I have a Harmon SF 1500 The blower Is rated at 1000 cfm but barely makes it thu my heat ducts . It’s an add on to my Propane furnace. At first I thought the heat was being recirculated thru the returns so I plugged them and still got very little velocity at any register. does anyone know if you can add a larger fan moter presently the moter is 1/5 hp. I would rather not put booster fans on the ducts.