On the Mastering Building Science discussion board, Allison A. Bailes III, PhD asks students: What is the difference between an ERV and an HRV?

Allison A. Bailes III, PhD: In the homework, I asked, “What is the difference between an energy recovery ventilator (ERV) and a heat recovery ventilator (HRV)?” Let’s address that here and extend it to ask when you might want to use an ERV and when an ERV might be better.

What say you?

Student 1: An HRV transfers heat between two air streams, while an ERV transfers both sensible heat and latent heat (carried by moisture) between the two air streams. An ERV would most likely be preferred over an HRV in a place like Valdosta, GA, because the ERV reduces the amount of outdoor humidity brought into the home, while also reducing the amount of energy needed to condition the new air once it’s been brought inside.

Student 2: While both exchange heat between incoming and outgoing streams of air, the ERV additionally provides a humidity barrier which with some degree of effectiveness rejects the passage of humidity from passing through the heat exchanger.

For example in the summer, if it is hot and humid outside (say 80%) and you would like lower humidity inside (say 50% RH) it will reject a percentage of that humidity from passing into the home. Of course, you would likely require a whole house humidifier under such conditions, but the ERV can reduce the latent heat load on your A/C. Likewise in the winter, when humidity is low outside (say 30%) and you may wish to maintain whatever latent heat humidity you have from showers and cooking in the home (say 40% RH) the ERV would reject the passage of some of that humidity from passing to the outside.

The ERV should be considered in more moist, mixed-humid parts of the country and areas where you may desire maintaining some latent heat benefits in the winter.

From a more personal or medical perspective, if someone typically needs supplemental humidification in the winter for allergies or asthma concerns, then the HRV might be worth considering.

Of course all of the above assumes that a home is well under control from an air leak perspectivem (say <3-5 ACH@50pa) i.e. the building is tight and mechanically ventilated.

Of course, I don’t mean to imply that the ERV is not useful in other zones, it certainly should be considered from the aforementioned design and need perspectives. Basically– the tighter the home, the wider its applicability might be considered. If you are building a passive tight home (below 1ACH) and winter humidity gets into the 30% area ERV could be well worth considering.

…What do you think the difference between an HRV and an ERV is? Under what conditions should either be used? Add your thoughts in the comment section.

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About Instructor Allison A. Bailes III, PhD – President, Energy Vanguard

Allison A. Bailes III, PhD, is founder and owner of Energy Vanguard in Decatur, Georgia. Like many in the field of building science and green building, he is multi-faceted: His academic credentials in physics (BS, MS, MST, and PhD all in that field) give him a solid foundation in the science that underlies buildings. Having taught physics at the high school and college levels, he’s adept at explaining technical concepts in a way that people new to green building can understand. In addition, he has practical, hands-on experience. He built a high-performance home out of structural insulated panels, doing much of the work himself, and ran a home performance contracting business. Numerous homes in the Atlanta area had their ducts sealed and crawl spaces encapsulated by Dr. Bailes himself. Between his first and second businesses in this field, he gained more green building experience by working as the regional manager for the EarthCraft House program in the Southeast. What Dr. Bailes has become most known for in recent years, though, is writing the Energy Vanguard blog. In it he covers everything from building science fundamentals to HVAC particulars to big-picture topics like energy security and peak oil. The blog has gained a wide readership in a short time and is often cited and linked to from other websites. In fact, it is the Energy Vanguard Blog that garnered Dr. Bailes an invitation to become a Green Building Advisor. As a result of his varied experiences and abilities, Dr. Bailes is a highly respected teacher, speaker, and writer, praised for his ability to turn what often comes across as dry and technical into something fun and understandable. As one of his readers commented, “Your blog is off the chain! Your writing is technical yet easily readable, such a rare combination.” He travels across North America, speaking, teaching, and leading workshops.

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