I have been able to (mostly) integrate my Lyric T5 with ST using IFTTT and CoRE (I ditched Stringify).
I set up a CoRE piston to more or less act as a thermostat (within CoRE, I have access to presence, temp readings throughout the house, etc). I created 4 virtual switches that CoRE sets based on the rules I defined. These switches are monitored by IFTTT. When they toggle from off to on, IFTTT sends the appropriate command to the thermostat depending on which switch changed.
Limitation 1:
IFTTT unfortunately does not support changing of thermostat modes (off/heat/cool). It will only allow you to change the “heating temperature to permanent hold” and “cooling temperature to permanent hold”. The thermostat needs to be in the proper mode (e.g., if the thermostat is in heat mode and you try to set the cooling temperature, the command is ignored). To get around this, I set up the following virtual switches, which are set by CoRE depending on how I want to command the thermostat. These switches are monitored by IFTTT:
a. Set Heat Low (If in heat mode, this command tells the thermostat to set the temp lower - ex. if nobody is in the house, etc).
b. Set Heat Normal (If in heat mode, set the thermostat to the normal heating temp - e.g. when people are present)
c. Set Cool High (If in cool mode, set high - ex. if nobody is in the house, etc)
d. Set Cool Normal (If in cool mode, set the thermostat to the normal cooling temp - e.g. when people are present)
CoRE determines if the thermostat is in heating or cooling mode based on the time of year and temp in the house. It then restricts output to either a and b or c and d appropriately.
Limitation 2:
Since there is no way that I know of to send an integer value from ST to IFTTT, you need to pre-define what you want the “normal” heating and cooling temps to be and the “high/low” temps to be within IFTTT. So you have 4 IFTTT triggers, one for each of a,b,c,d above, which have pre-defined temperatures.
Limitation 3:
There is no way to report back from the thermostat that the temp was changed manually, so depending on how often your CoRE piston runs, you will likely “override” any changes made by people setting the actual thermostat from time to time. I have mine set to run less often if presence is detected within the house.
This is just a first cut at a solution… If anyone has any suggestions or questions I’d like to hear them.