It occurs to me a flex duct could be run from the return duct (using the blockoff plate hole) to a pass-through in an unused door from the basement to outside, and a rodent/leaf/waterproof exterior air intake could be put on the outside. This would take supply air from outside the house and duct it to the air handler. Now here's the question: What would be the effect on system efficiency by doing so? I can think of two possible answers: Either the very cold outside air would take much more energy to heat up to household temperature, increasing the cost of running the system, or the greater temperature differential between the fire and air sides of the heat exchanger would mean more heat transfer to the supply air, reducing the amount of heat wasted up the flue and not significantly increasing fuel consumption to provide a given household ambient temperature. Obviously the reverse (cool/ warm) of either situation would apply during air conditioning season.
So...which is it?
Please and thank you,