I suspect the switching occurs when the "called for" temperature and actual room temperature difference exceeds a set amount. On my system it's 4 degrees. Example: Room temperature is 65 degrees and you want 70 degrees, if you switch on Elect. Heat (Heat Pump) the system defaults to propane heat as the difference is more than 4 degrees.
Hope this helps.
Fred
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Fred and Bonnie
2005 Dolphin LX 6375
Abby, Ruffles & Scarlett, "The Cats"
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