Hello. I have, and am pretty happy with, a Venstar T7850. I use it with a typical heat pump backed by an electric furnace (heat strips). Everything seems to be working right in terms of furnace control. But I am a bit confused about AUX heat.

I am told by Venstar tech support that any time the heat strips are engaged (be it because the heat pump is locked out at cold temps or if the difference between setpoint and current temp is too great for the heat pump alone to meet the need) that the thermostat should display "AUX". However, I have never seen that happen.

Yesterday morning, our heat pump was locked out because the outside temp (we have a wired sensor) was below our lockout point of 25. Of course, the electric furnace did run. But I never saw "AUX" displayed when, based upon the tech support statement, I should have seen it.

I have a program that polls the thermostat's local API every few minutes and records the runtime stats in a database. These stats include the number of AUX heat run minutes for the day at that time. During the entire morning that value remained 0.

I will say that I do, on occasion, see the runtime stats include non-zero AUX heat runtime minutes. I did so yesterday afternoon for example. But at that time, the heat pump was NOT locked out as the temp outside managed to get above 25. In this case, looking at my recorded data from the thermostat, I could see that these AUX minutes occurred between 1pm and 1:09 pm when the setpoint was set to 64 and the room temp had somehow dropped to 61.

I am not quite sure how this large a gap occurred. I set my 1st deadband at 2 degrees so I would have thought that the heat pump would have been engaged when the inside temp dropped to 62. I guess it is possible that the temperature drop could have occurred during a period wherein the heat pump was disallowed from running due to cycles per hour (set to 3) or minimum compressor time off (set to 5).

However the gap occurred, in this case the inside temp managed to fall such that the thermostat decided it had to engage the electric furnace to meet the short term demand. I do find this a bit strange as I have my second deadband set to 2 degrees and the second 2 degrees/2 minutes off the first deadband. So I would have thought the temperature would have to drop by 4 degrees below the setpoint for the AUX to be engaged. And none of my readings recorded show a difference greater than 3 degrees during the period in question. I suppose it is possible that it did so between the times of two successive polls.

So I am hoping that someone here can help my understand all of this non-intuitive (at least to me) behavior? I guess I (and maybe Venstar tech support too?) could be confused by the meaning of "AUX". Could it be that AUX is only used/recorded/displayed when the thermostat sees too large a difference between the setpoint and the current inside temperature?

Thanks in advance for your help.