Good article from Martin Holladay
http://www.finehomebuilding.com/2014/11/05/ducting-hrvs-and-ervs
Another one describing a damper so air doesn't leak into the house
http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/71488/This-Little-Ventilation-Mistake-Can-Lead-to-Comfort-Problems
And this comprehensive article from Building Science
http://buildingscience.com/documents/information-sheets/info-611-balanced-ventilation-systems
http://www.finehomebuilding.com/2014/11/05/ducting-hrvs-and-ervs
Another one describing a damper so air doesn't leak into the house
http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/71488/This-Little-Ventilation-Mistake-Can-Lead-to-Comfort-Problems
And this comprehensive article from Building Science
http://buildingscience.com/documents/information-sheets/info-611-balanced-ventilation-systems