Radon Update

Back in June 2015, I wrote a couple of posts about the radon issue that I discovered in my house.  I won’t rehash the details here, other than to say that the theory proposed by Marc Rosenbaum…that the problem may have been caused by they improper balancing of the ERV…has proven to be true over the past year and a half.  If you’re interested, you can read the details about the problem, and his theory with regard to the cause, in the posts that I wrote during that time.

What I want to communicate today is, since that time, and without exception, every time the house radon level has risen, I have found that the air pressure in the house (which I measured with an appropriate manometer) had changed from positive to negative.  And without exception, correcting the pressurization issue returned the radon level to the norm of 1 pCi/L, or less.  This has happened about a half-dozen times over the past year-and-a-half.

In each instance, the change in pressurization was caused by an accumulation of bugs and dirt on the ERV intake.  The ERV is balanced to pressurize the house at +1 to +2 pascals, so it takes little to change that positive to a negative.  So, in effect, the two radon detectors that I have in my basement have proven to be a definitive way to determine, not just when radon levels have risen, but also when my house pressure is out of balance and the ERV intake filter needs to be changed.


ERV Pre-Filter Update

A while back, I wrote a post on an ERV pre-filter that I built.  It’s been about a year-and-a-half since I did that and I’ve learned a few things in the meantime, so I thought I’d provide an update.

To recap, I built the pre-filter for two reasons.  First, the system was sucking in large amounts of small, gnat-like bugs.  Although the internal ERV filter was catching almost all of them, that meant that the bugs were still getting into the ERV, and therefore into the house, and I preferred to keep them out.  In addition, I found that the interior of the ERV (and the duct leading to it) gets fairly grungy over time.  When I disassembled the ERV to give it an annual cleaning this summer, I found a film of fine dirt on everything, including the intake fan.  As with the bugs, my preference is to keep as much of that grime outside of the house as possible.

I also built the pre-filter because the ERV filters are relatively expensive, at over $20 apiece, and of much more substantial construction; with a hard plastic frame.  It seems somewhat wasteful to throw them out so often.  And while Zehnder says they last about six months, with no external pre-filter, I found I was getting no more than three months out of them.  By that time, particularly during the warmer months, they were caked with bugs (both dead and alive) and dirt.  So my thinking was that, if a less expensive pre-filter could catch the bulk of the particles/dirt/gnats, the internal filter would last longer.

While the original pre-filter worked as planned, it soon became apparent that I should have made it bigger.  The original system used a 12″x12″ filter; not a size that you’ll find at Home Depot, and not particularly inexpensive on the internet (even for the disposable versions).  Also, all of the disposable versions that I saw had a higher MERV rating than the Zehnder filter  I use (Zehnder makes two versions; a MERV 7/8 and a Merv 13. I use the former).  It seemed to me that the right balance between filter life and my needs would be something less than a MERV 7.

So I purchased a washable aluminum filter online.  But they are flat, rather than pleated, and therefore have a relatively small surface area.  For example, the 12″x12″ filter has about 121 square inches of surface area, while the Zehnder filter appears to have somewhere around 500 square inches of surface area due to the pleated fabric.  Because of that, within a month or so they tended to clog enough to throw my house out of balance and cause the radon level to rise.  The sequence became very predictable.  I’d notice that the radon level had risen significantly.  I’d check the house pressure with a manometer and find that the house was negatively pressurized.  I’d pull the filter and clean it.  The house would regain positive pressurization.  And the radon numbers would decline back to their norm of about 1 pCi/L.

To solve these problems, a couple of weeks ago I built a new filter unit using the same design.  However, this unit uses a 12″x24″ disposable filter, which can be inexpensively purchased at Home Depot or online.  Because these filters are pleated, I estimate that they provide about four times more surface area.  To combat the gnat problem, I am using a Merv 6 antimicrobial “3-month” replaceable filter that runs about $6 online.  In this filter, the pleated filter cartridge is glued to the peripheral interior of the outer frame, which, according to the manufacturer, prevents air (and bug) by-pass.  Time will tell how effectively it does that job.  But I am hopeful that, with about 500 square inches of surface area, I’ll get far more life out of these filters than I was getting out of the 12″x12″ reusable version.

