I have a 40k furnace and with only one floor vent in the bedroom, which is in back, it can get quite cool. Our unit has the Polar package, hence the larger furnace. The rest of the unit stays nice and warm. The furnace unit itself is located in the front. My thoughts are after a 42 ft run of duct, there's not much forced air left to hear the back. We do have the second ac, which just feeds the bedroom and cools it very quickly.