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:

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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:

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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.