Don't have a Fuzion, but our Momentum is built the same way in that the kickplates under our main area cabinets are open to the basement. Ours does have ducting and heat vents to the basement. We had to replace our furnace this year, and in looking at the design, there really is no return air. It takes air from outside, heats it in the plenum, and blows it through the ducting to the heating vents. Exhaust from combustion is vented directly outside. Bottom line is I'm not sure why they do this as your basement is always going to be cooler than the main areas. Seems it could be sealed, but look forward to seeing other Reply's.