Air filter efficiency or air filter effectiveness: definition of measurements of air filter effectiveness:
This article explains and explain various measurements of the effectiveness of different types of air filters for HVAC systems. We explain how to use air filters on HVAC systems, how to reduce indoor airborne dust, debris, pollen, even mold spores or allergens.
We explain and define MERV, HEPA and how to use these air filter ratings when choosing and installing air filters in an air conditioning or heating system. We also discuss the problem of reduced air flow or strangled or air starved air duct systems from over-filtration and how to correct that problem.
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
- Daniel Friedman, Editor/Publisher - See WHO ARE WE?
Discussed here: How to Assess the Effectiveness or Efficiency of Air Filters for Heating and Air Conditioning Systems.
Other Steps to Reduce Indoor Airborne Dust & Debris. Air Filter Design Basics: MERV, HEPA, How Air Filters Work. Definition of Air Filter MERV and what MERV means when buying an air filter.
What are typical air filter MERV Rating Values?. Definition of HEPA Filter & HEPA. Should You Install a HEPA or High MERV Filter on my Air Conditioner or Furnace?
Is There Such a Thing as Too-Much Air Filtration? Air-starved or "strangled" air ducts.
Current air filter MERV ratings and air filtration suggestions useful during the COVID-19 pandemic were summarized by ASHRAE experts in December 2020, from which we excerpt here:
Along with the basic need for good IAQ for the good of the people breathing the air and any sensitive equipment in the indoor space, there are many reasons that are extremely relevant in 2020.
a. COVID-19
With the understanding that COVID-19-containing particles can stay in the air long enough to move into the breathing space and into HVAC systems, the desire to filter these particles out of the air is strong.
ASHRAE is recommending that MERV 13 filters be used where possible, with MERV-A 13-A or MERV 14 preferred.
This is pushing the use of higher efficiency filters in many buildings.
For locations that can’t upgrade the HVAC filter or increase outdoor air during winter, in-room units are recommended to add cleaning capability.
These units should include high MERV or better filters to remove most of these virus-laden particles.
b. Wildfires
With global warming, the wildfires seen on the West Coast are likely to be a continuing major problem. Air filters are needed to clean the air to help people breathe in fewer particles. Filters are also often used to help people with allergies and asthma.
c. Energy Costs, Especially with Global Warming
Using appropriate filters can lower the need for bringing in outdoor air. This lowers the costs for heating, cooling and conditioning outdoor air.
The filter used and its pressure drop will be important in determining how much energy savings can be achieved.
Watch out: in the same article the authors noted that just installing improved air filtration will not improve indoor air quality if the building air is not moved through the filter system. That means running the air handler in heating, cooling, or circulation modes.
For residential buildings occupants may want to simply run their air handler fan continuously for part or all of the day by making use of the FAN ON controls available either on the thermostat or at the fan limit control right in the air handler.
If you're not sure how to do that see
FAN ON AUTO MAN Thermostat Switch
Source: Kathleen Owen, Fellow ASHRAE, member of the ASHRAE Epidemic Task Force, and Carolyn (Gemma) Kerr, Ph.D., Life Member ASHRAE., Debunking Myths About MERV, Air Filtration [PDF], ASHRAE Journal Newsletter, 8 December 2020, original source: https://www.ashrae.org/news/ashraejournal/debunking-myths-about-merv-air-filtration
The air filter cascade system installation in our lab (photo at left) provided an opportunity for careful testing of its effect on indoor air quality in the lab, since unlike most building owners, we have an extensive set of indoor air quality monitoring equipment at hand for our IAQ and environmental work.
Details are provided at OPTIMUM INDOOR AIR FILTERS.
This cascade air filter heating system uses four levels of air filtration or cleaning:
1. a low-cost 1-inch fiberglass or pleated paper air filter at the return air inlet grille (shown at left and represented by arrow #1 in the photo above) - this filter at the front end of the return air flow collects the most dust and debris and is replaced monthly during the heating season
2. A washable metallic air filter - extraneous but included with the system and retained as airflow was not a problem
3. An electrostatic air cleaner - a removable washable element is cleaned every year, more often if inspection shows that's needed.
4. A high efficiency HEPA-rated air filter - because this is the most costly air filter to replace, it was located after the washable electrostatic air cleaner - reducing the frequency with which this filter is replaced to about once a year
The airborne dust levels of all particle sizes dropped dramatically when the new system was installed (and the existing ducts cleaned as part of the project). Airborne particles in the 1-20 micron range were reduced by about 70%.
Simple subjective visual inspection of lab surfaces showed that dust accumulated at a much slower rate. The system was so successful that we made use of the air handler blower assembly and filters to control indoor dust year round.
