Why does my old 1975 Lennox G12D2 that I'm about to replace show a AFUE of 80%??????

[steppinthrax]

It'a a 1975 Lennox G12D2. It's really old and I'm getting it replaced. The unit shows an input BTU of 82K and an output BTU or named "bonnet capacity" of 65600. This makes not sense. I believe furnaces of that era had a AFUE of 60% or so. If you divide those numbers I'm getting 80% or it's outputing 80% of what's comming in?

Is it because the older furnaces calculated AFUE Differently?

[hvaclover]

It'a a 1975 Lennox G12D2. It's really old and I'm getting it replaced. The unit shows an input BTU of 82K and an output BTU or named "bonnet capacity" of 65600. This makes not sense. I believe furnaces of that era had a AFUE of 60% or so. If you divide those numbers I'm getting 80% or it's outputing 80% of what's comming in?

Is it because the older furnaces calculated AFUE Differently?

There were many loses that took place like the heat lost up the flue and a myriad of other things.

[steppinthrax]

So what your saying if I went and put in a system according to the "Bonnet Capacity" (i.e. 65600BTU Output system) it would be oversized. The people that have came into my house for estimates are showing me the TRANE XR80 which is 80% AFUE at 80K BTU input that is 64000 output.

[beenthere]

AFUE is Annual Fuel Utilization Efficiency.

What your looking at on your current furnace is SSE/CE.
Steady State Efficiency/Combustion Efficiency.

Your current unit may be drawing a large amount of heated indoor air through it and out the chimney.
A new 80% AFUE won't do this. Even though it may still only be 80% SSE. It doesn't waste as much heat during its off cycle.

You should get a contractor that does a load calc to see how big of a furnace you need, instead of just putting in the same size.

Depending on your area, a 90% may be in your best interest. Gas ins't going to get cheaper 5 years from now.

[hvaclover]

So what your saying if I went and put in a system according to the "Bonnet Capacity" (i.e. 65600BTU Output system) it would be oversized. The people that have came into my house for estimates are showing me the TRANE XR80 which is 80% AFUE at 80K BTU input that is 64000 output.

You'd have to do a load calc to be posative, but if the practices that prevailed during 1975 were applied to you old furnace then it could quite possibly be over sized.

Only a load calc will tell for sure.