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  1. #1
    Join Date
    Sep 2006
    Location
    Tampa Fl
    Posts
    105

    MERV 8 vs 11 vs 13

    What are your thoughts on the difference in MERV ratings vs IAQ?

  2. #2
    Not to be too sarcastic.

    But 13 is a higher number than 11 which is a higher number than 8.

    MERV is not a rating system.

    What are you trying to accomplish with the filters?

  3. #3
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,266
    MERV:
    Minimum Efficiency Reporting Value, commonly known as MERV Rating is a measurement scale designed to rate the effectiveness of air filters.
    The scale is designed to represent the worst case performance of a filter when dealing with particles in the range of 0.3 to 10 microns. The MERV rating is from 1 to 16. Higher MERV ratings correspond to a greater percentage of particles captured on each pass.

    I picked this up on the web. Filters main function is to keep equipment clean. "Clean" means to avoid a buildup of particles on the cooling coil. Very small particles are flushed down the drain by the condensate leaving the coil. Most cooling coils are able to handle particles passing through a Merv +8 filter. Clean coils dry after cooling cycle and are clean therefore no mold growth. Dust does accumulate in the ducts and on the horizontal surfaces in the home regardless of the merv rating. Very poor filters allow large enough particles that build up on the coil. Thus poorer indoor air quality??
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  4. #4
    tb
    Thank you for providing a kinder answer than I gave.

    MERV is not a rating system. A higher MERV number does not mean that it is a better filter. The number is derived from a relatively complicated table that shows average efficiency by particle size within particle size ranges.

    The real question is what are you trying to accomplish? If it is just to keep the coil clean, then a MERV 8, MERV 11 or a MERV 13 all have about the same efficiency on larger particles (larger than 5.5 microns). Keep in mind that visible particles start at 20 microns. These larger particles would be the ones that would lead to coil fouling.

    Keep in mind that the filter is just one component of the building system. There are many things that contribute to the IAQ including air leakage of the building envelope and the HVAC system, humidity, system run time, indoor sources of contaminants, etc.. Upgrading a filter only changes one element and should not be looked at as a "cure-all." That being said I usually recommend a MERV 11 filter for those with respiratory issues.

    Hope this helps.

  5. #5
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,266
    Just to clarify the difference in MERV ratings. I copied this from filter mfgrs. web site.

    "MERV- filter efficiency simplified

    MERV, or Minimum Efficiency Reporting Value, is a number from 1 to 16 that is relative to an air filter’s efficiency. The higher the MERV, the more efficient the air filter is at removing particles. At the lower end of the efficiency spectrum a fiberglass panel filter may have a MERV of 4 or 5. At the higher end, a MERV 14 filter is typically the filter of choice for critical areas of a hospital (to prevent transfer of bacteria and infectious diseases). Higher MERV filters are also capable of removing higher quantities of extremely small contaminant (particles as small as 1/300 the diameter of a human hair). A higher MERV creates more resistance to airflow because the filter media becomes denser as efficiency increases. For the cleanest air, a user should select the highest MERV filter that their unit is capable of forcing air through based on the limit of the unit’s fan power.

    Most filters become more efficient as the filter is used in the system. Care should be taken when considering filters that incorporate an electrostatic charge. Although offering a reasonable MERV value these filters will actually drop in efficiency as the filter loads with contaminant. A number of electrostatic filters are presently being offered for residential as well as the commercial/industrial applications. Refer to the filter packaging or literature to determine if the filter you are purchasing relies on an electrostatic charge to boost its MERV. If it does, request that the manufacturer provide you with the “discharged MERV” for the filter to determine its actual in-use performance"

    Also consider that high Merv rated filters have more pressure drop and reduce the air flow from a given system.

