Work continues on the Knox, Maine, Passive House under construction by Chris Corson and EcoCor Design & Build. Last week, the crew started installing the high-tech energy recovery ventilator (ERV) that will supply the home with fresh air at a rate of one-third of an air change per hour — while recovering 92 percent of the heat from the outgoing air in wintertime, and maintaining a comfortable and healthy level of indoor humidity. (In summer, the unit can operate in bypass mode, which draws in fresh air while rejecting outdoor heat and humidity.) “I’m a proponent of energy recovery ventilators, not just heat recovery ventilators,” says Corson. “You get the best of both worlds. You get the capacity to control the latent load, to use the latent load of the house for heat exchange — you can convert that moisture into heat energy with the incoming air. It allows me to regulate humidity both in the peak seasons and in the shoulder seasons pretty effectively.” Corson considered several different brands of ERV before settling on his choice, a ComfoAir 200 manufactured by Zehnder. Corson says, “ Stirling Technologies was on the short list because it’s an American-made product that’s relatively inexpensive, and it is passive house suitable and has been used in passive houses. But it would not have been my first choice. I also looked at Venmar units, which are manufactured in Canada — they were never really on my short list but they were always in the mix for comparing and contrasting options based on cost efficacy and so on.” Corson also looked at Paul systems (pronounced “pow-ool”), another Zehnder brand: “The Paul had the highest efficiency rating, but it was also more expensive.” The ComfoAir 200, however, has a rated efficiency very close to the Paul’s, but with an easier price tag to swallow. “Zehnder’s ComfoAir 350 was my first choice when I started this project,” says Corson, “but then the ComfoAir 200 came into the U.S. market, and it immediately went to the top of my list.” Coastal Connection spoke with Barry Stephens, the national sales and marketing manager for Zehnder America, about Corson’s project and the comparison between the ComfoAir 350 and the ComfoAir 200. The 350 is rated at 84 percent efficient, Stephens says; the 200 is rated at 92 percent efficient (as defined by Passive House using a formula that measures heat loss to the outdoors as well as the electricity use of the systems). In practice, Stephens says, either unit will use about 20 or 30 Watts while running. On a freezing day outside, the units will take 70°F air from the kitchen and bath and exhaust it to the outdoors at about 35°F; incoming fresh air will rise to 67°F or 68°F before it’s delivered through 3-inch tubing to bedrooms or living rooms. The difference between 84 percent and 92 precent efficiency may have only modest effects on the comfort of house occupants; either way, they’re getting a steady flow of fresh outdoor air that’s barely cooler than room temperature. But as Stephens points out, marginal energy savings can have significant bottom-line effects when a builder’s working to the very strict Passive House standard. “Over the course of a year, that small improvement might allow the builder to use a few inches less insulation in his exterior shell, or purchase less expensive windows, and still keep the annual energy consumption low enough to pass the standard,” he says. “So in this very challenging application, those efficiencies can pay for the extra cost of the unit.” One benefit of Corson’s Passive House’s very low heating load is the simplicity and safety of the mechanical systems. On site, Corson assembled the ComfoAir unit himself, placing registers and running air tubing with the help of carpenter Rich Perry (see pictures). The open-web floor trusses made running the 3-inch air lines simpler, but in the tight mechanical space under the staircase, lining up the intake and exhaust vents, the manifold, and the supply and return lines took a little figuring out. “Next time it’ll go quicker,” said Corson.
