Unfortunately my house furnace is located in the attic.
How to build a box around heater in attic.
This video shows how to build a soffit around ductwork.
Make the box of 2 by 2 inch lumber with two side ladders with a top and bottom plate and vertical rails in between.
Cut a piece of foam the size of the exterior dimensions of the box for the lid.
Slit 3 inches in on the cord s outer insulation from the cut end using a utility knife.
Tape the lid to the top of the box along one of the long edges with foil tape.
Researching the web i found some opinions that folks say consider building a large insulated box around the entire furnace leaving an opening at the bottom for combustion air intake.
Clean the floor around the attic stairs so the tape will adhere well.
There are many different ways to build a soffit but this is definitely my favorite way.
Attach the two layers of foam board together and apply them to the hatch.
Build a framing box the length width and depth of the duct you will cover.
Cut out two pieces of rigid foam board that are 1 4 inch smaller than the hatch so homeowners can slide the attic access panel open easily.
Position the box over the opening making sure it is square.
Place the box fan face down on a work surface.
Elevate an enclosure in a basement or garage.
Cut off the end of the power cord using the blade of a utility knife.
Pull the insulation back from the two wires inside the cord.
I leave in nh so in the winter time it gets pretty cold up here.