So I'm in the US and have a standard (for the US anyways) central heating system. The way my duct system flows is we have an a/c and furnace in the basement. That is hooked up to ducts which sends the conditioned air (hot and cold) throughout the house. We then have a circulation return system. Each room has another vent that sucks up air through a second set of ducts (the return) and recirculates it back into the furnace. I was considering tapping into this duct just before the furnace.

Any thoughts if that would pose a problem?