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: