. It's a starting point, no more. Unit capacities being only available in substantially separated nominal capacities (2.5, 3, 3.5, 4,5,6 tons) means that capacity roundoff, ductwork, loop constraints, and total cost of ownership considerations may suggest a unit capacity specification such that aux heat comes on a bit above 'design day' temps.

Anyone using warmflo as aux. heat? Jumps up in I think 500w increments as needed...