Here is a code book question. Maybe a good laugh too. I live in New York State. I have a builder friend who is installing two propane fireplaces in his McMansion. He wants to install a return in the ceiling above said fireplaces and use his air handler in manual to circulate the 'free' heat throughout the house. Now keep in mind that there is 1 A/H for each of 2 floors and the A/H primary purpose is for A/C. I suggested that installing a hydro coil or a resistive heating element would be more economical, practical and safer than trying to Rube Goldberg this idea and try and capture the heat. Also, he is telling me that he wants to install a 14x30 register, (on the first floor) slap in a 7" flex and run this thing up a wall to the return, in the attic. Also making some kind of monster return so that it also feeds the 2 upstairs air handlers to put some heat up stairs as well. I understand what he is trying to do, but I don't think that it is practical, necessary or viable economically with all the work that is involved when weighed in with the expense versus benefits.
I think that the guy has a good idea, just not practial in my opinion or experiance.