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Filters are used in:
- Vacume Cleaners
- HVAC filters
- dust masks
- smoke hoods
- Auto air filters

This page is primarily about Heating Ventilating and Air Conditioning (HVAC) filters


HVAC Filters
Air filters serve several purposes:
  • Protecting the motors, fans and coils of HVAC systems from getting a buildup of dust, dirt and debris
  • Keeping your house clean by removing dust and lint particles
  • Protecting your health my removing allergens and some micro organisms

There several kinds of HVAC filters.

  • Fiberglass filters - Trap larger particels including some dust. They are cheap, but be replaced monthly.
  • Electrostatic Filters - Use static electricity to remove particles from the air. The front of the filter charges any particles with static electricity. Then, as the particles move through the they are attract and stick to the rear layers.
  • They are equivalent to a MERV 4-9, but claim to be better at removing allergens. They cost from $150 - $450, so are cheaper in the long run, but need to be washed every month or so.
  • Pleated filters - Made of a polyester fabrics or cotton with MERV ratings typically from 8 - 16. They gain efficiency because they have an increase in surface area.
  • UV Filters - Use ultraviolet lights to kill bacteria and viruses in your home.
Different Types of HVAC Filters Explained | Conditioned Air
Location:
Filters may be either
1. Behind the return duct grills in the ceiling or on a wall in the house. Usually a 1 inch thick filter.
or 2. Where the main return duct connects to the HVAC system in the basement or attic. These filters can be anywhere from 1 to 5 inches thick.








MERV Rating (Minimum Efficiency Reporting Value)

MERV is the primary rating system used in the industry, both domestically and internationally. Established by the American Society of Heating, Refrigerating, and Air Conditioning Engineers, MERV rates a filter’s ability to capture and hold particles and pollutants.
8 = Standard; 11 = Superior; 13 = Best
The company which installed my Lenox system recommends 11, but says you can use 16 if you want to filter many viruses.
MERV
Value
Av. Particle Size Efficiency Types of things trapped
0.3 - 1 Micron 1 - 3 Micron 3 - 10 Micron
MERV 1 - - Less than 20% Pollen, dust mites, standing dust, spray paint dust, carpet fiber
MERV 2 - - Less than 20%
MERV 3 - - Less than 20%
MERV 4 - - Less than 20%
MERV 5 - - 20% - 34% Mold spores, hair spray, fabric protector, cement dust, animal dander, and dust mite droppings (highly allergenic)
MERV 6 - - 35% - 49%
MERV 7 - - 50% - 69%
MERV 8 - - 70% - 85%
MERV 9 - Less than 50% 85% or better Good for homes with occupants who have COPD, allergies, astma and other respiratory conditions. Humidifier dust, lead dust, auto emissions, milled flour.
Almost as effective as HEPA filers
MERV 10 - 50% - 64% 85% or better
MERV 11 - 65% - 79% 85% or better
MERV 12 - 80% - 89% 90% or better
MERV 13 Less than 75% 90% or better 90% or better Viruses, bacteria, most tobacco smoke, proplet Nuceli (sneeze) and carbon dust. Used in commercial manufacturing operations. Bacteria,
MERV 14 75% - 84% 90% or better 90% or better
MERV 15 85% - 94% 90% or better 90% or better
MERV 16 95% or better 90% or better 90% or better
Source: What does MERV Rating mean | CoolRay.com

According to Sun Kool AC, Inc. a residential system will not accommodate filters with MERV ratngs above 12 without modifications.
The company which installed my Lenox system said that is not true.

True HEPA filters with a MERV between 17 and 19 are defined by the Institute of Environmental Sciences and Technology. (IEST) method as having a minimum efficiency between 99.97% and 99.999 % in removing 0.3 µm particles.
A MERV of 20 is rated for 0.1 - 0.2 µm particles.
Source: HVAC-Talk.com


Particle size:
Dust: 1-12 micron
Bacteria: 0.2 to 20 micron
Smoke: 0.01 - 1 micron
Virus: 0.02 to 0.4 micron [rhinovirus (cold) = 0.02 µm, Flu = 0.1 µm]
µm = micron
See Size in reference.

