Although I initially intended on using an UltimateAir ERV, I ended up changing my mind and installing a Zehnder Comfoair 350. It was a difficult decision. On the one hand, the UltimateAir ERV was relatively inexpensive; about $2,200, whereas the Zehnder ended up costing just under $8,000. But to keep the comparison fair, other things need to be taken into account.
Performance-wise, it seems that both manufacturers claim that their unit is the better “mousetrap,” and clearly both units have their share of proponents in the Passive House world. So I just assumed that either would do the job.
The UlimateAir unit price doesn’t include the cost of duct work, which is conventional (i.e. metal) and would (as far as I’m concerned) have to be installed by a professional. That would have been an added cost, but what concerned me was that the efficiency of the unit would, in part, depend on the care taken by the installer. Given what I’ve seen so far, I anticipated that I would have ended up with the burden of trying to ensure that the ducts were sealed up properly or I would have had to pay a premium to ensure that the installer took the time to do it right.
The UltimateAir price also didn’t include most of the other required parts (diffusers, grills, etc.), and I would have also had to find someone to “commission” the system to ensure that the correct amount of air was flowing into, or out of, each room. The potential problem I saw there was forecast by all the head-scratching from the HVAC contractor as he asked how anyone would measure such low airflow rates (20-25cfm).
On the other hand, the Zehnder unit was a complete kit, and included everything I needed (except some caulk (for the foam intake and exhaust venting, and some thermostat wire to connect the main controller to the ERV) to get the unit installed and running; and this included the cost of having someone from Zehnder come down and commission the unit (the $500 charge for this service was “mandatory”). But what really sold me was that collective “unit” was so well engineered that I was able to install the entire system myself, with very little assistance (My neighbor helped me lift the ERV to hang it on the wall and helped me put some of the intake and exhaust pipes together because someone had to hold one part while another person had to push the other part into it). But that assistance amounted to only about an hour of his time.
So taking everything into consideration, I figured that the true cost of the two units wasn’t as great as it initially appeared, although there’s no doubt that the Zehnder unit was more expensive.
The other thing that sold me on the Zehnder unit was the ducting, which consisted of 3″ flexible plastic hose, with one, two, or three run to each room, depending on the necessary airflow. My house used 20 runs; three to the kitchen, two to each bedroom, two to the office, two to each full bath, one to the laundry room, one to the powder room, and three to the basement (one exhaust and two supplies). There was no metal ductwork to install or seal, and the duct hose was relatively easy to snake from floor to floor and even through 2×4 walls. In my case, it was a one-man job.
The worst part of running the duct hose was that I had to unravel it in the basement as I was pulling it through the house (which required a fair amount of back and forth between the basement and the other floors). In other words, as when working with Romex, if you just start pulling it from the “coil,” it will kink in an instant and make your life miserable. To assist with pulling it through the basement, I rigged up some rollers using pvc pipe, wood scraps, and hanger iron, as shown in this photo:
Then I fed each line through, one at a time, starting at the ERV. Around the corner (in the first photo) is the central “chase” where I transitioned from a horizontal run to a vertical:
I drilled a 3″ hole through the floor for each line and fed the hose up through it. Here’s where the hoses come up through the first floor…
Since we used floor trusses for the second floor, I had a lot of flexibility in getting the hose to wherever I wanted it, whether it was a ceiling mounted kitchen exhaust (which required three lines)…
…wall mounted retangular supplies (on the second floor)…
…or a wall mounted round supply…
As you can see in the photos, there are round and rectangular diffusers. The round version comes with either two or three ports. The rectangular version only comes with one port. Both can be used in either a wall or a ceiling, although based on the plan that Zehnder put together for my house, the rectangular diffusers are used for the bedroom supplies, and the round diffusers are used for the exhaust lines, most of which are in the ceilings (although the 2nd floor bathroom diffusers are in the walls because I didn’t want to pierce the envelope). Both versions of the diffusers are very robust.
A few of the lines didn’t go through the central chase, but most did. I ran the lines for the guest bath exhaust and the laundry room exhaust through the laundry room. They will end up behind a built-in cabinet:
I could have run it through the exterior wall that is directly behind them, but that would have cut down on insulation.
As I said earlier, the lines are 3″ O.D., and will fit through a 2×4. But there’s not much meat left in the 2×4 when you’re done drilling the hole 1/4″ on each side if you are perfectly centered. I also had a slight concern that someone may poke a hold through the hose someday while trying to hang a picture. So I packed out one of the walls an extra inch-and-a-half…
I also ran a couple of the lines through a 2×6 wall that I installed on one side of the office (the only 2×6 interior wall in the house). But most importantly, I took care to think through where I would place each diffuser, and how the line would get there, before running the vent hose.
As suggested in the Zehnder video, I marked each line at the ERV end to keep track of where every line ended up. When I finished, the lines that you see in the first photo were a jumbled mess, tangled up with each other. So I removed all of them from the roller-racks, and replaced the one at a time, securing each to the I-Joists with hanger straps. That was pretty easy, and resulted in the much neater installation that you see in the (first) photo.
Pop a gasket on the hose, and it just plugs into the ERV (on one end) and a diffuser (on the other end).
One thing to keep in mind is the requirement for fire blocking, which I didn’t consider until the inspector raised the issue. Here are a few photos of that central chase. The first shows where I packed the 2nd floor floor trusses with rock wool insulation in the area where the hoses come up through the chase and into the 2nd floor floor trusses…
Then I boxed the rock wool it in with plywood, and foam it to death with fire-blocking foam to satisfy the inspector.
The irony is that, if the house ever catches fire, the hoses will probably melt and all (or most of) my work will probably be for naught. In fact, someone told me that he heard that some inspectors had concerns about the installation of these Zehnder units for that reason (I have no idea if that is true). Ironically, my inspector didn’t fail me (he did say that he had never seen a system like this before), but he did say that the hoses would probably melt in a fire, rending the fire-blocking ineffective. But on the other hand, someone else reminded me that the house is so tight that there really wouldn’t be much of a “chimney” effect. And then I’ve read comments by some who say that, in a fire, an ERV or HRV feeds the fire mechanically unless there’s an automatic shut-down feature (or the electric line burns through). So who knows?
In addition to running the duct hoses, as I mentioned earlier, I had to hang the ERV on the wall in the basement (it weighs about 75lbs). I also had to cut two 7+ inch holes in the rim board for the supply and exhaust ducts, which are made of 1/2″ thick foam; another slick park of the Zehnder package. I suggest that anyone contemplating the use of this unit really think about where it will be situated and where the two foam ducts will pierce the building envelope. One of my ducts was short and direct. But the other had to wind around a bit because I couldn’t cut a hole that big through the I-Joists and because the closest available spot for it was about 10 feet away from the ERV.
And finally, I had to run a 4-wire thermostat line from the ERV to the place on the first floor where the main control will go.
Every part of the Zehnder package appears to be top-drawer. And every part looks and feels like it was engineered to work as a component of a well-integrated unit.