    Your point is well stated about the other even more important issues of fresh air ventilation and humidity control. Homes with finest air filtering and no fresh air or high indoor humidity will have poor indoor air quality.
    I like a good air filter (+merv 9) to keep the equipment clean heating/cooling/dehumidifing/ventilating the home.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  6. #6
    tb
    This article provides a complete description of how air filters are tested, how the MERV system works, and additional information on the "electrostatic" charge issue. It appeared in the January 2009 Issue of Indoor Environment Connections:

    http://allergyclean.com/articles/?p=157

  7. #7
    Join Date
    Mar 2010
    Location
    All over Florida
    Posts
    25
    Commercial or Residential...M11 would be the maximum I would use in a Residential application due to pressure drops. Depending on other concerns, a stand alone HEPA may be a product to incorporate.

    In either application - Commercial or Residential - start with 'Source Removal'. EPA Guidelines to follow as well: http://www.epa.gov/iaq/pubs/sbs.html

  8. #8
    Join Date
    Jun 2007
    Location
    Bucks Co PA
    Posts
    364
    I found this at the EPA;

    True HEPA filters with a MERV between 17 and 19 are defined by the IEST test method as having a minimum efficiency between 99.97 percent and 99.999 percent in removing 0.3 µm particles.

    A MERV of 20 is rated for 0.1 to 0.2 µm particles. HEPA filters have higher efficiencies for removing both larger and smaller airborne particles. True HEPA filters normally are not installed in residential HVAC systems; installing a HEPA filter in an existing HVAC system would probably require professional modification of the system.

    A typical residential air-handling unit and the associated ductwork would not be able to accommodate such filters because of their size and increased airflow resistance. Specially built high performance homes may occasionally be equipped with true HEPA filters installed in a properly designed HVAC system.

    * Some air filters may be effective at reducing tobacco smoke particles, but they will not remove gaseous pollutants from tobacco smoke. While some gas-phase filters may remove specific gaseous pollutants from the complex mixture of chemical compounds in tobacco smoke, none is expected to remove all unwanted gaseous combustion products. Odorous and toxic organic gases may also evaporate from liquid tobacco smoke particles trapped by the air filter12.

    Manufacturers market HEPA filters to allergy and asthma patients.

    Experimental data and theoretical predictions indicate that medium-efficiency air filters, MERV between 7 and 13, are likely to be almost as effective as true HEPA filters in reducing the concentrations of most indoor particles linked to health effects.

    17 Available data indicate that even for very small particles, HEPA filters are not necessarily the preferred option. For these small particles, relatively large decreases in indoor concentrations (around 80 percent) are attainable with medium filter efficiency (such as a MERV of 13).

    Increasing filter efficiency above a MERV of 13 results in only modest predicted decreases in indoor concentrations of these particles.*

    Predicted reductions in indoor concentrations of cat and dust mite allergens carried on small particles vary from 20 percent with a MERV 7 filter to 60 percent using a HEPA filter.

    Increasing filter efficiency above a MERV of 11 does not significantly reduce predicted indoor concentrations of animal dander. Medium-efficiency air filters are generally less expensive than HEPA filters and allow quieter HVAC fan operation and higher airflow rates than HEPA filters because they have less airflow resistance.

    Pleated filters 1 to 2 inches thick that have a MERV of 12 are available for use in homes and may often be installed without modifying residential HVAC systems; however, manufacturer’s information should be checked prior to installation.

    http://www.epa.gov/iaq/pubs/residair...eir_Usefulness
    .

  9. #9
    Join Date
    Aug 2003
    Posts
    906
    MERV ratings present you with a choice of filtering efficiency vs cost and load on the furnace fan. If the highest MERV filters had the same cost and same resistance to airflow as the lowest MERV filters, there would be no reason to use the lower MERV filters. Aspects of the choice are how comfortable vs thrifty you are, your health, or hypochondriac (paranoid) you are, vs your environment and what you know of it.

    The % reduction quoted are guesses from some model house, with many assumptions. Real conditions can deviate greatly. Also BE is right that it depends on what you're exposed to and what you're sensitive to.

    Discussion of HEPA vs MERV is meaningless if you don't consider the different flow rates in cfm. HEPA filters are usually installed in bypass mode because they need a separate motor and they have a low cfm, which results in a much lower benefit than expected just from reading the efficiency of the filter alone. A lower MERV rating can be compensated by a higher cfm, although at some point you end up living in a virtual tornado inside a sealed room.