My guess is that I might have had less of a problem (at least with the bugs) if the ERV intake was located in a higher location.  But given that the ERV is in the basement, that would have been a bit difficult.

I also suspect that some people may never look inside their ERV until something goes wrong.  Of course, in any house, that could turn out to be an expensive mistake.  But in a passive house, it could also be potentially harmful.

Here are some photos of the new unit.  Like the old unit, I built it out of 1/2″ Azek with a removable front that helps keep the rain off of the filter:

img_1235 img_1236 img_1261 img_1262


Update on Power Usage – One-Year of Detailed Data

As of December 7th, the eGuage energy monitor has been installed for a full year, giving me the ability to know how much energy it took to heat our house for a full one-year period.  The data was actually a bit better than I anticipated:

Total kWh used to heat the house for the period December 7, 2014 through December 6, 2015:  2,549.6534

At 15 cents per kWh (winter rate) that resulted in a total cost of $382.45 (This includes all charges on our electric bill).

Knowing that natural gas is generally much less expensive than electricity, I thought it might be interesting to compare the two.  In other words, if we heated the house with natural gas, what would the cost have been?  Here are my calculations.  Feel free to let me know if you think my numbers are incorrect:

One kWh of electricity will generate approximately 3.14 kBtus of heat energy.  Therefore, our house used 8,007 kBtus of energy to heat the house during the one-year period.

8,007 kBtus equals 8,007,000 btus.

A “therm” of natural gas produces approximately 100,000 btus and costs approximately 79 cents (of course, this varies by location).

Therefore, if our house had a 100% efficient natural gas furnace, it would have used slightly more than 80 therms of natural gas for heat (8,007,000/100,000) and that would have resulted in a cost of $63.25 for the year.

Now, of course, we can’t use natural gas for several reasons; natural gas is not available where we live, they don’t make a natural gas furnace small enough, and a Passive House is so tight that burning any fuel in it is not advisable.  But I thought the comparison would help put the energy efficiency of a Passive House in perspective for those who use something other than electricity to heat their home.

Some other highlights from the one-year data:

The water heater used 715.516 kWh; about 31 cents per day.

The clothes dryer used 634.7463 kWh; about 28 cents per day on average.

The ERV used 369.02674 kBtus; about 17 cents per day.  However, it should be noted that the ERV was off for an estimated 50% of the three summer months because we had the windows open.

Raw Data on Electric Usage and Temperature/Humidity

In this post, I’m including links to files that contain the historical electrical and temperature/humidity data for our house from February 1, 2015 through the date of this post, October 23, 2015.  The data begins on February 1st because although I started recording in early December 2014, the heat pumps were not working correctly until late January.  My intent is to offer this data to anyone who desires to use it for analytical purposes.

The electrical data, which was recorded by an eGuage energy monitor, contains daily numbers for total electrical use, the first floor heat pump, the second floor heat pump, the combined total of the two heat pumps, the dryer, the water heater, the ERV, the well pump, and the family room, which basically includes the lights and TV in that room.

The temperature and humidity data was recorded by Hobo data loggers that were placed on each floor (basement, first floor and second floor), and outside on the north side of the house under the porch roof.

eGauge Data 2-1-15 thru 7-31-15






I will update this data on February 1st of next year.

In early December (once I have a complete year of eGuage data), I will post the yearly total electrical usage for the appliances and mechanicals that are listed above.  As I indicated, the heat pump data won’t be totally “accurate” (because they were “short cycling” until the end of January).  But the information should still provide a reasonable indication of how efficiently the house is operating.

A Pre-Filter for the ERV

I made a pre-filter for the ERV. So far, it looks like it’s going to work fine.  It uses a 12″ washable filter element and is protected from the elements with a removable cover.  I’m confident that it will keep the bugs out and extend the life of the internal filter.  The big question is how long it will last before it reduces the airflow enough to require cleaning.  I’ll have more data on that this fall.  Here are a few photos:

IMG_1052 IMG_1051 IMG_1050

ERV Balancing – The Trim Setting

A couple of posts ago, I talked about how the ERV was out-of-balance, which placed the house in a state of negative pressurization and increased the radon levels.  When I rebalanced the system, I became a bit concerned because I had to completely open all of the supply vents and slightly close a couple of the exhaust vents to get the house into a state of positive pressurization.  That just didn’t seem right.  I installed the system, and I knew that all of the supply and exhaust lines were hooked up properly.  So I couldn’t understand how the supply flow could be that much lower than the exhaust flow.