Fix the air quality problem at its source: Properly installed air filtration can definitely reduce the level of airborne dust in buildings, as we have tested and measured in our lab and in other buildings.
But air filtration should not be relied on as a "cure" for sick buildings or for mold contamination.
Rather, it is more important to find and remove the problem source and to correct the conditions that caused the problem in the first place.
For mold concerns, see ACTION GUIDE - WHAT TO DO ABOUT INDOOR MOLD.
The US EPA takes a similar view:
However, air cleaners may not reduce adverse health effects completely in sensitive population such as children, the elderly, and people with asthma and allergies. For example, the evidence is weak that air cleaning devices are effective in reducing asthma symptoms associated with small particles that remain in the air, such as those from some airborne cat dander and dust mite allergens.
Larger particles, which may contain allergens, settle rapidly before they can be removed by filtration, so effective allergen control measures require washing sheets weekly, frequent vacuuming of carpets and furniture, and dusting and cleaning of hard surfaces.
There are no studies to date linking gas-phase filtration, UVGI, and PCO systems in homes to reduced health symptoms in sensitive populations.
That said, good air filtration on central air handling systems combined with our "fix the problem" advice above, can improve indoor air quality significantly.
MERV
is an acronym for "Minimum Efficiency Reporting Value".
The MERV rating on an air filter describes its efficiency as a means of reducing the level of 0.3 to 10 micron-sized particles in air which passes through the filter. Higher "MERV" means higher filter efficiency. The purpose of the MERV standard is to permit an "apples to apples" comparison of the filtering efficiency of various air filters.
Don't expect the thin washable plastic air filters used on window or split system or portable air conditioners (shown at left) to operate with the same efficiency as more sophisticated filters.
Al Veeck, executive director of the National Air Filter Association has pointed out that MERV includes a broad range of small airborne particles described in 3 ranges:
An air filter that is effective within the range of 0.3u to 10u will collect even the smallest mold spores (such as Aspergillus sp. & Penicillium sp. and larger particles such as pollen, and upwards to fiberglass insulation fragments.
Air filter efficiency
refers to the relative ability of a filter to remove particles of a given size or size range from air passing through the filter. If a filter were 100% efficient, none of the particles in a given size range would escape the filter and air which has passed through such a filter would contain zero particles.
Also see AIR FLOW RATE TYPICAL CFM Specifications for HVAC equipment.
MERV "Ratings":
Mr. Veeck also points out that MERV is a reporting value, not a filter rating value, although we find frequent referrals to "MERV Ratings" in various publications, including a Wikipedia entry on this topic.
The MERV Efficiency Rating [reporting] Scale
ranges from 1 to 16, with 1 being the lowest efficiency and 16 describing the highest efficiency. The particle size range addressed by the MERV scale is 3 to 10 microns. A logical inference is that if an air filter is removing particles down to 0.3 - 10 microns, it is certainly also at least that efficient at removing larger sized particles.
A very low-cost disposable fiberglass furnace filter may have a MERV rating of 1 to 4 - pretty low.
We provide examples of specific air filters and their efficiency
at Air Filter Efficiency, a section of our discussion
of AIR FILTERS, OPTIMUM INDOOR
A medium efficiency MERV pleated air filter
in the range of MERV 5-13 is, according to the US EPA, reasonably efficient at removing small to large airborne particles.
EPA also opines that: "Filters with a MERV 7-13 are likely to be nearly as effective as true HEPA filters at controlling most airborne indoor particles." We can't explain the overlap of these two statements except to speculate that the EPA writer was thinking that the MERV of an air filter is not constant - particle filtration improves as the filter gets dirtier.
EPA adds that medium efficiency air filters are generally less expensive than HEPA filters, and allow quieter HVAC fan operation and higher airflow rates than HEPA filters since they have less airflow resistance.
A high efficiency high MERV pleated air filter
in the MERV 13-16 range would be expected to remove smaller particles between 0.3u and 1.0u in size at about a 75% efficiency.
But before getting too excited about 75% looking not as good at 99.97% (see HEPA filtration discussed below) remember that in a heating or air conditioning system air is being recirculated through the filter system. So if each pass is removing 75% of the particles we want to capture, over time the heating or air conditioning filter system will do a good job of removing a very large amount of airborne dust.
This is exactly what we saw when we tested some filtering approaches in our forensic laboratory. The longer we ran the air handler fan the more dramatically we saw the airborne dust level drop in the test area.
The US EPA points out that higher efficiency filters with a MERV of 14 to 16, sometimes misidentified as HEPA filters, are similar in appearance to true HEPA filters, which have MERV values of 17 to 20. As you'll read below, you may not want to rush to install a HEPA filter on your central air conditioner or heating system.
