how to wire an in-line duct fan on an arcoaire twostage furnace
First off, never wire two motors in parallel you will cause problems. Secondly an inline duct fan can't make up for inadequate duct work, if the air isn't there it isn't. If you are dealing with a run that has a lot of resistance from too many fittings, a fan pulling through them may help. If you want to do it right, power up a switching relay with a 24volt coil off the control circuit that powers the blower. You could also use a amperage pick up switch that are available for humidifier control. It would sense when the blower motor is running and energize your relay coil. The line voltage power coming from the furnace service switch.