More BS* + Beer
This episode of the BS* + Beer show features Enrico Bonilauri, co-founder and chief product officer of EMU, and Cramer Silkworth, a licensed engineer and Passive House consultant, discussing HRVs and ERVs—their differences, mechanics, and applications.
Cramer kicks things off with a presentation titled “Introduction to ERVs and HRVs: Ventilation Without Taxation.” He describes the appliances as a means for ensuring good indoor air quality (IAQ) at close-to-room-temperature in a reliable way without incurring huge energy penalties. He shows us the inside of a unit and explains the way the intake and exhaust air streams move through the heat recovery core to remove stale air and deliver fresh air. He illustrates the principles of crossflow and counterflow, and points to the physical properties of an HRV core vs. those of an ERV core. (HRV cores conduct heat; ERV cores conduct heat and moisture.) He answers the question: Which do I need? Because moisture recovery in places that are cold/dry in winter, and hot/humid in summer is ideal, he says, “I think the entire eastern half of the United States would be best served with an ERV.” He also mentions a handful of manufacturers’ products—RenewAir, Panasonic, Zehnder, and Lunos—their price points, pros/cons, main features, and ideal applications. The second half of his presentation is about locating, installing, and ducting the units; what to think about when making selections; code considerations; commissioning; and client communications.
Enrico describes ERVs and HRVs as appliances for delivering fresh air. He talks about the need to balance systems, which is done by first, measuring the total intake and total exhaust, and then balancing individual registers to get the right airflow rates for each room. By way of clean and dirty Merv 13 filters, he illustrates the key to keeping running units properly, emphasizing that common but often-ignored refrain, “Change the filters regularly!” He also notes the difference a HEPA filter can make—a particularly relevant point in light of the current wildfires polluting much of the West Coast’s air. Unlike conventional construction, he says, which allows pollution to come in from nearly anywhere, supertight building envelopes offer more control—the point of entry is the HRV or ERV. Through a series of graphs and studies, he demonstrates the ways in which heat-recovery measurements can be inflated; and explains the need to insulate both intake and exhaust air supply ducts. He shows us different ducting materials, pointing out the pros/cons of each; stresses the importance of frost protection in new builds; and gives us a look at an installed unit in a 2×4 wall, noting duct sizes from the unit to the registers to the manifold.
The big takeaway? HRVs and ERVs deliver filtered fresh air—that’s all they are truly designed to do. But their nuances are many.
Enjoy the show!
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Join us on Thursday, September 24, from 6 to 7:30 pm, when guests Bruce King and Chris Magwood of Ecological Building Network (EBNet) will launch our first book club discussion. The topic is The New Carbon Architecture: Building to Cool the Climate, which Bruce wrote and Chris contributed to. Hailed as a “paradigm-shifting tour of the innovations in architecture and construction” that promise carbon-sequestering solutions to the planet’s health crisis, this book ignites the environmentalist’s imagination. It also provides data-backed information on the many building products and methods at our disposal for combatting climate change. We plan to come to the show with a bunch of questions in hand—hopefully you will too.
Bruce King is the founder of the EBNet, and a registered engineer with 35 years of worldwide experience in structural engineering and construction. He is the author of Buildings of Earth and Straw, Making Better Concrete, Design of Straw Bale Buildings, ASTM International E-2392, earthen building guidelines, and dozens of papers and articles for conferences and journals. He has organized three international conferences on ecological building, and is the founder of BuildWell Source, a user-based collection of low-carbon materials knowledge, and of the BuildWell Symposia.
Chris Magwood is obsessed with making the best, most energy-efficient, carbon-sequestering, beautiful and inspiring buildings without wrecking the whole darn planet in the attempt. Chris is currently the executive director of The Endeavour Centre, a not-for-profit sustainable building school in Peterborough, Ontario. He has authored numerous books on sustainable building, including Essential Hempcrete Construction (2016), Making Better Buildings (2014) and More Straw Bale Building (2005). In 1998 he co-founded Camel’s Back Construction, and over a period of eight years, helped to design and/or build more than 30 homes and commercial buildings, mostly with straw bales and often with renewable energy systems.
