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Building Science

Do High-MERV Filters Always Reduce Air Flow?

A look at the research and the real world

High-MERV filters can remove a lot of pollutants from indoor air. But can you use one without suffering a drop in air flow? [Photo Credit: Energy Vanguard]

Over the past few months, I’ve taken a look at some critical issues for indoor air quality.  Starting with the issue of how much time we spend indoors, I then wrote about kitchen ventilation, the panoply of indoor air pollutants, problems with filters in general and with high-MERV filters.  Now let’s take the next step:  a look at what research has been done on high-MERV filters and what can be done to overcome those unintended consequences I wrote about.

Article:  Residential AC Filters by John Proctor, ASHRAE Journal, October 2012.

In this article, John Proctor summarized some of the results he and his colleagues have found from studying California homes.  One of the concerns in their research for the California Energy Commission was homeowners changing out 1-inch-thick standard fiberglass filters with 1-inch-thick pleated filters.

The Air Conditioning Contractors of America (ACCA) protocols for HVAC design assume a pressure drop of 0.10 inches of water column (i.w.c.) across the filter.  (Keep that number, 0.1 i.w.c., in mind as a reference point.  I’ll be coming back to it.)  If a system is designed with a standard filter for that pressure drop, the pressure drop with a pleated filter of the same size will most likely be higher.

In addition, poorly designed and installed duct systems already have external static pressures that are too high.  The typical furnace or air handler is rated for 0.5 i.w.c. but many run much higher pressure.  David Richardson of the National Comfort Institute says that in the testing they’ve done, the average system is running at about 0.82 i.w.c.

Result:  In the California study, Proctor et al. found that the pressure drop across the filter in 34 HVAC systems was 0.28 i.w.c.  That’s nearly three times what ACCA protocols assume. …

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4 Comments

  1. Steve Grinwis | | #1

    What I want to know is how much is .2 wc is an impact on my heat pump performance for my Mitsubishi split?

    1. User avater GBA Editor
      Allison A. Bailes III, PhD | | #2

      Steve,

      If you're talking about a 0.2" w.c. pressure drop across the filter in a ducted mini-split rated for 0.2" w.c. total external static pressure (the SEZ model for Mitsubishi), it means you've used up all of the available pressure just to move air across the filter. The result of that will be higher energy usage because the ECM blower will ramp up to maintain the air flow. It'll probably also be noisier.

      1. Steve Grinwis | | #3

        It's actually a PVA air handler. It'll do up to .8". But I've got a .2" filter, a .2" hydronic coil, and who knows how much resistance in actual ductwork...
        I'm curious how much it'll effect my COP.

  2. Walter Ahlgrim | | #4

    Out of curiosity let’s say a house has a 4 ton system and the owner wants to use a filter commonly found at the local store say the 20”x25”x1”Filtrete 1500 MERV 11. How many filters are needed to have enough surface area to get .010 i.w.c. and .003 i.w.c.?

    Walta

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