MPR (Micro-Particle Performance Rating) - A rating system developed by 3M (sold at Lowes). It rates the manufacturer’s filters and their ability to capture airborne particles smaller than 1 micron

FPR (Filter Performance Rating) - A rating system developed by The Home Depot for brands sold through their stores, including Honeywell. It utilizes a color and number scale from 4 to 10 that closely resembles MERV rating

MERV MPR FPR Efficiency Frequency
6 300 n/a Captures dust and pollen 30 Days
8 600 5/Green Dust, pollen, dust mites, lint 90 Days
11 1000 7/ Red MERV 8 + mold spores, bacteria, smoke, pet dander 90 Days
13 1900 10/Blue MERV 11 + virus carriers and odors 90 Days
Source: How Often Should I Change The Air Filters In My Home? | AADC
The replacement frequency depends on the size of a filer.
The 5 inch 1500 MPR 3M filters sold a Lowes say 12 months.
The 1 inch 1500 MPR 3M filters say 3 months.

Replacement:
Fiberglass filters need to be replaced every 30 days.

Recommended furnace filters replacement is as follows: MERV 8 every 3-4 months, MERV 10 and 11 every 6 months, MERV 16 every year. If you have pets it is recommended that you check your filter monthly because having pets tends to send more particulates into the air. Source: MillersHeating.com
Conditions Filter thickness
1 - 2 in 4 in 5 in
Baseline 3mos 6 mos 12 mos
Animals 1/month 2 mos 3 mos
1 animal 2 mos 4 mos 6 mos
Dusty 1/month 2 mos 3 mos
Moderate dust 2 mos 4 mos 6 mos
Always leave fan on 1/month
Occasionally leave fan on 2 mos.
How Often Should You Replace Your Furnace Filters? | Newport News, VA


Technical Specifications - Flow Restriction - Dependency on :
Honeywell FC100A1029 - 16" X 25" r Static Pressure Drop (in. w.c.) - 0.23 at 500 FPM

FC100A1003 - Honeywell FC100A1003 - 16" X 20" Media Air Filter MERV 11 (at 492 FPM)


Replacement Size:
Not all filters are the same actual size.
20x25x5 Air Filters | AC and Furnace Filters at Filter Buy "Confused with all the choices of 20x25x5 filters? You’re not alone. Even though these filters carry the same nominal size, they range from only 4.25” thick to over 5.25” thick. The difference in these sizes is just enough to make them unique and not interchangeable.

Unfortunately, the fact that they are not interchangeable is not a coincidence. The major HVAC brand manufacturers would prefer you to purchase their (overpriced) OEM replacements for your unit. Given there are so many different varieties, no local box store can stock them all."

My Honeywell MicroDefense CF508A1625/E 16x25x5 was actually
15 3/4 x 24 3/4 x 4 3/8

A Lennox OEM Merv 11
15-3/4" X 24-3/4" X 4-3/8"

A replacement 3M Filtrete 1550 MPR Ultra Allergen at Lowes was.
15 3/4 x 24 1/8 x 4 7/8 with 1/4" foam at the top and bottom, so it could fill a 16 1/4' opening..
It said it was designed to fit Honeywell and Lenox.
It was too wide to fit in my HVAC system. I had a new Lenox unit but the duct work still included the old Honeywell filter slot designed for for an electrostatic filter.

At FilterBuy.com they say.
"Even though these filters carry the same nominal size, they range from only 4.25” thick to over 5.25” thick.
Unfortunately, the fact that they are not interchangeable is not a coincidence. The major HVAC brand manufacturers would prefer you to purchase their (overpriced) OEM replacements for your unit. Given there are so many different varieties, no local box store can stock them all."
FilterBuy.com has a large selection of replacements.

Filter at Lowes (click to enlarge)


Links:
Air Filter Particle Size chart
National Air Filtration Association (NAFA) User's Guide
Filter Effeciency
Quick Reference Comparison of ASHRAE Std. 52.2 and Std. 52.1
A Guide to Understanding HVAC Filter Selection
Ultra-web technology

last updated 13 Mar 2008