The Definition of A "HEPA" rated air filter:
a HEPA filter (High Efficiency Particulate Air [Filter] or better put, High Efficiency Particulate Arresting [Filter])
has to meet more demanding US DOE standards than a high MERV air filter.
True HEPA rated air filters have a MERV of 17-20.
A HEPA filter is required to remove at least 99.97% of airborne particles down to 0.3u in size (in diameter or in their longest dimension). Any filter, high MERV or HEPA, can be expected to filter out larger particles with an efficiency above its rating.
So do we want to use a high MERV air filter or do we need to install a HEPA filter On our air conditioner or heating system air handler?
A high MERV air filter is much less costly than a HEPA filter and can be more effective over time in a residential or office air conditioning or heating system, but there are two warnings that have to be considered:
Smaller airborne particles
than those covered by the HEPA or MERV rating might be filtered out with still
higher efficiency.
Their smaller mass means they may "stick" to the filter surface well but some filters, such as the budget lowest-cost fiberglass furnace filters, will pass these small filters right through when they are new, but filter out more of them as the filter surface becomes clogged and dirty with debris (leaving smaller and smaller openings through which air must pass.
We have a HEPA filter on both our downstairs and upstairs units. When we had a high efficiency Rheem furnace put in this summer, the technician told me that since I have HEPA filters, I don't needs filters on the return air vents. That it suffocates the system. Can you confirm this? - L.F.
A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem. That said, here are some things to consider:
So in sum what you were told is reasonable, but it was not the whole story.
This article series answers almost any question you might ask about air filters for heating or air conditioning systems. We explain how an air conditioning service technician will diagnose certain common air conditioning system failures or defects.
In these articles we are referring to filters installed on central air conditioning or central heating systems that move air through air handlers and duct systems. Standalone "air cleaners" are generally ineffective in buildings.
We include photographs to assist readers in recognizing cooling system defects.
CONTSACT us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. [The page top photograph is of a low-MERV HVAC filter in an air handler.]
...
InspectAPedia is the creation of Daniel Friedman, based on over 50 years of experience in building construction & inspection, building mechanical systems, indoor environmental hazards, and forensic investigation. InspectApedia also includes contributions from other experts and technical reviewers.
Daniel holds memberships in many professional organizations, has authored countless publications, and has presented classes and seminars on many related topics. For more details of Daniel’s experience, expertise, and qualifications, please see WHO ARE WE?
Below you will find questions and answers previously posted on this page at its page bottom reader comment box.
On 2016-09-07 by (mod) - by (mod) - oor air filters can lead to the evaporator coil needing cleaned
Bob:
It would be odd to see no dust whatsoever, ever on an air filter unless the indoor air dust level was very low - such as a nice clean rental without carpeting, pets, and so on.
Even a rough filter that doesn't filter out ultra fine particles down in the 1u range will still collect house dust as that includes larger particles, hair, skin cells, fabric fibers, etc.
Inspect the cooling coil: if you see that it's dust-clogged then the filter may have been left off or may be leaky. Air bypass leaks around a filter can clog the coil as can damaged or dirty duct interior or duct insulation.
A leaky air filter can lead to dust and dirt on the evaporator coil - that needs to be cleaned
On 2016-09-07 by Bob C
In a rental with supplied air filters that never collect dust.
Have read that some of the least effective filters can actually lead to dust build up on the evaporator coil as they trap little to no dust at all.
Also have heard that they actually only work well once they start to trap dust, which could be months.
Are there instances where poor air filters can lead to the evaporator coil needing cleaned and or replaced even if the filters don't get dirty?
On 2014-04-03 by (mod) -
Angela, I haven't seen this issue come up before either, but I don't assume the tech is necessarily blowing smoke as you put it.
I may not understand the question correctly but it seems to me that the air filter is an add-on level of filtration beyond what the electrostatic filter is doing. Take a look at our article on cascading air filters to see if that describes your system:
https://InspectAPedia.com/aircond/Best_Air_Filter.php
Give us the brand and model of your electrostatic air filter and we (or you) ought to be able to read its specifications and operating and maintenance instructions - or to obtain them if you've not already got a copy on hand.
On 2014-04-02 by Angela
We have an electrostatic filtration system on our 90% furnace/14seer a/c system. The tech who was doing our a/c tuneup last week told me the replacement filters we purchased online were only 5000V system approved and our system was 7500V.
It sounds to me like he's shooting smoke up my patoot, as I can't find a single bit of information on the great Google about electrostatic filtration voltage ratings.
But in the interest of not frying my brand new system system, is there any validity to his statement or should I find a new HVAC company to work with?
