Aux heat not kicking in until 5-6 degrees lower.

[Chris010590]

It seems like my aux heat does not kick in until it gets 7-8 degrees below my set temperature. I wake up some mornings and it’s 62 in the house and the thermostat is set to 67 per schedule but it’s just using regular heat. I know the aux heat works because sometimes it kicks in and it’s not emergency heat. Help?

[CoolingWizard]

@Chris010590, With a heat pump, the auxiliary heat is not typically energized until the outdoor unit goes into defrost mode.  If the Nest determines the outdoor ambient air temperature is not too cold, it will determine the heat pump is working efficiently enough that boosting the heat with auxiliary heat is not needed.  
If the heat pump is running and the indoor temperature drops 3-degrees quickly,  the Nest can turn on the auxiliary heat. If your heat pump is running and not keeping up with heating demand, you need to have the system serviced.  Auxiliary heat is electric heat strips typically at 50 to 60 AMPS @240V AC.  This is very energy inefficient and that is why the Nest will try not to activate unless it is necessary.  What is you current overnight outdoor low temperature?

AC Cooling Wizard

[Georgiaava]

It seems like my aux heat does not kick in until it gets 7-8 degrees below my set temperature. I wake up some mornings and it's 62 in the ... Cold weather monkey type means HVAC systems need a little push from secondary heating, so auxiliary heat turns on. In areas of the United States that get a lot of snow, it's common for auxiliary heating to run all winter long.

Aux heat turns on when the indoor thermostat struggles to reach and maintain the temperature setting. If your aux heat operates for an exceedingly long time, you may have an issue with your heat pump. Specifically, a faulty fan motor, bad compressor, or malfunctioning defrost control board may be to blame.

[Chris010590]

When it was 62 in the house, the overnight low was around 20-25 degrees F. The AUX heat does work because sometime nights it has kicked on when it’s been in the teens outside and it has no issues getting up to 67-68.

[CoolingWizard]

@Chris010590 , as I stated, with a heat pump, the outdoor unit will tend to ice up on very cold nights and when it does, the sensor notifies the defrost control board that ice is present and that starts the defrost cycle.  When the defrost cycle starts, the reversing valve switches and the heat from inside the house is used to defrost the outdoor unit.  When this defrost cycle starts the outdoor unit will send the indoor control board a signal to activate the auxiliary heat. In short, the heat pump becomes an air conditioner when in defrost mode. Since the heat pump is sending cold air indoors during defrost, the auxiliary heat warms the cold air so as not to send cold air indoors.  Again, the auxiliary is very energy inefficient so we try use it when absolutely necessary.  
Now then the Nest Thermostat has software that analyzes the heat pump performance and when the heat pump fails to keep up with the demand over time, it will turn on the auxiliary heat as a supplemental heat.  The intelligence of Nest Thermostat is that of energy efficiency so it patiently evaluates the performance of the heat pump using past knowledge of the heat pump performance given similar outdoor and indoor ambient temperatures.  This same software can notify the homeowner if it determines the heat pump is not performing at its previous best.  This is less than optimal performance can indicate a system needing servicing. 

AC Cooling Wizard