Editor’s note: This post is one of a series by Eric Whetzel about the design and construction of his house in Palatine, Illinois, a suburb of Chicago. A list of Eric’s previous posts appears below. For more details, see Eric’s blog, Kimchi & Kraut.
Building with Passive House principles in mind, we knew that in addition to maintaining a tight building envelope and installing substantial amounts of insulation, we also would need continuous mechanical ventilation.
Our system, using either a heat-recovery ventilator (HRV) or an energy-recovery ventilator (ERV), would have to be highly efficient. It would need to hold onto some of the heat in the conditioned air even as it introduced fresh and, oftentimes, cold air by means of heat exchange as the two streams of air (fresh and stale) passed by one another inside the main unit (without actually mixing together).
After researching the many options, we ended up going with Zehnder’s ERV, in our case the ComfoAir 350. We only considered two other brands, UltimateAire and Renewaire. In all the research I did, these three brands showed up the most.
Another interesting option would be the CERV system. Because it’s a smaller, newer company, we didn’t feel comfortable pursuing it, but it does look like a viable option worth considering if building a Passive House or Pretty Good House. (For more information on the CERV system, see “A Balanced Ventilation System With a Built-In Heat Pump.”)
I was also familiar with Panasonic units, but I had read that they weren’t efficient enough in terms of the heat exchange function (or heat recovery) to seriously consider using it in a Passive House or a Pretty Good House in a predominantly cold climate region like ours here in the Chicago area.
A high-efficiency system
Our Zehnder ComfoAir 350 is said to be 84% efficient in terms of heat recovery (the same principle applies in summer, only working in reverse, when you’re trying to hold onto cooled, conditioned air). Based on what I read, the consensus seems to be that while more expensive, the Zehnder has a strong track record of performance and durability.
The Zehnder also came with its own ductwork, which we knew would simplify installation and save us some money by allowing us to do it ourselves rather than hire someone else to run more conventional ductwork. Conventional ductwork would’ve taken up a lot more space as well.
As far as the ERV/HRV for northern states debate is concerned, we decided to opt for the ERV because it was supposed to help us hold onto some humidity in winter months — especially important when most structures in the Chicago area are exceedingly dry for most of the winter (and our winters are long). Although I read repeatedly during the design stage that ERVs can also help control summer outdoor humidity entering the house, this has not been our experience at all. In fact, the ERV seems pretty useless in this regard (more on this below).
The system quote we received was easy to understand, and Zehnder was nice enough to essentially design the system, both in terms of layout (i.e., where we should put all the supply and exhaust points) and recommendations about the quantity of air for each point. The system is designed to be balanced, meaning the unit should be bringing in as much fresh outdoor air as it is expelling stale indoor air.
Installation is not a DIY-friendly experience
As far as Zehnder units being DIY-friendly in terms of installation, in our opinion, this is highly debatable, since the installation manual is far from comprehensive. Our installation manual ended at physically installing the main unit on the wall. Not very helpful.
Without a detailed installation manual showing step-by-step how all the individual pieces fit together, you end up with a pile of what initially seems like random parts.
This was incredibly frustrating, especially since Zehnder units come at a premium price when compared to other brands. It never occurred to me to ask for an installation manual before purchasing the unit. It seemed a fair assumption that no one would sell a premium product without detailed instructions on how to put it together.
We were only able to proceed because of numerous online videos, Googling Zehnder unit photos, and by staring at and experimenting with the various parts to try to figure out how they were all supposed to come together. It was an unnecessary and torturous puzzle that shouldn’t have needed solving, and it wasted hours of my life that I’ll never get back. If you do an internet search and type in: “Google review Zehnder America,” the experience Sean Hoppes had with his installation wasn’t all that different from ours.
The videos we found were especially helpful but still leave out quite a bit of information necessary for any first-time installer.
