What happens if your blower speed is too high?

The previous owner had the furnace blower set on the med-high setting, but this could have been because the bearing was going out. I have no clue. I recently replaced the motor and wired it the same way as the old one.

Tonight it's very cold out (-5F) and the furnace is having trouble keeping up. Currently 64F in here. Which I'm fine with if this is just an occasional thing... adding insulation is on this year's todo list. The air coming out of the vents has always been just lukewarm. I checked the LP pressure setting with a manometer last time I replaced the gas valve.

What I'm mainly curious about though, is what about that blower speed setting? Should I change it back to low as recommended? I don't know if the PO added duct work for the 2nd floor himself or if that was there when the furnace was sized. If the fan speed is too high, what's the worst that would happen? I can't see running a cooler heat exchanger as being all that bad...? Would it cause increased propane consumption?

Oh, and I should mention... the PO for some reason cracked off the intake PVC pipe mid-run in the basement and the furnace is currently sucking air from the basement. I don't know if it ever went outside or not but it certainly isn't now. I suppose I should fix that one of these days?

Model GMNT060-3

 

On the furnace data plate should be some temperature rise spec's. Measure the return air temperature near where it connects to the furnace & measure the supply air temperature either aprox. a foot from the furnace. Adjust the blower speed to bring the difference within the temperature rise spec.