I tested ours when it was below 30 degrees by turning on the heat pump. The heat pump didn't start but the furnace did. I usually switch it to furnace anyway when we go to bed if it's going below 40. One time the forecast was for 40 degrees and rain, but it went to 32 and rain. The heat pump still ran but with 100% humidity it is very subject to frost so I switched it to furnace. Besides, the floor gets really cold when it's that cold outside and we're using the heat pump.
2011 Sunseeker 3100
1997 Pathfinder Toad