Carrier 58sx120 specs/replacement- questions for the pros

[lorim1999]

Looking for some advice from the pros- thanks in advance for your time.

I have a Carrier 58SX120 circa 1989 heating my 3700 sq ft house. We are looking at replacing it, and my HVAC guy has recommended an Amercan Standard variable speed 95 AFUE (AUY120). Now on really cold days I have felt that the Carrier hasn't kept some of the upstairs rooms of my house as warm as it should have. Unfortunately my house is not set up for zoned heating or multi units. So all that being said, here are my questions.

1. My understanding was that the Carrier had 120 BTU output, while the American standard has 120 BTU input. Do I have the Carrier info correct?
2. If my numbers are correct, would the Carrier still be expected to be putting out 120 BTU, or is efficiency lost over time?
3. Is the Amercian Standard an adequate replacement because of the 2 stage heating and variable speed, or am I going to be even colder if go ahead with the replacement? If not, what size unti should I be looking at?

It is all very confusing as a homeowner. I want t omake sure I've done my research before moving ahead with such a costly project. Thanks again for your time.

[BaldLoonie]

The AUY is a discontinued 92% model. If in the US with the available tax credit, be nuts to go with that. The AUH2 is 95% and saves 15% up to $5000 purchase price.

Yes, I think the Carrier might have been 120K out where the A-S is more like 114K out. Not a real biggie. Individual upstairs rooms being cold is often due to poor duct design not undersized furnace. In fact the longer run times (with nice 2 stage stat) with the new furnace likely will improve on that. Furnaces don't really lose capacity with age except if they have dirty blower wheels or secondary heat exchangers.

[yourairman]

1. The 120 in the model number refers to the btu input not output on your existing carrier.

2. Furnaces do not lose as much efficiency over time as air conditioners due. If it has been maintained properly than it should be close to the original rating.

3. The two stage furnace should have longer run cycles which helps even the temperature throughout the house. The thing to consider with your situation is make sure the ductwork to these upstairs rooms is adequate. 1 system on a multilevel house usually requires manual duct dampers for the different load requirements of each floor. The variable speed motor will help with airflow to a certain level but is not a fix all solution. If the house requires 120,000 btu furnace you should not be colder with 2 stage equipment. It should actually be the other way around due to run times.

[lorim1999]

Baldloonie-
Thank you for your observation. I checked my number and the one he gave me the quote for was AuH2 D120 A9V5, so you are correct.

Yourairman-
Thanks for the info, it is helpful!