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Well, almost all new manufacturer's air handlers have ECM blowers, so many, it's likely mandated. They sound great, especially in setups that allow for long duty cycles. Some people have allergies and leave the blower fan on 99% of the time and thats where the ECM blowers shine with their 30% better efficiency. Where they don't shine is in systems with high static pressure, or lousy ducts, as then they will only supply as much air as its pressure sensors allow.
Alas, it's possible a HVAC contractor will assess your actual needs, possibly suggest a 60-80k BTU high efficiency heat pump unit that will work in a 3.5 ton unit...which means...a 3.5k blower would be about 1400 cfm....possibly at 1400 cfm, your ducts would be sufficient.
And, there is still time to do return and supply Temps for the furnace to calculate temperature rise. Should be in the range of 40°-70°. Say, if the rise was close to 40°, you could lower the fan speed which could possibly circulate air better...being that the blower is operating longer.
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