Several days later, with that issue still on my mind, I pulled out the report that was prepared when the system was commissioned, and read through it again.  It was then that I noticed a box in the lower corner called “Trim,” which seemed to indicate that the supply and exhaust fans could be, and were, adjusted independently, so the supply fan ran at a lower speed than the exhaust fan at each of the three speed settings (low/medium/high).  This was particularly interesting because up until that moment I assumed there was only one fan in the unit.

That prompted me to read the ERV manual, and several things fell into place.  The ERV does, in fact, have two independent fans, and yes, these fans can be adjusted independently of each other.

I then checked the actual settings and found that, as the Trim box on the report indicated, the fans were adjusted to the stated levels during the commissioning process.  For each speed setting (low/medium/high), the tech set the fans to operate at a certain percentage of full power.

One (i.e. me) might think that the fans would be set to operate at the same level at each of the three speed settings.  But that wasn’t the case.  For instance, at the “low” speed setting, the supply fan was set at 39% of full power and the exhaust fan was set at 45% of full power.  At the “medium” speed setting the fans were set to 56% (supply)  and 62% (exhaust), and at the “high” speed setting, they were set to 78% (supply) and 95% (exhaust).  So basically, the intake fan was operating at a level that was 10% to 15% lower than the exhaust fan.

I’m sure there was a reason for setting the supply fan to run at a lower speed than the exhaust fan at each power level, and ultimately I’m sure that reason was to ensure that the system was in balance.  I suspect that one may need to adjust the fans independently because the supply lines and exhaust lines are rarely, if ever, going to be the same lengths, and the difference in lengths play a role in the amount of resistance.  The same goes for the ducts that go from the ERV to the outside.  In my house, the exhaust duct is at least twice the length as the supply duct.

But regardless of the reason, the goal clearly wasn’t achieved.  Not only was the goal not achieved, but the difference between the settings left the system far enough out of balance to make it impossible to bring it in balance without completely opening all of the supply vents and slightly closing some of the exhaust vents.  In fact, even when the system was initially balanced, the tech didn’t install baffles in several of the supply vents, which should have been a clue that something was amiss.

Fortunately, changing the trim settings is a simple process that is explained in the manual. Once I adjusted the fans so they were operating at the same level at each speed setting, balancing the system (and creating a slightly positive pressure in the house) became a lot easier.  And since there is no longer any need to fully open all of the supply vents to achieve that goal, I can now set the flow at each supply and exhaust vent to the proper setting.


ERV Maintenance – Keeping the Bugs Out

While learning about radon and ERV balancing, I also learned a bit about ERV maintenance that no one had previously mentioned.  Specifically, on my house the ERV is in the basement and the exterior intake and exhaust vents run through the rim joist just below the first floor, which places them less than two feet above ground level.  On some of the houses I’ve read about, the ERV is placed about the living space and therefore vents in a location that is much less accessable, but may be advantageous when it comes to bugs that commune near the ground.

The exterior vents on my house look like this with the covers on:


Once the cover is removed, you’ll see that Zehnder provides a screen to keep critters out; it has about a 1/2″ mesh.   That clearly wasn’t sufficient in my case because there are a large number of bugs both large and small, that can fit through it.  On the exhaust side, this isn’t a problem because the air is constantly pushing things out (at least while the ERV is on).  But on the intake side, these bugs are attracted to the opening and end up inside the ERV.  I spoke with Zehnder, and they said that they don’t make any external pre-filter. Evidently, bugs are a lesser problem in Europe (I’ve seen that many places in Europe don’t even use window screens).  Also, this might be an issue that is more problematic to those of us who live in more rural areas and/or those of us who have the supply vent near the ground.

