I live in Michigan. Home was built in 1997 and is a modular design. We have a 90+ furnace a/c unit. Is it necessary to have a fresh air return that goes to my main return air trunk. If you need fresh air return wouldn't that interfere with the heating and cooling by reducing it's efficiency? Is there a way to increase efficiency in way of special damper? Our water heater is a Rinnai tankless. Thanks for any suggestions.