The lack of a comprehensive installation manual makes no sense. I’m not sure how even a licensed and competent HVAC installer would fare much better without direct experience installing the units. My guess is they would be searching online for missing info, much like we did.
Start by mounting the main unit
After we got the main unit installed on the wall, figured out how all the parts fit together on top of the unit, and got to installing the 3-inch ComfoTubes and the large ComfoPipe, the process became much easier.
If you try to piece the tubes together one piece at a time in mid-air, it’s much harder to judge when the pieces are actually tightly assembled. With each connection point of pipe clearly marked with a Sharpie, it gives you an obvious goal to shoot for once you have the pipe almost in its final position. More to the point, it’s obvious when sections of pipe get out of alignment, or the connection isn’t nearly tight enough.
Using a piece of ComfoPipe, we outlined on the interior side of our Zip sheathing on one basement wall exactly where we wanted the pipe to end up. After a hole was cut with a 3-inch hole saw, we cut out the rest of the hole with a jigsaw.
Once we started using the Sharpie, it was relatively easy to get all the ComfoPipe installed and air sealed around the Zip sheathing. We added a Roflex gasket to make the air sealing easier.
Following the directions, we kept the ComfoPipe exit points for supply and exhaust more than 10 feet apart where they enter and exit the structure. This is so the two air streams won’t mix.
On the outside, we made sure to extend the ComfoPipe out farther than we needed, giving us some leeway once insulation and siding were installed over the Zip sheathing. We cut the ComfoPipe back to the proper length before installing the permanent covers supplied by Zehnder.
Mounting diffusers on walls, not the ceiling
During the design phase, and even after we brought the Zehnder unit to the job site, we had intended to place the diffusers for supply and exhaust points on the ceiling. But after really looking at all the cuts in our ceiling service chase that would be required to make this happen, we decided to place them on walls instead.
It proved to be one of the better decisions we made during construction. Not only did we avoid having to make many cuts in the ceiling, which would’ve meant a struggle to appropriately map them out around conduit, ceiling lights, and plumbing vents, it had the added benefit of making it much easier to do ongoing maintenance at the diffusers. Once we had moved in, that consisted mainly of checking on and cleaning filters.
During commissioning, our Zehnder rep told me they have issues with homeowners not keeping their exhaust diffuser filters properly cleaned, effectively undermining the efficiency and overall performance of the units. This is understandable if the diffusers are on ceilings. We were also told that placement of the diffusers is extremely flexible — almost anywhere can work. (Check with Zehnder directly just to make sure your proposed placement will work.)
By keeping the diffusers around 7 feet off the finished floor, it’s easy for me to check and clean the exhaust filters on a regular basis — once or twice a month. I keep two sets of filters, so it’s easy to remove the dirty ones, put in clean ones, and then rinse and dry out the dirty ones.
Once we decided to go through walls, it was just a matter of deciding where in each wall we wanted the diffusers to go, then cutting the corresponding hole through the bottom plate and the subfloor — being careful to check, and re-check, in the basement for any floor joists, plumbing, or electric conduit that might be in the way.
For bathrooms, we placed the diffusers between the shower and toilet, slightly cheating towards the shower to ensure maximum moisture removal.
All of the diffuser boxes required at least two ComfoTubes, except for the laundry/utility room, which only required one. Using one of the supplied black plastic caps made it easy to block off one of the outlets in the diffuser box. These black caps are also handy when pulling the ComfoTubes around into position since they help to keep out any construction debris.
In the kitchen, we located the diffuser across the kitchen, basically on a diagonal, from the stove. So far we haven’t had any issues with cooking grease or odors, and our range hood (recirculating) seems to be doing its job just as well.
Using scrap lumber, we were able to give each diffuser its proper stability inside the wall cavities. Although the mounting hardware for each diffuser box seems rather fragile, we didn’t run into any problems.
Applying a bit of hand soap around each opening in a diffuser box made getting a solid fit between the ComfoTube, the black O-ring, and the diffuser box fairly straightforward.
