Problem with thermostat controlling heat pump with auxiliary resistance heat

[Cody84]

I have a Goodman heat pump system with an A18-05 air handler and a White-Rodgers 1F58-72 2-stage mercury thermostat. The label on the outdoor unit is illegible, but I believe the entire system is original equipment from when the house was built in 1991. I keep the stat in the "emer" position so that only resistance heat is used (I also keep the breaker for the outdoor unit open). The resistance heat is controlled by the upper mercury cell, and the "aux. heat" LED comes on whenever the stat is calling for (resistance) heat.

This works fine until the outside temp goes below freezing. Then for some reason that I cannot comprehend the thermostat "switches gears", for want of a better term, due to what seems to be a physical change in the position of the mercury cells and bimetal coil. It will NOT keep calling for (resistance) heat in the "emer" position, even if the lever is raised to the warmest setting. I have to change to the "heat" position, and then the LOWER mercury cell (which usually controls the outdoor unit relay) controls the resistance heat, so I have to set the lever at a lower temp than when the switch is in the "emer" position, to maintain the same room temp.

How does the stat know how cold it is outside? And why does it care? I could understand if I was using the heat pump that it would want to switch to resistance heat when it got below freezing outside. But why does this affect the way it functions in the "emer" position? I set it before I go to bed and it works fine at first, then in the middle of the night it "switches gears" and I wake up cold and have to get up and reset the switch and lever positions. Then of course in the morning it "switches gears" again and starts blowing cold air because now (still in the "heat" position) the lower mercury cell is controlling the outdoor unit relay, when in fact the outdoor unit is not running because the breaker is open.

Is there any way I can prevent the stat from "switching gears"? Would it help to disconnect the red and white wires from the R and W2 terminals on the outdoor unit? So far as I can see these are only used to operate the defrost system which is not needed so long as I do not use the outdoor unit for heating. All I want is for the system to work as a regular resistance-heat only system, without have to screw around with the thermostat.

I apologize for the length of this post but I wanted to try to explain the situation thoroughly. This is the first time I've had to deal with a heat pump system. Any explanation and advice would be appreciated.

[Bill Arden]

The two outputs from the thermostat are ran to a relay someplace that is then controlled by the outdoor temperature.

Normally you would leave it in "heat" and the heat pump would run until it got cold and then it would switch modes to use the electric heat when it get's cold.

Edit: One thought would be to look at the wires at the back of the thermostat since the label there is likely to be still readable.

You might also be able to find the relay that switches modes in the unit.