If the statement IS valid, how do I determine the voltage rating on the filters, I can't seem to find it anywhere. draken.krafte@gmail.com
On 2012-11-20 by (mod) -
Zack,
Assuming you mean does it catch more fine particles, yes ... Up to a couple of thresholds.
Eventually the filter becomes so dirty it reduces the airflow capacity of the system,
And / or
Eventually some filters start releasing chunks of particles from their overloaded surfaces.
On 2012-11-18 by Zack
When an air filter is dirty is it working more efficiently?
Thanks
On 2012-08-04 by (mod) - To keep efficiency at its peak, use the most basic air filtration available
Jake, thanks for an interesting comment. But I'm not sure I agree with your thinking - or at least it may be that some other factors need to be considered.
Example: if we allow excessive dust to enter (through a marginal air filter) the duct work system the effect is deposition of dust on the squirrel cage blower fan blades, resulting in up to a 50% loss of air movement in the system.
So while I agree that housekeeping to minimize avoidable dust is a great idea, I must add that even in a clean home there weill be airborne dust and debris - fabric fibers, skin cells, pollen, road dust and debris; And the more we can keep this out of the ductwork and off of the blower fan the more efficient and effective will be the operation of the equipment when it is cooling or heating the building.
Finally, it is a fundamental part of at least some HVAC systems to include filtration specifically designed to improve indoor air quality by reducing the airborne particulate levels. Rather than subverting the system and giving up those benefits, I prefer to see a design that uses a cascade of air filters (appropriately matched to blower and duct capacity).
This allows a high level of filtration while putting a cheap, disposable air filter at the front end of the system to catch most of the debris - thus reducing system operating cost, keeping performance high, and making maintenance easy and cheap.
On 2012-07-31 by Jake
To keep efficiency at its peak, use the most basic air filtration available with the goal of protecting HVAC equipment only, and a good cleaning company to keep the dust down at the source. Unless overrated blowers are installed to deal with resistance in multiple filtering stages, it's false economy to rely on HVAC equipment to keep dust down.
On 2012-07-10 by (mod) -
Bing, your question reviews an important point - we can't install so much filtration that we block the aystem air flow - doing so reduces cooling effectiveness and efficiency. A flat fiberglass filter indeed offers low restriction on airflow (until it gets dirty);
In our lab, faced with this problem, when we were renovating the system and ductowrk, we made room to install a deep, pleated, high efficiency filter.
The pleated design, by giving more surface area to the moving air, can get back the loss of airflow that would otherwise occur through the more dense, high-filtration surface of the more effective filter.
For a still more effective approach take a look at "Cascading Air Filters" under OPTIMUM AIR FILTERS linked articles at upper page left.
On 2012-07-09 by BingBong
We have been using a cheap fiberglass filter for 7 years. Today we had the pleasant AC guy come out to tell us that we need to not only have our blower motor cleaned, but also our coils pulled out and cleaned as well. Cost .. ridiculous.
When we used the high efficiency filters we couldn't enough air through the system. We used the cheapies.. and they cost us by not filtering out the air enough. Of course the house is 12 years old and system has not ever been serviced. So.. I guess instead of paying yearly, we paid all at once.
On 2012-05-08 by Cooltech
We replace more blower motors, because people use wrong filters. Too much filtration will burn up blower motors, as will not changing your air filters as needed. Over filtering causes lack of air flow and can cause heating failures "over heating exchanger" and cooling failures "indoor coil freezing". More is not better.
On 2012-02-01 by cool
how can i measure the effectiveness of my air filter in aircon. im trying to do a study for my research. the one that have calculated results
On 2012-01-26 by georgegrund@msn.com
I caugh in the morning and there is this white dust on everything. Is this Abestos. I know the building has some in it somewhere but I don't know where?
What should I do. I have One Orek air purifir and one hepa and one inoic breeze still this white dust.
On 2011-07-12 by haitham
rich informations.... thanks alot
On 2011-06-22 by Banatu
Wow, just the information I was looking for and a whole lot more. Thanks!!
(July 16, 2015) Daniel F (mod) said:
Filters:
According to Brendon Reid's nice article on this
Filters
If you are really quoting a properly-made HVAC duct system static pressure measurement in inches of water column " wc then your number is a bit shocking. Most residential blowers won't create more than 0.9" wc. Your number sounds high, wrong, and likely to mean that the system is noisy and also return-air starved.
Typically we want to see 0.5 wc static pressure as a general rule of thumb. - see www.achrnews.com/articles/103710-interpreting-duct-static-pressure
- Brendan Reid is President of the Comfort Institute (CI), Bellingham, Wash. CI provides building science training and instruments for whole-house and air-distribution diagnostics and repair. He can be reached at 360-420-5049.
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