I’m getting ready to take on a similar project but I don’t have the option of returns in each room. I plan on offsetting this by dual temp sensors in the room placed to limit external influence (windows, etc). I also have the added joy of 2 separate HVAC units so that should make it even more exciting to get adjusted correctly to ensure the systems are not fighting each other. I read somewhere you said the code and such was available for both the arduino and linux systems? I don’t see it available and thought I’d ask and see if you were willing to release it?

Overloading the motor isn’t the only issue you can run into. When heating if your not passing enough air over the heat exchanger it could possibly overheat. Causing it to crack, then CO gasses can get into the house. Although the furnace should shut itself down if you go above the allowable temps.

As well it’s possible to ice up the evaporator.

I’d add a temp sensor to both your supply and return vents. Watch the temperature differential…

Another spec on your furnace is the “heat rise” you don’t want to go over this number. This is the differential from the return to supply (measured immediately out of the furnace) while heating.