Tom G after some measurements on 11/20/17.

The switch that switches between external and internal themostat was permanently modified by removing the black jumper from the center of the switch and attaching it directly to the external thermostat terminal. The other poles of the switch had a black and red wire going to the controller board. This was unplugged from the controller and a 2 pin jumper put in it's place.

When the unit is powered up the power switch displays nothing or a “P” on the LCD. When the “P” is displayed the fan runs all the time. When there is no “P” the external thermostat can be used to turn on the resistance coils heating them up without the fan. The overheated light soon comes on. This in general is not a good situation. I did some measurements on the control board and found that the “P” can be detected with the #5 relay. When a “P” is displayed the relay is driven by -12 VDC. There is no voltage when there is no “P”.

The strategy is to intercept the thermostat closure and apply it to the T1, T2 terminals with a relay only if the #5 relay is selected indicating a running fan. If there is no fan and heat is called for by the thermostat, the power switch will be activated by a relay. When the #5 relay indicates power it is safe to close the T1, T2 terminal relay.

When no heat is called for the power switch will be toggled while monitoring the #5 relay for a ground.