Joseph Meehan wrote:
snip
Night setback tends to kick in the backup for two reasons. First
consider when the set back is likely to be canceled. Usually in the morning
at the coldest part of the day. Far more likely to kick in if for no other
reason that it is not too cold outside. I don't know about their control
systems, but I would not be surprised if they detected the difference
between the set temp and the current temp and would decide that it was going
to need to kick in the backup source to help bring up the temperature in a
reasonable amount of time.


Especially if the OP was setting back the thermostat ~13 degrees at
night as could be inferred from his post. No way a heat pump is going
to recover from that on a cold night without some help.

I almost feel like I am using too little gas with my heat pump. I live
in NC and have used a whopping 6 cf this year, since the gas backup on
the downstairs heat pump is the only gas appliance. With the base
charges it works out to about $11/cf Don't know why they only
installed gas on first floor and no gas water heater or stove when they
built the place, although I guess it was the cheapest way for the
builder to be able to claim "gas heat".