Please do not overlook the information about pressures pointed out by
Butch. The DC creates a negative pressure to collect the dust from
your saw or planer or whatever. If the DC is in the same area as the
furnace or water heater, you need to restore the exit air from that DC
back into the room to equalize the pressure. If you vent the exit air
to the outside, you effectively keep a negative pressure in the shop
anytime the DC runs. If the furnace is on at the same time, flue gases
can be sucked back down the chimney rather than going their normal flow
and therefore cause you extreme danger for CO poisoning. DCs can be
vented outside in climates where no furnace is needed or where the
furnace is in some other part of the house.

Rocky