Your post is a little confusing. It is not clear which equipment is for the first floor and which is the second floor. I think it is this: 1st floor: Comfortmaker G114000075-12M Rheem Classic X - RPKA030jaz 2nd floor: Rheem Criterion II - 80 afue Amana Distinctions - CKL24-IL - 1.5 ton There is nothing wrong with having one furnace be a single stage, and the other furnace being a 2-stage. The multiple stages are for comfort. A variable speed furnace will allow you get a more efficient AC. This would be attractive for your hot summers. If you are going to have only one 2-stage furnace, then it should be the one you use the most. I would think that would be the first floor furnace. You have mismatched equipment which means the efficiency is unknown. The proposed furnaces seem oversized. It could be these are the smallest sizes Rheem offers, or the smallest with the required blowers for the AC. I think the Rheem Classic X - RPKA030jaz is a 2.5 ton heat pump. The proposal is to replace it with a 1.5 ton unit. Did the contractor do a Manual J calculation to confirm the sizes?

One thing which might eliminate the need for serious duct revamping, is to have a proper heat calculation (manual J) done. In days of old, most furnaces were oversized. It's quite possible that you could get by with a smaller sized furnace, whether modulating or not. I would question the statement about the gas burner not being able to function due to 'back pressure'. Rather, an undersized duct might not allow the required CFMs to flow, and might be noisy. I went from an oversized 125,000 BTU beast, to a 90,000 BTU Rheem Modulating furnace, which might run in medium to medium-hi speed during mornng setback. I probably could have gotten away with a 75,000 unit, but needed the blower capacity of the 90 for air conditioning. The comfort level is superb. The Rheem modulates in 5% steps, but as BaldLoonie has stated, you would be hard pressed to tell the difference. Most importantly, the Rheem uses a special thermostat to control modulation in a way that the others don't. He has also pointed out other differences between the units. V

Two stage furnace isn't any more prone to breaking than a single stage. The difference between 2 stage and single stage? Gas valve has two different modes of operation. One is partially open, the other is full open. Both types of gas valves would have similar lifespans. There isn't anything more overly complex with a two stage gas valve. Cost to replace 2 stage gas valve out of warranty would be marginally higher, but by then you'll likely be in the market for a new one anyway. gas valves are probably one of the most reliable parts to a furnace at least in my experience. Provided of course the installation is performed properly. If you get garbage in the gas line, you can't expect the gas valve to work can you? You're in an extreme heating market (Chicago) so naturally it will seem like you're always having to do something to that furnace. The more you use something, the more it is likely to break, need maintenance etc. I doubt you will see much difference in repair cost vs a single stage furnace.

Don't believe I have dampers. Let me ask you this. If I decided on single stage (forget two stage) , could a 3 ton really cause any issues? I realize bigger isn't always better but I'm thinking (layman here) that the increase would be more positive than negative and this is the one opportunity I have to "enhance" what I have now. Wondering since I'm not looking to increase size dramatically it would probably be fine?????