My initial attempt to deal with the situation involved adding a piece of window screen over the opening:


That’s about a week’s worth of accumulation.  Typically, I found beetles and moths on the screen trying to find a way in.  The screen did a good job of catching them.  But when I checked the internal filters, there continued to be a significant number of small living flying bugs (and quite a number of dead ones) buzzing around or on the internal supply filter.  At the intake, I could see these bugs were small enough to work their way through the screen to get in.

In addition to the bugs that got into the system despite the window screen that I added, the supply filter continued to gets dirty very quickly.  Zehnder says that the filters should generally last about six months with periodic vacuuming.  While the exhaust filter appears to easily last that long, the supply filter turns pretty black within weeks.  And as I indicated earlier, we live “in the country.”  It’s almost shocking.  And unfortunately, periodic vacuuming doesn’t do much more than remove the dead bugs.

Two things annoyed me about this situation. First, I don’t like to see bugs flying out of the ERV when I pull the intake filter.  And second, although I can afford it, the filters can only be purchased from Zehnder and cost about $22 each.  So changing them often (say once per month) is both costly and wasteful.

In my second attempt to remedy the situation, I purchased a roll of “pollen proof” window screening from Home Depot for about $10.  It’s basically the same as regular window screen, but the mesh is much finer; fine enough to keep even the small bugs out of the system.  It worked well, actually too well.

Although we generally keep the windows open (and the ERV off) during the summer, we do occasionally close things up and turn on the ERV and a/c when the weather gets too humid.  We recently went through a period like that.  Following that period, the humidity dropped, but my wife and I were both out of town for a week, so we turned the air conditioning off and left the ERV on.

When we returned, I noticed that the radon level in the basement had risen.  This was surprising, and the fiirst thought that entered my mind was that maybe the whole “pressurized house” theory of keeping radon out wasn’t true after all.  So I pulled out the manometer and took a reading. To my surprise, the gauge showed that, instead of being positively pressurized to about two pascals as it had been when I last checked it, the house was negatively pressurized to about eight pascals.  At first, I thought that something was wrong with the gauge.  But then I thought to check the pre-filtering pollen screen that I had placed over the ERV intake.  Sure enough, it was completely clogged with bugs and “dirt.”


I removed the screen and took another reading with the manometer.  As I anticipated, the house had returned to a state of positive pressurization.  Over the next few days, the radon level sank back down to just above 1.

Obviously, I needed to find a different way of pre-filtering the incoming air.  Currently I’m using this product, which I also purchased at Home Depot:

IMG_1039 IMG_1038

It seems to stop most of the bugs and dirt, although not as much as the pollen screen. The mesh is more open than the screen, but it is a “maze” that is about an inch thick.  So while some of the smaller bugs still appear to get through, I suspect it will be much less prone to clogging to the point where it will affect the pressure balance inside the house.

But I now have an idea for a removable, cleanable, pre-filter with a larger surface area.  Once I build and install it, I’ll report on how well it works.

As I mentioned earlier, maybe in those situations where the supply vent is located high on the exterior of the house the bugs and dirt aren’t an issue.  But if they are, remedying the situation becomes a whole lot more difficult.

Radon – Part 2 – The ERV

Several months ago, I posted about the radon problem that I discovered, and how I solved it.  Shortly after writing that post, I was contacted by Marc Rosenbaum, Director of Engineering at South Mountain Company,  a design/build firm in West Tisbury, MA.  Marc was interested in the radon issue, and offered a thought that had not been raised by anyone else I had spoken to. In short, he suggested that an imbalance in my ERV could possibly have been the cause of, or at least a contributing factor to, the relatively high levels of radon in my house.

His theory went something like this…The ERV is supposed to be balanced; it is supposed to draw in the same amount of air as it expels.  When that is the case, the interior of the house is neither positively nor negatively pressurized (with respect to the outside air).  It is neutral.

If the ERV is drawing in more air that it is expelling, the interior of the house will be in a state of positive pressurization.  In this state, the excess inflow of air will try to find a way out through any gaps that are present in the building envelope because it cannot all get out via the ERV exhaust duct.

