Right now we're in the middle of a kitchen remodeling. During demolition there was a lot of dust. Unfortunately, we forgot to cover the intake vents and so our ducts are pretty dirty now, and we feel it even with MERV 11 rated furnace filter. Come spring, we'll have pollen and other crap get into the ducting too, and we have allergies. Of course we can do duct cleaning but it's expensive, requires special arrangements and can't be done as often as we change a furnace filter. If the additional filters strain the furnace and AC system, how can we calculate whether our system is powerful enough? If we decide to get a new furnace -- ours is very old -- then how much additional power should we add to account for the additional load?