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CeloNC · 10-16-2012, 05:08 PM

Hello out there! I am installing a new Suburban NT 30SP furnace in my 1970 Sovereign. Thanks to the many forum posts about this subject, I have modified the old plenum and about ready to install the unit. I have 3 questions: (1) do I have to fabricate a new metal cabinet to replace the old one which was not deep enough, (2) do I close off the three open, round, duct holes, and (3) what do i do about the 1/4" air space between the bottom of the unit and the top of the plenum? See photos below. Thanks, Bill

pgr32e · 10-16-2012, 06:42 PM

Given that a picture or two are worth a lot of typing, attached is how I did mine. I blocked all the side holes with a sheet of aluminum held in place with rivets, You shouldn't need to build a cabinet over it as it would restrict airflow to the furnace. The pictures show how I modified the old furnace front cover and plenum to take the new one. It works well, lots of hot air.What is the shiny stuff with the red writing on it?....Phil

idroba · 10-16-2012, 06:53 PM

I don't understand quesiton #1 about a new cabinet. The new furnace comes with a new cabinet already.

Yes, you want to close the round holes as all of your air wants to go down to the old ductwork. The round holes are for a different duct system type.

I would use sticky aluminum tape to seal off the area of the gap from the furnace to the old ductwork.

CeloNC · 10-16-2012, 09:34 PM

Dear Phil and idroba,
Thanks for answering my questions and providing photos. The shiny stuff with the red writing is heavy duty HVAC foil tape that I bought from a heating and plumbing store. It's about twice as thick and sticky as the basic aluminum tape. I was concerned about the "cabinet" because the old furnace cabinet was solid and the new one has lots of slots in it. Anyway, thanks to you guys, I am ready to install!!
Bill