Heat pump auxiliary heat does not turn on

Lux thermostats are not very good in my opinion, a heat pump is an AC unit that reverses operation so if the AC works and the Heat works then it is OK as far as the refrigeration cycle which is good, replace with a better thermostat such as Honeywell , try Lux 800 # too just in case.

If it's a heat pump, it means you either bumped the heat up to a temp that the heat pump cannot reach quickly OR it's so cold out that there is no heat for the heat exchanger to pull from the air outside.
Both conditions turn on the heat strips in the air handler. The heat strips are your 'auxiliary heat.'

If you look on the install manual. It says it will work for a heat pump without backup heat. No heat strips.........If you do not have strips the guide says as follows

"PLACE A JUMPER (PIECE OF WIRE) BETWEEN Y AND W IF YOU ARE USING A HEAT PUMP WITHOUT AUXILIARY/BACKUP HEAT." if this dosnt work check the fuse in the air handler you may have blown it if you did not turn the unit off before installing.

See if it happens only when your temperature of the room is more than 3 degrees of your setpoint temperature. Since you have a heat pump, the auxiliary (electric heat strips) is use for backup of the heat pump when it can't keep up. The heat pump will run by itself if it can maintain temperature within a couple degrees. If it is too cold outside to handle the load or you set the temperature more than 2 degrees past room temperature, the auxiliary will turn on. The thermostat has a memory of how long it takes to reach desired temperature. If it goes into auxiliary mode trying to satisfy setpoint during the last call for heat, it will start the next call for heat in both aux & heat pump mode until the thermostats algorithm (timed program) allows only the heat pump to run. Not sure if you have an air to air, or geothermal heat pump. The outside air temperature will affect only the air to air system ie. when the temps drop outside, the heat coming out of your vents will be cooler...the air to air systems drops efficiency when the outdoor air drops.

IT SOUNDS TO ME LIKE EITHER YOUR SYSTEMS LOW VOLTAGE WIRING IS INCORRECT, MOST LIKELY @ THE FURNACE. IT COULD ALSO SIMPLY BE THAT WHEN THE TECHNICIAN THAT INSTALLED THE T-STAT AT START-UP DID NOT GO THROUGH THE INSTALLER SET-UP & CHOOSE THE CORRECT SETTINGS FOR A DUEL FUEL SYSTEM. TRY THIS " SET THE THERMOSTAT TO EMERGENCY HEAT, WAIT A MINUTE OR TWO & SEE IF THAT TURNED GAS FURNACE ON. IF SO THEN IT'S A SIMPLE FIX IN THE WIRING OR PROGRAMMING & NOT SOMETHING MAJOR! GOOD LUCK!

Auxiliary Heat is used to turn the electric heat strips ON, If the heat pump is not working. Or if the heat pump can't bring the space temperature up to Set Temperature it will display Aux Heat ON....... Assuming you have a Heat Pump System with Auxiliary Electric Heat as Back up...... If the tech didn't install a jumper wire from AUX to W on the subase which is in back of the Display on the Thermostat.....you won't get Auxiliary Heat if the Thermostat is set for Auxiliary Heat. Other wise the thermostat will turn the heat pump on first and then the electric heat (Aux Heat) if it needs it to satisfy the set temperature....... Hope this helps

Auxiliary heat is electric heat that is put into the air handler on the inside of the house. Its used when your heat pump doesn't provide enough heat, but heat strips are not always used. To see if you have auxiliary heat, look inside and see if you have breakers on the front panel of the air handler or anywhere close. No breakers, no electric heat. Hope this helps!

Honeywell Chronotherm thermostats have always had a great reputation for longevity and reliability. They are also much more user-friendly than the other types of programmable thermostats out there. Make sure you get one that states 2-stage heat, heat pump. T8611G will offer that for you. There are many out there that claim to be suitable for heat pumps; however, they do not have the ability to utilize your auxiliary heat strips (2nd stage of heat).
Before you attempt to replace, be sure to write the wire colors and their corresponding terminal marking on a sheet of paper........of your old thermostat. This is very important to make it easier to hook up the new thermostat. Also, before you disconnect the wires, be sure to shut off the circuit breaker to your air handling unit. You can ensure you have the right one by putting the fan switch in "ON". When you turn off the right breaker, the fan will stop running. :-)

I hope you find this information to be very helpful to you. Please don't forget to leave a rating.

sounds like a thermal fuse

The smell is usually dust on the wires for electric resistance heat.The thermostat shouldnt have anything to do with it.