Any good Tech should already know what I am going to illustrate here; this is for Homeowners & any who can use the least costly way to deal with this too common a problem in the cold North Country.
We are moving into the cooling season; however, any changes in equipment should consider how every component matches with optimal airflow efficiencies.
My lowest cost solution to the cold climate oversized Oil furnace with a small tonnage A/C evaporator coil which requires nearly half the airflow as heating:
I ran a check on my late brother’s home & the summer cooling heatload is about 14,000-BTUH.
The original scenario, had much less airflow than required for 1.5T cooling; its 2-T A-Coil wouldn’t flow 1150-cfm for heating
I did a lot of other figuring; 45,000-BTUH output should heat that small single story home.
However, with the 140,000-BTUH input, 112,000 output Oil furnace installed; the nozzle size can be dropped from one gal an hour (we’ll use 139,000-BTUH input) to .75 @100-psi, the BTUH drops to 103,500 input, this furnace tested at .74% efficiency; *103,000 = 76,590-BTUH.
Using a 90F heating temperature rise, which Thermo Pride can stand; (90*1.06 here) 76,590 / 95.4F is 803-cfm.
Therefore, the 2-Ton A-Coil will handle the CFM; mounted +6 inches above the furnace, it has the flow capacity to work okay in both heating & cooling modes.
With a third HP belt-drive motor, you could simply adjust the RPM down by turning the adjustable motor pulley out enough to get to 700 or 600-CFM for the 1.5-Ton A/C.
The other solution is to install a multiple speed direct drive motor with the fan relay energizing the cooling speed tap.
This rather common situation in cold climates seems not to be properly addressed my Techs, & the HO does not know why things don't work well.
We should all work to improve efficient use of high cost energy ... so America & everyone wins.