But if the ERV is expelling more air than it is drawing in, the interior of the house will be in a state of negative pressure.  In this instance, the ERV is going to pull air into the house through whatever cracks/gaps exist to make up the difference between the smaller volume of air coming through the ERV intake and the larger volume of air moving through the ERV exhaust.  Since some of these entry points are inevitably going to be in the foundation, if the soil contains radon the incoming air is going to bring that radon with it. And this is essentially what Marc thought might be happening.

I have to admit that it was difficult for me to believe this could happen.  My house tested out at .3 ACH @ 50 pascals.  I saw how the basement slab was constructed and sealed.  I just couldn’t imagine that an imbalance in the ERV could suck enough (i.e. any) air through the foundation to make the radon level increase.  I assumed that radon particles are so small they can move through things that air cannot.

Marc then made the idea more mind boggling by saying that the radon fan might be contributing to the negative pressure situation even as it solved the radon problem.  This also made no sense to me.  How could that little fan – one with the lowest airflow I could find – possibly be powerful enough to suck so much air through minuscule gaps in the foundation that it would change the internal pressure in the house? It just didn’t seem possible.

Well, as I have come to learn, Marc appears to have been correct on all counts.  The ERV was out of balance and causing the house to be negatively pressurized.  That negative pressurization was causing the radon problem.  And the radon fan was increasing the negative pressure even as it solved the radon issue.  It was doing that by sucking air out through the gaps in the foundation floor, which in turn pulled air into the house through whatever gaps are present above the foundation floor.  This of course isn’t good, because none of that air is filtered. But on the upside, at the same time, the radon fan was pulling the radon from the soil under the house and expelling that radon to the outdoors, thereby reducing the radon level in the house.

To determine whether Marc’s theory was correct, Marc suggested that I check to see if the house and the ERV were balanced by using a manometer, like the one pictured below:


The manometer measures the relative difference in pressure between the interior of the house and the outside air.  Basically, you do this by running a flexible tube from one valve on the manometer to the outside, and leaving a second valve on the manometer open to the inside of the house.  The manometer does the rest, with the results displayed in “Pascals.”  Ideally, if the ERV is balanced, the manometer should display a reading that will probably fluctuate, but stays close to zero (The calmer the wind outside, the more stable the reading will be), thereby indicating that the pressure in the house is the same as the pressure outside the house.

With (once again) the help of my Passive House Rater, I was able to take measurements with the radon fan off, the ERV off, both off, and both on.  This enabled me to see the effect that each was having on the interior house pressure.  The reading we got supported Marc’s supposition. The manometer showed that the house was negatively pressured by eight to ten Pascals with both the ERV and radon fan running.  Most of this (about five to six Pascals) was due to the ERV.  About three to four Pascals were due to the radon fan.  So, both the ERV and the radon fan were pulling air into the house through the cracks and gaps that were simply too small to find and close.

This led to the question of “why?”  Why wasn’t the ERV balanced?  The purchase price for a Zehnder ERV includes a $500 fee for commissioning (i.e. balancing).  Zehnder mandates that their own person commission the system because they want it done correctly.  Well, it appears that the problem with my house wasn’t the person who did the work, it was the equipment that he used.

The Zehnder ERV that I installed in my house has ten individual interior supply ducts (which bring fresh, filtered air into the house) and ten individual interior exhaust ducts (which expel stale air from the house).  Those two airflows need to be the same for the system to be in balance.  [Note: This is different than interior supply and exhaust vents.  In my house, there are eight supply vents and six exhaust vents.  Some vents have one duct attached (e.g. the powder room exhaust vent), some have two ducts attached (e.g. the main floor supply vent), and one has three ducts (i.e. the kitchen exhaust vent).]

To balance the system, the tech person went from room to room and opened or closed each vent until a pre-determined amount of air flowed through it.  This was done by checking the flow through each of the vents, one at a time.  Since adjusting the flow at one vent can change the flow at another, the complete process must be performed a number of times.  So once all eight supply vents were checked and adjusted, the tech started at the beginning and checked/adjusted them a second time, tweaking each to get it as close to perfect as possible.  And then he checked them a third time.  In fact, I think he ended up checking them five or six times before he was satisfied. The same went for the six exhaust vents.   The process was completed when the tech felt that he had each vent adjusted as close to perfect as possible.  And at that point, he concluded that the system was balanced, with (in my case) about 120 cfm being supplied and 120 cfm being exhausted when the ERV is set to medium speed.