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-You can contact Kiley Jacques at [email protected].
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7 Comments
I enjoyed this discussion very much and I think it is a valuable contribution. One thing I would like to clear up is that Enrico mentioned that an ERV he uses has a selected bypass mode where filtered incoming air does not go through the heat exchanger core. It can be used to bring in cool filtered air at night to cool the house passively.
Me being somewhat slow on the uptake sometimes I tried to apply that principle to my whole house fan by installing three round 8" diameter spring loaded vents throughout the house. It didn't work at all because the opening size of the vents were insufficient for the CFM of the WHF, which is over 1000 cfm. There was so much air resistance through those vents that there was no sensible cooling from the fan. You need a LOT of airflow to create measurable temperature change because there is just a very small specific heat difference in air between outdoor and indoor temperatures in summer. I ended up having to remove, reinsulate, and re-drywall all three openings. Lesson learned. One really needs many large windows opened wide to change the temperature in a home with a WHF.
For that same reason an ERV does not, and cannot, replace a WHF. I admit that it is an attractive idea to bring in filtered night air to cool a home but an ERV just doesn't supply enough air to change the temperature in the house. A typical house might use a 2000 CFM whole house fan. But an ERV, even on the high end, only supplies 200 CFM though one small 6" inch duct.
This is an important point you make, Eric. Thank you for taking the time to share it. I can't help but wonder if most people view ERVs as sufficient and opt out of whole-house fans. You might find this Q&A thread on the subject interesting.
Eric, a 2000sf house with 8' ceilings is 16,000 cubic feet. At 200 cfm, that air would (in theory) change every 80 minutes. Of course not every molecule of air is going to change over in 80 minutes, but on an order of magnitude basis, that seems more appropriate to me than a 1000 or 2000 cfm fan that can replace all of the air in a house in 10-20 minutes.
Michael, it would work the way you're describing if one had a much longer time between sunrise and sunset in summer. Temperature inside will always equalize to a hotter or cooler temperature outside if one is given enough time for that to happen. Unfortunately the specific heat of air is very low. That means you either have to have a very long time at a very low air flow (much longer than the time between sunset and sunrise) or a shorter time (the actual time between sunrise and sunset) at a much higher flow rate. I didn't know this myself prior to my experiment with small vents. The only reason I've explained my mistake is to prevent others from making the same mistake. I don't usually go around describing my mistakes unless I've made a conscious decision that my discomfort about doing that is worthwhile to have others avoid the same mistake!
I think it's important to keep in mind that H/ERVs are intended only for ventilation, and WHFs are for cooling (but you just happen to get lots of ventilation, as well). In dry climates where the summer nighttime temps drops well into cool or even cold temps (<55ish), ERV bypass air is cold enough to help cool the building, a little. The way I usually phrase this is that it's not so much cooling the building as it is removing an unnecessary cooling load - recovering heat when you don't need to. It helps, but very climate-sensitive as to how much cooling it really does. In more humid climates where the summer nighttime temps are only slightly below comfortable levels - maybe only 60-65F at best - then you need a lot more airflow to provide any substantial cooling, which is where the WHF comes in. Here in the northeast any many other humid summer climates, you're lucky if it's in the low 70s overnight - but it's still muggy and ERV bypass is basically useless.
It's important to keep climate in mind as well as the different intended functions of each device.
Enjoyed the episode. My bucket list included a Zenhnder(sp?) ERV in my bucket list whenever I build a home.
Does anyone have any experience with these Minotair (or Equinox) units? They seem intriguing, but I imagine it's hard to balance temperature, humidity, and fresh air all in one unit... ?
I'm getting started on a passive house design, to be built next year. My current plan is to use a Zehnder ERV, mini splits, and potentially a whole house dehumidifier - but I hadn't heard of these multifunction units until watching this show.
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