Because they are so small (at least compared to traditional sheet-metal ductwork), the tubes are easy to manipulate and move around, whether over a basement beam, around plumbing, electrical wiring, or any other structural component that’s not easily relocated. As long as you don’t need to make a short 90° turn, the tubes are easy to work with, so I imagine they would be ideal for renovation work in older homes.
Once all the ComfoTubes were installed at all the diffusers and at the main unit in the basement, we were able to pull all the lines tighter for a less messy final installation.
The commissioning of the unit, after drywall was complete, was fairly easy and straightforward, apart from a couple of wiring and electrical issues that had to be dealt with by phone with a Zehnder rep. And ordering filters from the Zehnder website has also been a straightforward and painless process so far. (They’re not cheap, but they do seem to be highly effective.)
The only issue we’ve really noticed with the unit is during the summer when outdoor humidity levels are high. Since the ERV is constantly running, there’s no way to avoid bringing in some humid air.
And, unfortunately, it’s enough so that our Mitsubishi heat pump set-up (a future Part 2 of 2 for HVAC details) can’t properly get rid of the excess humidity either, even as it keeps the interior more than adequately cooled. We tried setting the heads to “dehumidify,” but they just dropped the temperature (to almost 60°F) without budging the humidity in the house very much — the rooms were freezing and clammy. As noted earlier, an ERV just can’t handle elevated levels of humidity in the summer on its own.
By having meters in various areas of the house, it’s easy to see when humidity levels become a problem. (We’ve been happy with our AcuRite gauges). Last summer our solution was to buy a couple of small dehumidifiers, one for the first floor and one for the basement. They worked, but they also ate up a lot of energy. Setting the Zehnder fan speed to low seemed to help somewhat, but not enough to avoid using the dehumidifiers. This summer we’re going to try a stand-alone Ultra-Aire whole-house dehumidifier, which should use less electricity — and it should perform at least as well as, if not better than, our current equipment at removing excess humidity.
Having read that anything above 60% indoor humidity can be problematic, especially in tighter, high-performance homes, it was disheartening to see the numbers move towards 70% in early summer. This is what prompted the purchase of the dehumidifiers.
From everything I had read during the design phase regarding Passive House, I knew that indoor humidity in the summer could be a slight issue, but having experienced it firsthand, it now seems obvious that incorporating a dedicated dehumidifier in any structure that will see elevated levels of summer humidity, even if it’s only expected to last for just a few weeks, is a necessity. Based on what I’ve read recently, it sounds like Passive House designers, who were already doing this for southern states, are moving towards doing it in states much farther north. Presumably this would also hold true for anyone designing a Pretty Good House as well.
Granted, 60-70% indoor humidity (or even higher) for a couple of weeks probably won’t ruin any structure, but for us, at least, keeping it in the 50-60% range during the hottest days of summer not only gives us some added peace of mind, regardless of the hit we’ll take in terms of overall energy use, but it’s also an issue of comfort. (I grew up in a house without air conditioning and still have vivid memories —all of them bad — of enduring hot and humid summer days and, even worse, long summer nights.)
Much like the initial complaints of overheating due to excessive or improper placement of glazing, especially on southern facades, this issue with excessive humidity seems to be part of the evolution in understanding how Passive Houses, or high-performance homes generally, actually work in real-world conditions. Although the concept has been around since the 1990s, anyone building to or even just leaning toward the Passive House standard should know they are guinea pigs to some extent, no matter how well established the idea may be in building science terms.
Winter has not been a problem
In the winter, we’ve had no issues. When temperatures fall below 20°F, we set the Zehnder to low in the hopes that it will reduce demand on the heat pumps slightly. The system seems to retain humidity somewhat when the cold air being introduced would otherwise be excessively dry.
