Since it has been a while, here is my original post for context:
...This means that the only way for me to currently have heat when it is colder than the heat pump can handle is to use the original thermostat and manually move it to the aux heat setting. I do not see a way in Indigo to make this switch, though. Is there any kind of workaround that you guys are aware of, or am I stuck with the old thermostat until I pay $500 or so for the installation of that control board?
Basically, I can set a variable to pull the outside temperature, and if I can find a way to tell the thermostat to manually change to aux heat when that variable drops below a certain temperature I would be set, and I could forego having to add equipment. Any help would be greatly appreciated!
You can probably do what you want, but, it will involve some additional hardware, As Jay has noted, the Venstar Thermostat cannot do what you want on it's own.
You might be able to use a relay to switch the input wires to the Venstar between your heat pump and the Fuel burner, or simply place a relay in an appropriate wire between your thermostats and their respectively controlled systems and turn off one or the other system as necessary.
In both cases the relay(s) would be controlled by Indigo based on the outside temperature. To control the relays you could use an I/O-Linc or EZIO* interface, or there are a number of boards that can control an output: EasyDAQ, Weeder and Phidgets all come to mind).
It's hard to be specific not knowing more about your system. But, with a little creativity, and some knowledge of how things are wired, you should be able to achieve your goals, and probably a lot less than $500.
After contacting Venstar I found out that there is actually a much simpler solution. Venstar makes a remote sensor that you wire into the T1900 and place outside, so that it knows what the temperature is and can control the changeover from using the heat pump to using whatever secondary heat source is set up on the thermostat (via jumpers, wiring, and programming).
The sensor can be found online for around $30, and in my case I just had to change a jumper on the thermostat to place it into a dual-fuel operating setup (then program the cutover temperature to be 45 degrees). It was relatively painless.
It would be nice if the remote sensor temperature could be pulled and displayed in Indigo, but it isn't a necessity. I realized I never updated the information here with the fix, though, and thought it might be beneficial for someone who encountered the same problem I had. The sensor in question is the Venstar Indoor/Outdoor Remote Temperature Sensor (ACC0400).