How to Convert a Downdraft HVAC to Updraft

Install an upflow heating/cooling unit to replace a downflow in a utility room in the house or outside a wall where a duct can enter the house. Put an upflow furnace in a central utility room or a large closet with easy access to an attic. Place a unit outside on a concrete slab if necessary; build a form and pour concrete or buy a preformed pad designed to support a heating/cooling unit.

Draw a rough floor plan to locate the existing ducts. Show where they attach to the present system and how they are routed through the house; a downflow unit probably will be in an attic, near the center of the house. Use the existing ductwork if at all possible; an upflow unit can distribute conditioned air through attic ducts. Be sure to find and mark the return, usually on a floor in a central hallway.

Cut a hole in the outside wall with a reciprocal saw, if installing outside, to attach the plenums or entry points for the new unit; you need two, a supply for conditioned air and a return to collect "used" air and bring it back to the unit. Run the sheet metal piping from the plenum through the wall into the utility room or other suitable space to connect to a duct going to the attic.

Seal the entry around the plenums with concrete patch or other appropriate material, depending on the type of siding on the house. Caulk around the edges where the plenum enters the house to make it airtight. Skip this step if installing the unit inside the house.

Cut a hole in the ceiling to run a supply duct from the unit into the attic; connect this duct to the supply plenum and run it up a wall into the attic. Fasten the appropriate size duct to the supply plenum with adjustable plastic or metal straps and seal the connection with heat-resistant tape. Secure the vertical duct to wall studs with metal straps around the flexduct, nailed to studs with galvanized nails and a hammer.

Connect the upflow unit supply duct to the existing interior supply duct, usually a main duct through the center of the attic, with branches to individual vents. Cut a piece of flexduct with tin snips long enough to connect the two ducts. Fasten ducts with metal sleeves inside the ductwork, secured with adjustable straps tightened around the outside of the flexduct and sealed with tape.

Locate the existing return; you may have to eliminate this and create a new one. Close off the existing return if it cannot be used. Remove the filter grill and ductwork for the old unit and patch the wall where they were installed. Install a new filter frame and connect it to return ducting or use a built-in return, which most upflow units designed for interior installation have at the bottom.