Equipment:
Trane XL1400 4 years old
Auxilery heat: Strips... 2 -60Amp circuits
Filters changed every 30 days
I cleaned outdoor unit summer 2006, as every year.
Thermostat: Honeywell Chromotherm II

Problem: I have been noticing at times the aux heat kicking in at times I think it should not.
It is obvious as I can feel the very warm air genrerated and then look at the thermostat and see the green (aux) lamp on.

Just noticed it a while ago as well. It is 38 degrees F damp and windy today.
What are the possibilities?

There is a differential in the thermostat for the 2 stages of heat. So if you set the stat at 38 when it gets down to 36 the aux heat comes on. (this differential can usually be adjusted in the stat) Also when the heat pump goes into defrost it is actually air conditioning your house, so to counter act that, it turns on the aux heat so you don't feel the cold air blowing.

There is a differential in the thermostat for the 2 stages of heat. So if you set the stat at 38 when it gets down to 36 the aux heat comes on. (this differential can usually be adjusted in the stat) Also when the heat pump goes into defrost it is actually air conditioning your house, so to counter act that, it turns on the aux heat so you don't feel the cold air blowing.

OK I can understand that but need to ask further.

To clarify the thermostat, I set it at hold 70 degrees for the winter season 24 hours a day. I only use setbacks in the summer hot weather. I just do not remember in past winters seeing the aux kick in at any time in previous years until the outside air was in the low twenties. One suggestion that I had was to give the coils a good cleaning, but unsure of best method. Should I go with compressed air, water hose, or what. I also have to figure out how to get the cover off the unit itself.

The outdoor unit is in the rear of the house somewhat in a pocket to protect it from the cold front high winds that we get here.

To add more info, I just checked the outside unit. The aux heat was on. It is forty five degrees outside. The fan was not spinning outside, but the unit was humming. I'm guessing defrost cycle. After it finished cycle, I reset power, thermostat etc, but think I have done this a couple days ago.

I am pretty sure this is getting out of bounds of normal operation.

Wind doesn't afffect a heat pump it actually helps. Yes it was in defrost if the fan was not running in the condenser. What I'm talking about on the differential is. When the heat pump can no longer keep up, and the inside temperature starts to fall below set point the aux/emergency heat turns on. I don't know why you reset the unit. And do not clean the resistive heat elements, they will clean themselves, that is why it smells when you first turn it on. Make sure all the connections are tight, that none of them have hot spots, the evaporator coil, blower wheel, motor and filters are clean.