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Building Science

How to Ensure a Low Pressure Drop Across a High-MERV Filter

A simple rule of thumb will get you there

Using a high-MERV filter doesn't have to result in a large pressure drop. [Photo credit: Energy Vanguard]

Last August I began a series of articles on filtration and indoor air quality.  You can find them listed in the related articles section but let’s do a quick review here:  We spend a lot of our time in buildings.  A lot of indoor pollutants are generated in the kitchen and not removed by the range hood.  The consensus among indoor air quality researchers is the particulate matter that’s 2.5 microns or smaller (PM2.5) is one of the worst for health.  And finally, good filters (i.e., MERV-13) can remove a lot of the PM2.5 and other pollutants but experience as well as studies show they often don’t, for a variety of reasons.

So how do we fix this situation?  How can we get a high-MERV filter and have it do a good job of filtration without causing problems with the air flow in the heating and cooling system?  The answer is simple:  Make the filter big enough.  John Semmelhack, owner of building science firm Think Little in Virginia, spoke about this topic last year at the North American Passive House Conference in Boston.  Also last year, we got a new Mitsubishi ducted minisplit system here in the Energy Vanguard office in Georgia (meticulously installed by PV Heating and Air).  Let me show you what’s possible.

Semmelhack spoke about ducted minisplit heat pumps in Boston and towards the end of his talk he got to the topic of filtration.  (Download the presentation and see the filter section starting with Slide 25.)  He used a 2-inch deep MERV-13 filter in a filter grille.  The ducted minisplit they used was a one-ton system moving 400 cubic feet per minute (cfm) of air.  The pressure drop across the MERV-13 filter was 0.0274 inch of water column.  Yes, really!

What was their secret?  They…

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