No offense, but your post doesn't make sense. What you need to know is what your home's heat loss is (manual J).

Once you know what that number (BTUs) is, then you know what size unit (input) x % efficiency, will yield the number of BTUs required.

Having that number, you can then research various brands, determine what size you need, then check their specs to see how many CFMs of air are delivered. Then, you will know whether your duct system is too large or small.

Actually, duct layout is probably more important.

If you need X BTUs, it doesn't make any difference if it's a single or double stage unit. The ductwork is sized for the maximum (CFM) output. Also, a single stage unit does not run in low fire; it runs at maximum output of heat and CFMs.

I went from an oversized (125,000 BTU input) unit to a 90,000 BTU unit (modulating furnace). The duct system wasn't the greatest; had some minor changes. Probably could have used a 75,000 unit, but needed the larger blower for a/c.

After mornng recovery, the unit runs at a very low speed (you can't hear it) and the air barely trickles out of the registers. While I thought I would miss the hot blast of heat, the new unit is a world of difference (more comfort). The house is uniformly warm with little temperature fluctuation. Gotta love it.

What is your location, and the size (input) and age of your present unit?

V

If your ducts are to large, they make the furnaces with different blower packages in the same size btu of heat So if they checked you ducts close they should be able to match a furnace with the right size blower, each blower has like
4 speeds.So it can be a two stage or single . Have the dealer
prove to you by showing you the charts on your duct work and the blower performance chart of the new furnace so you know he is matching it right. later paul

More info.

General contractor hired a sub to install heating system. Sub installed two 95,000 btu oil furnaces and two 4 ton ac units.
Had a smell of oil in house. End result was that the units installed were supposed to be chimney vent units and he power vented them.
As part of this I also discovered that what was needed was to 60,000 btu furnaces an two 3 ton ac units.
Insurance is covering removing the old units and installing new units.
When I went for quotes everyone told me that since their was already gas in the house I should install gas.
The problem is that the ductwork throughout the walls was designed for the larger btu units.
One installer is telling me that because the ducts are larger that I should install a single stage furnace because at the lower fan speed I wonÂt get any air flow.
The other says because they are larger I need to get a two stage furnace.

I am in Boston. The current system is two years old.