Building with Passive House principles in mind, we knew that, in addition to maintaining a tight building envelope, and incorporating substantial amounts of insulation around the structure, we also needed to install continuous mechanical ventilation in order to have adequate levels of fresh air, not to mention the ability to expel stale air.
We also needed our system, either an HRV or an ERV, to be highly efficient, meaning it could 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 (the various Zehnder units are based on overall cfm demand of the structure).
We only considered two other brands for our mechanical ventilation (HRV vs. ERV):
In all the research I did prior to construction, these three brands showed up the most in the projects I read about.
Here’s a good debate on the Green Building Advisor website discussing brand options: ERV Choices
Another interesting option would be the CERV system. Because they’re 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.
I was also familiar with Panasonic units, but I had always 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.
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 during the design phase, the consensus seemed to be that, although 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, allowing us to do it ourselves, rather than hire someone else to come in and run more conventional ductwork through the house (conventional ductwork would’ve taken up a lot more space as well). Even though the unit itself was more expensive, we thought we could offset some of the total cost for a ventilation system by installing the Zehnder ourselves, thereby saving some money on labor costs.
As far as the ERV/HRV debate for Northern US states, 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 ERV’s 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), along with the quantity, or cfm’s, of air for each point. In the end, after commissioning the unit, the system should be balanced, meaning the unit should be bringing in as much fresh outdoor air as it is expelling stale indoor air.
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 are purchased at a premium when compared to other competitive brands, and with the expectation of durability and design precision. It never occurred to me to ask before purchasing the unit for an installation manual, since 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 and figure out how it all was 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.
Looking on the current Zehnder website (February, 2019), I can’t find a more detailed set of instructions, either written or in a video format, which is disappointing. This seems like a pretty glaring oversight on Zehnder’s part, and one that should be remedied immediately.
Having lived with the unit for almost a year now, overall we’re happy with its performance, and we feel like we could install one fairly easily now that we’ve gone through the entire process, so it’s a shame we can’t say only nice things about the product simply because the installation manual was so limited or, more to the point, non-existent.
With each video and each photo, it was possible to glean one more crucial nugget of information, which took hours, whereas a detailed written manual or a step-by-step video would’ve made the process straightforward, and by comparison, frustration-free.
The videos below were especially helpful, but, nevertheless, they still leave out quite a bit of pertinent information necessary for any first-time installer (especially regarding all the parts that need to be installed on top of the main unit):
Unless there are no DIYers in Europe installing these units, and this is the expectation Zehnder has for its units both for overseas and here in the US, not having 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.
Once we got the main unit installed on the wall, and we figured out how all the parts fit together on top of the unit, by the time we got to installing the small, white 3″ ComfoTubes and the large, gray ComfoPipe, the process became much easier.
In regards to the gray ComfoPipe for the main fresh air supply and the main exhaust, both of which pass through the wall assembly, we found it more effective to put individual sections together on the floor, and, once fully connected, we marked the points at which the pipes met with a permanent marker.
If you try to piece the tubes together one piece at a time in mid-air it’s much harder to gauge when the pieces are actually tightly put together. 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 — it’s much more difficult to accurately gauge if only going by “feel” once the sections of ComfoPipe are off the floor.
Using a piece of ComfoPipe, we outlined on the interior side of our Zip sheathing exactly where we wanted the pipe to end up (trying to get as close to center as possible — makes air sealing around any penetration much easier). After a hole was cut with a 3″ hole saw, we cut out the rest of the hole using 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.
Following the directions, we kept the ComfoPipe exit points for supply and exhaust more than 10′ apart outside, where they enter and exit the structure, in order to avoid any possibility of the two air streams mixing, which would undermine the effectiveness of the system.
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. This allowed us to cut the ComfoPipe back to the proper depth before installing the permanent covers supplied by Zehnder.
As far as the white tubing is concerned, we really enjoyed how easy it was to put the 3″ ComfoTubes together.
During the design phase, and even after we brought the Zehnder unit to the job site, we always intended to place the diffusers for supply and exhaust points on ceilings. But after really looking at all the cuts in our ceiling service chase that would be required to make this happen, we decided to opt for placing all of 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 our ceiling structure, 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, mainly checking on and cleaning filters, once we moved in.
