John in La had a good answer and he was dead on. This is a pic of my thermostat. The newer ones are electronic but still do the same thing. It has 2 little vials of mercury. The top one turns on the heat pump while the bottom one is 2 degrees behind the top one and turns on the strip heat (electric heat coils in the air handler). I leave the cover off the thermostat and can watch it tip over and the heatpump turn on etc. If it can't keep up finally the second vial tips and the strip heat comes on. No time involved in mine. It's worth noting that if the heatpump can't keep up it runs all the time with the strip heat cycling keeping the inside temp 2 degrees below the thermostat's set point. In the picture you can see in the top vial the mercury has tipped to the right side turning the heat pump on. In the bottom vial the mercury hasn't yet tipped over to turn the strip heat on.
I've been impressed with the sub 10 degree weather the past two nights as it ran most of the time but it never quite got 2 degrees below my setting of 66.
John's explanation of emergency heat is dead on too.
Don't want to muddy the waters but I was just reading somewhere where some folks have disabled their strip heat with a separate breaker or some other way. That might not be a good idea. Although not controlled by the thermostat the heat strips are turned on when the heatpump goes into defrost mode. In defrost mode the heat pump is switched to air conditioner mode even though it's set to heat. In ac mode it begins to take the heat from the air in the house and heat those outside coils to melt the ice off of them. It turns off the outside fan during this so no air is being pulled over them and they heat fast. In doing this the inside coils are getting cold--ac is on--thus to prevent cold air blowing into the house it turns on the strip heat to counter act the cold coils. If you somehow disconnect the strip heat then you will experience really cold air blowing into the house when the unit is defrosting.
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