Downflow furnace reducer duct?

[sonnyb3]

Considering an upgrade and will definately seek a professional contractor to change out my 80K BTU 80% AFUE.

I will choose a 90% AFUE plus unit but am disaapointed because of my 17.5 wide opening in the slab most manafacturers offer a 60-70K BTU furnace .
I understand that the effeciency is better and might end up with the same output btu as my old unit. I was soooo interested in getting some additional BTU and CFM by a larger unit.
Is there such a thing as a reducer plate to fit a 21 incher into a 17.5 opening or is that like shoving 5 pounds of meat in a 2 pound bag.

Thanks Very Much

[SBKold]

The industry is moving towards lower Btu output and longer run times and proper cfm to be very comfortable.

[beenthere]

80,000 times 80% is 64,000 BTUs. Roughly 987 CFM at a temp rise of 60 degrees.
70,000 times 95% is 66,500 BTUs. Roughly 1026 CFM at a temp rise of 60 degrees.
80,000 times 95% is 76,000 BTUs. Roughly 1172 CFM at a temp rise of 60 degrees.

A 70 or 80,000 BTU input 95%er will get you more heat, the question is can your slab/crawlspace duct flow the additional air they need.

[George2]

80,000 times 80% is 64,000 BTUs. Roughly 987 CFM at a temp rise of 60 degrees.
70,000 times 95% is 66,500 BTUs. Roughly 1026 CFM at a temp rise of 60 degrees.
80,000 times 95% is 76,000 BTUs. Roughly 1172 CFM at a temp rise of 60 degrees.

A 70 or 80,000 BTU input 95%er will get you more heat, the question is can your slab/crawlspace duct flow the additional air they need.

Again, B.T. is right on......I have had cases were the ductwork in the slab was not large enough for the higher efficiency furnaces. In other words, they would "bump" the limit and shut off.

The newer furnaces need more airflow. You don't want to be surprised if it doesn't work. Also, the return air on counterflow furnaces is historically short.....just an F.Y.I.

[54regcab]

The industry is moving towards lower Btu output and longer run times and proper cfm to be very comfortable.

+1. Many installed furances are way above the capacity needed. 60k-70k heats a lot more house than most people think. If 60k isn't enough to replace the heat leaking from the house, focus on fixing the house rather than installing a larger furance. Then there is also the 5lbs of meat into the 2lbs bad thing going on with the ductwork as other have mentioned.

[Joehvac25]

Tighten your house up then stuff 1 lb of meat In a 2lb bag, works better lol.