Hole in ceiling around Central Heat

[zebsdad]

I have central air and Heat (nat gas) installed in a closet in my hallway.

Where the ductwork (plenum?) goes through the ceiling, there is a space all around so that I can see all the way to the rafters.

Shouldn't this space be closed in and sealed to keep the cold attic air from falling in on the furnace and to keep the warm air from rising into the attic space?

[pushkins]

Is this duct work for the A/C and heat air to the vents, a return duct or is it for gas venting?.....
Is there an air supply to the furnace?.....louvered doors etc...etc...
In any case it should be sealed, but the manner in which to do it will vary depending on the answer to the above.
If it is just duct work to vents or a return then you could seal it as simply as apply a good quality duct tape if the hole is not to large. If it is big then you can get some sheet metal from Lowe's/ HD and fabricate a collar then seal off with the duct tape.