hey guys, i got a 600w hps that i got venting via a 400 cfm 6" inline fan into my house ducting, i just took off the face plate and flipped one of these"Register boots upside down and attached my 6" ducting.... trying to keep the rest of my house warm via the heating ducts (its -32 outside) but was wondering if back pressure is bad for a furnace to have.....
Well usually the furnace is just a heat exchanger with a electric element and a large blower pulling air first through the exchanger and pushing it through your ducts out the vents. Venting you lights into it is fine i would say. This may be a problem: does your light exhaust fan stay on 24/7? Im guessing it does. The blower from the furnace will fight with it when it kicks on. I bet it will even force air the other way back through your light ducting. You need a backdraft damper and an extra vent in your light ducting so when the furnace comes on the damper blocks backflow to your light ducting and the other vent opens to let the light exhaust out elsewhere. You could use a motorized damper.