Indoor humidity levels have been pretty consistent: When it’s above freezing they typically stay around 40%, and when temperatures plummet towards zero or below they’ve still stayed in the 30-35% range. We’ve rarely seen indoor humidity drop below 30%, even on the coldest days, which definitely makes a difference on overall comfort levels. I’ve also noticed that wood flooring and wood trim doesn’t shrink nearly as much as it did in our last, conventionally built home.
Also, even when we experienced record low temperatures in January (hitting -24°F without windchill), the Zehnder kept on running without any issues. As far as we know, it never shut off to try and protect itself from the cold (our minisplit system did, but more on that later). The product literature is somewhat vague, only noting that low temperatures could cause a unit to shut off, but it’s unclear at exactly what temperatures or what combination of other environmental conditions might cause this to happen.
Most people either tape or use sealant on the gray ComfoPipe seams to block air leakage. During our blower-door test, no air leakage showed up, even with a smoke pen test. Nevertheless, during our recent cold snap, some frost was evident on the ComfoPipe seams, so I’ll eventually caulk these seams with Pro Clima’s HF Sealant. There must be some air leakage, be it ever so minor.
Using the boost function
When the boost function is turned on, the ERV pulls from all the exhaust diffusers, not just a particular bathroom or the kitchen. Again, for the kitchen, even if we’ve been roasting garlic or cooking something else that’s equally pungent, by the next morning any cooking smell is usually completely gone. There have never been any lingering smells emanating from the kitchen.
For the kitchen, when we want to utilize the boost function we just set the ComfoSense wall unit to high. The bathroom boost switches run on a timer (set at the main unit in the basement). But in the kitchen, when we’re done cooking we have to remember to lower the fan speed; otherwise it just stays on high.
The ComfoSense unit can display error functions or tell you when filters at the unit need to be cleaned. It also has an “away” setting, meaning you can have minimal fan speed to exchange air while you’re on vacation instead of just unplugging the unit altogether.
The boost switch in a bathroom is set to run for 30 minutes on the highest fan speed. So far, this seems to be plenty of time for it to work properly. Unlike a normal bath fan, which tends to be quite loud, even when the Zehnder is in boost mode it’s still incredibly quiet. Guests need to know they only need to press the switch once — it is indeed working.
The boost function has been working really well at removing moisture after showers. Nevertheless, in the winter, when temperatures are below 20°F and we decide against using the boost function after showers (again, hoping to hold onto some of the added humidity), the bathroom humidity levels still quickly drop from the 60s and 70s back to the mid-30s in less than an hour (and this is even when the Zehnder fan speed is set to low).
The diffusers are unobtrusive
We’ve also been happy with the diffusers, in terms of installing/removing them when necessary, but also in terms of their overall look. Whether on more neutral colored walls, or something bolder, they just look nice in our opinion.
They’re subtle enough to blend in to the background, but attractive enough so when they are noticed they don’t stand out in a negative way.
As far as changing filters at the unit, or even cleaning the core itself, so far it’s been a trouble-free experience.
During the summer, of course, they look much worse after a month, with so much more stuff floating around in the air (pollen, debris from landscaping, insects, etc.). Also unsurprisingly, the exhaust-side filter always takes much longer to get dirty as stale air makes its way out of the structure. (It probably helps that we don’t have any cats or dogs.)
Overall, then, we’ve been extremely happy with our Zehnder ERV unit.
Other posts by Eric Whetzel:
- Choosing Windows
- Attic Insulation
- Installing an Airtight Attic Hatch
- Air Sealing the Exterior Sheathing
- Installing a Solar Electric System
- Prepping for a Basement Slab
- Building a Service Core
- Air Sealing the Attic Floor
- Ventilation Baffles
- Up on the Roof
- A Light Down Below
- Kneewalls, Subfloor and Exterior Walls
- Let the Framing Begin
- Details for an Insulated Foundation
- The Cedar Siding is Here — Let’s Burn It
- An Introduction to a New Passive House Project