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Furnace Filters

MERV, HEPA, CADR – picking an air filtration system quickly brings you into a confusing world of acronyms and information overload.  We’re going to wade through the data and provide you with actionable methods to improve your indoor air quality.

First, a few definitions


Particulate Matter.  Air quality is often measured in terms of several standard PM sizes.  For example, PM10 and PM2.5 .  The number denotes the size in microns of the diameter of particulates being measured.  A particle counter measuring PM2.5 is counting the number of particles equal or greater than 2.5 microns.  The smallest particles are generally thought to pose the greatest health risk.  Therefore, you will often see local air quality reports displayed in PM2.5 or PM10.

A brief overview of air filters & purifiers

Air flow is king

You can have the best air purifier on the market, but if it doesn’t push enough air through the filter, it isn’t going to improve your indoor air quality.  There’s simply no getting around the need for substantial air flow.  This is where the effectiveness vs. the efficiency of a filter comes in.  The efficiency is simply the measurement of the filters ability to reduce the quantity of pollutants in the air.  For example, a HEPA filter must remove at least 99.97% of particles greater than or equal to 0.3 microns.  The effectiveness of a filter considers all the other factors – air flow rate of the fan, how often it’s running, location with the home, etc.


Portable air purifiers vs. whole house filters


Common Questions

I’m replacing my furnace – what type of filter should I have installed?

We typically recommend a 4 or 5″ pleated air filter.  While other systems such as electrostatic precipitators and oxidizers can be effective, they are far more finicky.  Unless a strict cleaning schedule is maintained, they quickly lose their effectiveness.

What is a HEPA filter?

HEPA is a rating given to filters that meet a certain criteria.  In the U.S a HEPA filter must removed 99.97% of particles equal or greater than 0.3 microns.  Interestingly, this size of particulate is used because it’s the most difficult to remove from the air stream.  Although it’s quite counter-intuitive, the smallest particles are not the hardest to remove.

How does a HEPA filter compare to other filters?

HEPA filters generally equates to a MERV 17 filter.  Most residential furnaces cannot handle anything above a MERV 13.  The restriction of the airflow caused by the filter will place extra stress on the blower motor.  HEPA filter systems require a secondary motor to draw the air through the filter.





Project Report > Poor Filter Maintenance

Work #: 103037

Furnace filters

HVAC Intake Screen Extremely Dirty

General Information

  • Building was recently renovated to be used as a child care facility.
  • Upper level rooms contained open ceilings which were a part of a larger, single air space above the entire upper level.
  • Two HVAC systems were visible in the open ceiling area.

Furnace / HVAC

  • Furnaces noted in upper level appeared to be high efficiency, sealed combustion units.
  • Technician was unable to determine actual ventilation due to inaccessibility of the HVAC units.
  • Visual inspection of the units revealed a piece of screen installed over an intake duct opening.  Screen was caked with dust which appeared to be thick enough to possibly cause airflow restrictions.


  • Recommend having all ducts and HVAC units professionally cleaned.
  • Recommend having all HVAC filters serviced and replaced where necessary.

Upstairs Bathrooms:

  • Ventilation Fan in bathroom is very noisy and may appear to be close to failure.
  • Grill of ventilation fan is extremely dirty and requires cleaning.
  • Strong fecal odor present when entering bathroom.  This appears to have been caused by a dirty diaper left in a garbage can.
  • RH: 67%     Temp: 70 deg F.     CO2 = 824 ppm     CO = 1 ppm


  • Recommend cleaning and/or replace the ventilation fan in bathroom wall.
  • Recommend disposing of dirty diapers into a dedicated sealed disposal bag and empty every night.


  • Strong odor of breaded chicken tenders present due to the inspection being conducted near lunch time.
  • Ventilation hood and fan is present and functional over the stove.  Fan was not in use at the time of inspection.
  • Kitchen was occupied and in use during much of the inspection which limited the inspectors access to many areas.
  • RH: 45%     Temp: 73 deg F.     CO2 = 911 ppm     CO = 1 ppm


  • Recommend conducting a thorough cleaning of the kitchen to minimize any potential locations where offensive odors could be generated.
  • Recommend use of the ventilation hood whenever cooking.
  • Recommend operating the ventilation hood and closing the door overnight to determine if the initial odor present when entering building is coming from the kitchen or not.

 Upstairs – Great Room

  • Walls and ceiling were scanned using a thermal imaging, infrared camera to determine any areas of temperature differential which would indicate moisture intrusion, missing insulation and/or other structural defects.  Several locations were noted on exterior walls where insulation appeared to be minimal or missing.
  • RH: 41%     Temp: 70 deg F.     CO2 = 755 ppm     CO = 0 ppm


  • A previous moisture intrusion event had occurred in the sprinkler room.  Water damaged materials appeared to have been removed and patched up.
  • Slight water stains were noted on the concrete wall behind the washing machine.  This appeared to be from the previous moisture intrusion incident.
  • Threshold to sprinkler room was very spongey and, when stepped on, moisture was squeezed out onto the concrete floor.
  • Elevated moisture levels extended approximately 1-2 feet into play room from threshold.
  • Technician suspects moisture is a result either from the previous water intrusion issue or may be a result of a spill/overflow from the washing machine.
  • Dust levels appeared to be much lower in the base ment area.
  • Walls and ceiling were scanned using a thermal imaging, infrared camera to determine any areas of temperature differential which would indicate moisture intrusion, missing insulation and/or other structural defects. No areas of concern were noted.
  • Technician suspected a third HVAC system behind a closet door.  Unit was not inspected due to the door being locked.
  • RH: 36%     Temp: 70 deg F.     CO2 = 853 ppm     CO = 0 ppm


  • Recommend removal of water damaged floor tiles in front of door to sprinkler room.

Main Level

  • Heavy dust was noted on many unaccessable surfaces (tops of walls, door jambs and ledges)
  • Dust patterns were noted on surfacing paper of attic insulation. Dust was heaviest around cuts for joists and plumbing perferations.
  • Dust was also noted around openings for large roll up doors.
  • Suspect much of the dust is being generated by ongoing road  and building construction projects nearby.
  • RH: 39%     Temp: 70 deg F.     CO2 = 860 ppm     CO = 1 ppm


  • Recommend air sealing around all perferations to the exterior.
  • Recommend regular dusting and servicing of the HVAC filtration unit.

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