In fact, 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, whether at 8′ or 9′. It would be easy to forget about them, or even if you did remember, one can understand the reluctance to drag out a 6′ step ladder every time they needed to be cleaned. 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 them around 7′ off the finished floor, it’s easy for me to check and clean the exhaust diffuser filters on a regular basis (1-2) times a month. I always have 2 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 (both 2×6 and 2×4 framed walls), it was just a matter of deciding where in each wall we wanted the diffusers to be placed, and then cutting the corresponding hole through the wall’s 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 showers and toilets, slightly cheating towards the showers to ensure maximum moisture removal.
At the unit itself, Zehnder supplied us with blue (fresh air) and red (stale air) tags, to mark each ComfoTube as it leaves or returns to the main unit. This should make any potential maintenance or repair issues in the future easier to resolve, as well as helping to avoid confusion as you set in place each pipe at a diffuser.
Since we’re leaving the basement ceiling unfinished, it’s an ideal place to see how all the components come together: ComfoTubes meet at the diffuser box, along with the final cover for the diffuser, in this case for supply air. As you can see in the photo, there’s plenty of room in the metal tube of the diffuser box for deciding exactly where to cut it off in order to establish the finished height for the diffuser cover. In the basement we left them at their full height since there didn’t seem to be much incentive to cut them back.
All of the diffuser boxes required at least 2 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.
Our kitchen required the most cfm’s, at 36, so it required a special diffuser box and 3 ComfoTubes.
Again, since we didn’t place it in the ceiling, we put it 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 in the wall cavities. Although the mounting hardware for each diffuser box seems rather fragile, we managed to avoid any issues.
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.
It was also fairly easy to get each ComfoTube exactly where we wanted it. Since they’re 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, electric, 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.
With most of the ComfoTubes in place, we just needed to add a couple of walls in the basement before finishing up the last few ComfoTubes.
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.
Using 2×4’s, we created a little window for the ComfoTubes to pass through under the floor joists. This structure helped to get the ComfoTubes moving away from the main unit in an orderly way that made it much easier to organize all the tubing once it was all installed:
Using plumbing hangers also kept the ComfoTubes under control and organized.
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 beforehand. 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 summer when outdoor humidity levels are high. Since the ERV is constantly running, there’s no way to avoid bringing in some humid air in the summer.
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 (almost to 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 dehumidifies, 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, if not better, 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 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 simply a necessity. Based on what I’ve read recently, it sounds like Passive House designers, who were already doing this for Southern US 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 1990’s, anyone building to or even just towards 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.
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, and it seems to hold onto humidity somewhat when the cold air being introduced would otherwise be excessively dry. Indoor humidity levels have been pretty consistent: 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 last month (January, 2019), 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 mini-split 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, since there must be some air leakage, be it ever so minor.
In terms of the boost function, when turned on it 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’s never been any lingering smells emanating from the kitchen.
For the kitchen, when you want to utilize the boost function you just set the ComfoSense wall unit to HIGH (the Zehnder equivalent to a standard wall thermostat). Unlike the bathroom boost switches, which run on a timer (set at the main unit in the basement), when you’re done cooking you have to remember to go back and lower the fan speed, otherwise it just stays on HIGH.
The ComfoSense unit also can display error functions or tell you when filters at the unit need to be cleaned. It also has an AWAY function, 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, so guests need to know they only need to press the switch once — it is indeed working.
For the bathrooms, 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 decline to use the boost function after showers (again, hoping to hold onto some of the added humidity), the bathroom humidity levels still quickly drop from the 60’s and 70’s back to the mid-30’s in less than an hour (and this is even when the Zehnder fan speed is set to LOW).
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.
Here’s a photo of a supply-side filter after one month of exposure in winter:
During the summer, of course, they look much worse after a month with so much more “stuff” floating around (e.g. 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).
And since we didn’t need the framed-out HVAC chase in the corner of our Master Bath for all the ComfoTubes that we initially planned to send up into our ceilings, we ended up using this area for some much needed niche shelving for various toiletries and even some towels.
Overall, then, we’ve been extremely happy with our Zehnder ERV unit.