A Few More Notes on the Zehnder ERV

It’s late October, and we moved in on the first of the month.  A couple of weeks ago, we had the Zehnder ERV “commissioned.”  The commissioning was included in the purchase price (it added $500 to the total), and Zehnder sent one of their “commissioners,” Gary Bagget, down from Connecticut to do the job.

“Commissioning” basically involved several tasks.

The first task was to inspect the system to make sure it was installed correctly.  In doing this, Gary discovered two issues.

The first issue pertained to the ceiling registers that were used in the basement.  I installed them between the floor joists, and as seen in the photo below, they extended downward about 13 inches, terminating just below the bottom of the joists.


The problem was that, since we didn’t drywall the basement ceiling, there was no  surface against which to place the “funnel” that fits over the diffuser to measure the flow. Fortunately, I had a partial sheet of rigid foam laying against the wall, and was able to quickly cut a hole just large enough to slide up over the diffuser.  Problem solved.


The second problem pertained to the supply register that was used in our first-floor office.  In the system design that was created by Zehnder, this register was to be placed in the ceiling (like the one in the photo above).  But when I was installing the system, I had a brainstorm and decided that it would be easier to place it high on a wall.  To me, it seemed that it would make no difference. However, according to Zehnder, it does; at least with regard to supply diffusers.  It is preferred to have a ceiling supply diffuser expel air in a (more or less) horizontal fashion.  A wall diffuser should essentially do the same.  By using a ceiling diffuser on the wall, I changed that dynamic, causing the air to be expelled vertically, along the surface of the wall, rather than horizontally, out from the wall and toward the center of the room.  So while the proper flow of air (in CFM) could be still be achieved, the distribution of that air was less than optimal.  Fortunately, this was also an easy fix, although it cost about $70.  Here’s the new diffuser:



And here’s the ceiling diffuser that I erroneously put on the wall:


Had I notified Zehnder of my intended change to the plan, they would have sent me two rectangular wall registers…two because the ceiling register can accept one or two supply lines, while the rectangular wall registers can only accept one (and the office supply register required two supply lines).  So in the end, the mistake probably didn’t cost me any additional money.

After checking the system, Gary set the overall airflow to the correct number at each of the three power levels.  These flows are based on the volume of the house, which is calculated in a specific way for a passive house.  Our calculated volume of 23,610 cubic feet results in a required flow of 118 CFM on medium speed.

Once the overall airflow rate was set, Gary proceeded to balance the individual exhaust and supply diffusers.  This took the bulk of his time; at least several hours.  Each time he adjusted one supply (or exhaust) diffuser, he would have to re-measure the flow for  all of the other supply (or exhaust) diffusers. With each iteration of the process, he came closer to the correct flow rates for each diffuser.  And it took many iterations before he was satisfied with the results.

That was basically the process, and all-in-all it went smoothly.

There were two other points about the Zehnder system that I feel are worth noting.

First, as noted in the Zehnder installation video that you can see on their website, the unit has a condensation drain.  The ERV comes with a special J-trap that has an air block (in case their is no water in the trap) to ensure that the ERV doesn’t suck air through the drain.  I needed to run a condensate line from that trap to a condensate pump, which I thought would be a relatively easy task.  A trip to Home Depot and I returned with a couple of fittings that were intended to take me from an inch-and-a-half trap to a 5/8″ i.d. clear tube.  The problem was that the trap isn’t an inch-and-a-half.  It is a close, but larger, metric equivalent.  So the transition fitting didn’t work.  But like the other problems, it was a fairly easy fix.  On the advice of Aubrey Gewehr of Zehnder, I purchased a Fernco rubber transition fitting that did the job.


The second point pertains to the registers used for round diffusers.  As seen in the first photo above, they’re about 12″ long.  The intent is that they are placed in the wall/ceiling, and cut flush once the drywall is up.  My recollection is that, in the Zehnder installation video, Barry Stephens mentions that he uses an oscillating multi-tool to do the job, and the feeling that I got from his comments was that it was a fairly easy task.  I guess it depends on your definition of “easy.”  All of the Zehnder registers (both round and rectangular) are extremely robust.  As you can see in the photo below, I butchered the ceiling drywall before getting it done.


On my second attempt, which happened to be the wall diffuser in the office, I purchased a new half-round metal cutting blade, and used a piece of metal roof flashing under the blade to protect the wall.  You can see the results in the fourth photo, above.  It saved the wall from deep gouges.  But I’ve still got some painting to do to cover the marks left by the flashing as it vibrated against the wall.  And it pretty much destroyed the new blade in the process.










Septic Tank Leak

Earlier this summer, as I was working on the interior framing of the house, I noticed the sound of dripping water.  The weather was clear, but it had rained the prior day.  At first, I ignored it.  But after a couple of hours of listening to it, I decided that I needed to track it down.  In a few minutes, I was able to zero in on the source; the first floor toilet drain.  The plumber had finished installing his system a few days earlier, and, in doing that, connected the drains to the septic line in the basement.  Since the noise was coming from the drain, I went outside and lifted the two concrete access covers on the septic tank.

Although no toilets or faucets were installed, the tank had at least a couple feet of dirty water in it.  The problem was the two risers that connected the tank access holes to the surface of the ground.


As it turned out, the installer hadn’t sealed the joint between the risers and the cement tank with a butyl cord gasket, and the water in the soil was leaking into the tank through the seams.  When I pointed out the mistake, he was quick to dig up the risers and re-install them with the gasket.  Problem solved.  But it made me wonder what would have happened if I hadn’t heard that leak, and muddy water leaked into the tank for months or years.  And it made me wonder how many people ever think to check such things…