Has there ever been a side by side comparison of the difference between a home with underground air returns and one with attic returns. We build very high end homes and would like some solid industry numbers about which system provides the best return. All I have been able to learn is that there are nuances and systems that hvac people like because they have used them for the last 20 years, but there appears to be no definitive study which indicates one system is beteer than the other.