MERV 13 Filters
Are all MERV 13 rated filters the same? Are they really all rated MERV 13 or is there fudging going on in this space?
Visually, the filters all appear different – some have cardboard grids on one side to prevent the filter from collapsing into the airflow, some have metal grids, some have cardboard grids on both sides. Which is better/worse for HVAC operation and the environment? Are they all equally sealed at the edge where the filter media meets the cardboard frame?
I learned about flanged filters on this site a while back, but the only ones available seem to be the 4″ deep that fit into a 1″ frame. Given the warning by Allison Bailes in this article: https://www.greenbuildingadvisor.com/article/designing-return-filter-grille-boxes
I’m concerned a 4″ deep would be too restrictive to airflow, but I’d love to try a 2″ deep filter. Do they make 2″ deep flanged filters for install into a 1″ deep frame?
Not knowing the answers to the above, I have generally been ordering 1″ deep MERV 13 filters made in the US, from any company (other than the ubiquitous delivery company) generally based on price alone. This made me think that IF there were a company whose filter quality was vetted by this site, and their price was reasonable, I’d probably just buy those…could be a potential source of revenue…
As far as install, per the photos from Allison Bailes articles, I’ve been taping the filters in to prevent bypass. Wish there was a quicker solution…
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Replies
I'm not aware of anyone fudging their numbers on MERV ratings, but I wouldn't be surprised if some off brand was doing something shady. If you buy filters from reputable sources, you're not likely to have any problems in this regard.
The frame of the filter is there for structural support. I usually see the carboard frames, not the metal mesh. I don't think there is likely to be much difference in back pressure betweek the two, since the cardboard type tends to have less structure with larger openings, and the metal mesh is more structure with smaller openings. Overall open aperature is pretty large either way. I don't think there is any real enviornmental difference between the two, either, unless you're trying not to use some particular material (the "metal" used in the metal mesh is just aluminum, as far as I know, so pretty benign just like cardboard).
It has always been my experience that the deeper filters offer less backpressure than the thinner ones. You do need to watch out for pleats though -- thinner filters may have more pleats, which means they could have more filter media inside even in their thinner profile. Ultimately what matters for backpressure is the total area of filter media, not the depth of the filter frame. For a given MERV rating, a larger area of media should mean less backpressure for the same rate of airflow through the filter. In practice this means you generally want the deepest filter you can fit that also has as many pleats as possible.
All filter housings tend to be leaky. Tape works, weather stripping works too. Either is tedious to put in place. Ive sometimes through about trying backer rod (round foam cords that have no adhesive), but that's tedious too. What I usually do in my own system is to just keep the flange around the filter mount clean and smooth, so the airflow tends to push the filter tightly against one side of the frame. I get a seal that is good enough for my purposes, and I don't worry about a little bit of air leaking around the filter. If a little air leaks past, you just reduce your overall filter efficiency a little bit. With multiple passes of air through the filter, you end up doing a pretty good job filtering the air anyway, and that's what happens with multiple run cycles of your HVAC system.
Bill