When I purchased my Koker, installation was part of the deal. The installer insisted that pushing the Koker hot air into the cold air return on my propane furnace was the best way to do it. He wired it so whenever the Koker blower came on it also turned on the blower in the propane furnace. This system worked but it had some real drawbacks. As I now understand how the Koker works, I realize the installation he made was simply the easiest way for him to get the Koker operational; it certainly wasn't the best way to get heat through the house.
I now have the Koker putting heat into the top of the plenum on the propane furnace and drawing the cold air out of the main cold air return duct. I do not run the propane blower; the Koker blower does the whole job. I would not
put heat into the cold air duct. That doesn't work well.
By the way, you'll love your Koker!