I have a 1983 contemporary home which backdrafts air down the fireplace chimmeys and allows air in any other crack or gap. I have upgrades to a closed combustion furnace and a power vent water heater with a fresh air supply in the furnace room. I would like to add outside air into the HVAC air return duct. I would like it to open when we are exhausting air with stove and bathroom exhaust fans and when ever pressure drops inside versus outside due to stack effect. I am considering a powered damper intake operating on a switch based on differential pressure inside the house versus outside. I currently run the HVAC fan 100% of the time to limit stratification from our many cathedral ceilings.Has anyone ever tried this? Any thoughts on pros and cons?