How do I set my thermostat's system type?
Press SYSTEM, then press and hold the center blank button for approximately 5 seconds to enter system setup. Cycle through functions using the arrow buttons to find function 0170 (Select System Type), then use the right arrow buttons to choose your system type (options include heat/cool, heat pump, heat only, cool only, or multistage systems). Press DONE to save your selection.
From the official Honeywell RET97B5D100 Home Programmable Thermostat manual, page 54
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