The problem is that there is a margin of error in the equipment used to measure the flow at each vent, and some equipment has a greater margin of error than other equipment. More specifically, Zehnder used a rotating vane anemometer on my house.  It looks like this:


One evaluation of that equipment indicates that it could result in an error of up to 25%:

Download (PDF, 2.05MB)

That margin of error is further complicated by the fact that the anemometer display never stays static.  Rather, it bounces around a bit.  For example, it may be that the best that can be done is get a reading that bounces from 11.1 cfm to 13.3 cfm for the single duct powder room exhaust vent, leaving the operator with the task of interpolating the results.  In that instance, it might be fair to say that the results look like 12 cfm, on average.  But who knows what the actual reading really is?  And at airflow levels as low as that, being off 1 or 2 cfm each time you check a vent can add up.

And therein lies the problem.  If the equipment is inherently inaccurate and the tech is off in his interpolation of the data, it is not difficult to see how the ERV could end up being unbalanced.  That is essentially what appears to have happened at my house.  But I digress…

After seeing that the house was negatively pressurized, my Passive House rater, who has been a big help to me throughout the entire building process, measured the flow at the exterior vents using a more accurate devise known as a “power hood.”  Although the wind was preventing us from getting a steady reading, we estimated that the intake was bringing in 10 to 20 cfm less than the exhaust was expelling.

Unfortunately, Zehnder doesn’t check the overall flow at the exterior vents, primarily (they say) because those vents often are not easily accessible (although on my house they couldn’t be more accessible) .

Fortunately my Passive House rater helped me use his equipment to rebalance the system.  Essentially, what I ended up doing was opening up all of the supply vents and slightly closing a couple of the exhaust vents.  By monitoring the effect these changes had on the house pressure with the manometer, I was able to bring the interior of the house in balance with the outside air.

While doing that, I found that the balance and pressure are also affected by the speed at which I run the ERV fan.  When the ERV is set on “low” speed (i.e. Level 1), the house is now pretty much neutrally pressurized, with a reading that fluctuates between -1 and +1 Pascals, and probably leans more toward the +1.  But when set on medium (i.e. Level 2), the house is clearly positively pressurized, with a reading between +1 and +2 Pascals.

Once I completed the rebalancing, I waited several days to see what effect it would have on the radon level.  Prior to rebalancing the ERV, the radon fan had brought the radon level from about 6 pCi down to about 1pCi, and within three days of unplugging the radon fan (which I did a couple of times just to check), the level consistently rose back to 6 pCi.  But this time was different.  With the radon fan off and the ERV on low speed, the radon never rose above 1.4 pCi, comfortably below the mitigation threshold of 4pCi.  When I set the ERV to medium speed, the radon level dropped further (although, so far, never to zero).

One final note…All of this occurred this spring, and we rarely use air conditioning.  So as it turned out, we started opening windows a couple of weeks after the rebalancing.  Some days we only opened the first floor windows.  Some days only the second floor windows.  And some days both the first and second floor windows.  Also, on some days, we opened a window (or windows) for only a short period of time.  At other times, we left windows open all day or for several days at a time.  But regardless of the number of open windows or the amount of time they were left open, it soon became clear that opening windows had an effect on the radon level (which we measure in the basement).  The longer a window or windows were open, the higher the radon level rose.  The up-tick in the radon level is very slow, but given enough time (i.e. days) the level eventually rises as high as 2.4 pCi (still not bad).  And if the windows are left closed, the radon level slowly comes back down to about 1 pCi.  Subsequent testing with the manometer showed that opening even one window threw the house into a slightly negative pressure situation.  Whether its the “stack” effect, the wind, a combination of both, or something else, I don’t know.  But the effect seems to be consistent.

With regard to the stack effect, even on a hot day the temperature on the second floor is no more than one degree higher than the first floor.  But the temperature difference between the first floor and the basement is more substantial.

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.










Energy Recovery Ventilation (ERV)

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:

IMG_0652 IMG_0651

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.