The Carrier 58STA090 furnace is a single stage model. The input is rated at 88,000 BTU which yields an output of 71,000 BTU. As far as I know the numbers you have provided are not correct and it is not a 2-stage furnace.

This furnace would be very over sized your house. It will short cycle and I suspect will be very noisy due to the ducts are sized for a smaller furnace.

What size furnace do you currently have? Who is going to do the installation? You said the condenser "rambles forward". It is going to make it through another summer?

Buying the wrong equipment at a discounted price usually ends up costing more then buying the right equipment at the market price. If you plan to live in this house several more years then invest in the right equipment now.

Thank you! I'll forego the use of the Carrier. I can't tell any info on the current furnace as it is older than dirt and the label was messed up years ago. Assuming the condenser is a 2.5 ton, what size furnace would I need? This house is older construction, in the south, not good insulation, but a new roof.

The furnace output is determined by the heating requirements of the house. The fan (blower) needs to be sized so it can provide enough air for either the AC or the furnace when heating. The guideline is 400 CFM of air flow per ton. A 2.5 ton AC would require 1000 CFM. Small furnaces usually can provide this amount of air flow.

You could probably use a 40K BTU furnace.

Thank you! I have found a 45,000 Ruud UGPH-05 furnace. You don't happen to know anything about these Rudd units, do you? Any noteable reputation?

The Rheem/Rudd equipment has a good reputation on this forum.

So the 45,000 Ruud UGPH-05 would be large enough for my 2.5 ton ac?

The datasheet I found states it has a blower capable of up to 1200 CFM. It would work with a 2.5 ton AC.

You have been wonderful! Thank you so much!