    On top of that, you need to consider the sources of particles or dirty air, and how much you need the particle counts reduced. Submicron particles penetrate deep into the lungs and beyond, and can damage mitochondria, regardless of allergic reactions. If you live near a busy highway or smoggy city where there are lots of submicron diesel and tire particles and such, you may benefit more from a higher MERV filter than someone living in the country. High PM2.5 measurements by the EPA monitoring stations in your area could suggest that a higher MERV filter may be a good idea.

    Duct configuration (arrangement of supplies and returns) and pressurization also have a large effect.

    In addition, you can't control what you don't measure, so the average consumer pretty much has to guess and try things out more or less blind. There are no standard and practical tests to tell you how much cell damage you are personally getting from submicron particles, or how much benefit you would personally get from a X% reduction in particles of size Y. Personally, all other things being equal, I think MERV 11 filters are cheap enough that everyone who can afford them comfortably should use them, if even just as a courtesy to sensitive visitors or as a precaution.

    I find it fascinating to see how particle counts go crazy with what are probably carcinogens when we cook and burn something I use oversized MERV 13 filters and monitor particle counts, and I believe that they're worth it for my situation.
    -If you won't turn it on then nothing else matters.

  10. #10
    Join Date
    Oct 2009
    Location
    Murfreesboro, TN
    Posts
    108
    Let's not forget the loss of filtration efficiency due to air by-pass around any filter that is not sealed into the frame. All filters will be de-rated at least one MERV rating or more depending on the velocity and filter rating for that specific installation. Another consideration is the total cost to own a filter.

    The cost of energy to draw air through a furnace filter usually exceeds what the filter cost at the store. Some filter manufacturers don’t even tell you how much resistance their filter have when compared to similarly rated filters. The typical life-cycle cost of a filter system breaks down like this: 18% is the initial filter price and the installation. The energy consumption during the life of the filter equals 81% and the final disposal of the filter is valued at 1%.

  11. #11

    how do you determine when your merv needs changing?

    We bought a Healthy Climate air cleaning system from Lennox last year and are very pleased with it.

    But we don't know how to tell when we should change the MERV 16 filter.

    Also, we are considering buying a replacement filter from an online source, which seems to be a credible outlet, and the pictures of the 'genuine' Lennox products look legitimate. But their filters cost about half what our local Lennox dealer charges.

    So do you think we can trust the filters? They are from iaqsource.com

    Thank you so much. We really enjoy this site.

  12. #12
    We installed a MERV 16 to improve the quality of air that we breathe. We live in a very polluted city, and we are aware that indoor air quality is a problem in its own right.

    We never expected the filter to be a miraculous house-cleaning tool, but there is NO DUST in our home anymore. So I feel great about the job it's doing.

  13. #13
    Join Date
    Aug 2003
    Posts
    906
    Quote Originally Posted by sylkron View Post
    We bought a Healthy Climate air cleaning system from Lennox last year and are very pleased with it.

    But we don't know how to tell when we should change the MERV 16 filter.

    Also, we are considering buying a replacement filter from an online source, which seems to be a credible outlet, and the pictures of the 'genuine' Lennox products look legitimate. But their filters cost about half what our local Lennox dealer charges.

    So do you think we can trust the filters? They are from iaqsource.com

    Thank you so much. We really enjoy this site.
    It's normal for online purchases to be much cheaper. As for when, there is no perfect answer, just a continuum of more or less optimal points. Time based is simple. The GeneralAire G99 Air Filter Gage is a little better. Some people look at the filter and "how dirty" it looks. The system should be noisier too with a dirty filter, so that's another indication. Smell and dead insects is another one. Some people install a pressure switch that turns on a light. When I had an Aprilaire 5000, slightly dirty filters made me ill. Even if you have a precise gauge, it just lets you pick a precise point in that continuum -- it doesn't locate a sharp optimum point. Waiting longer costs more in electricity and wear, and later there's a chance of failure. Changing too often also costs more (obviously) so somewhere in the middle is a shallow optimum.
    -If you won't turn it on then nothing else matters.

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