Missing or Wrong Sized Air Filters
on Heating Furnaces or Air Conditioning Systems

What are the effects on heating or air conditioning systems or on indoor air quality when the filter is missing, the wrong size, or not maintained?
- Air Conditioning & Warm Air Heating Operating Cost Savings by Proper Air Filtration
Questions & answers aboutair filter mistakes: wrong size, wrong fit, leaks, bad air filtration, dirty ducts, extra expense
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The cost & trouble caused by missing or leaky air filters on HVAC systems: this article explains the effects of operating a heating or air conditioning system without proper air filters in place. 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. We include photographs to assist readers in recognizing cooling system defects.

Improperly-Fit or Wrong Sized Air Filters Cause Bypass Leakage

Leaky HVAC system air filters mean increased system operating costs, energy wastage, and ultimately increased service and repair costs.

Photograph of a dirty air conditioning filter

Bending over the end of an air conditioner or heating air handler filter such as shown in the photo at left above is a bad idea. If the filter does not fit there will be bypass leakage past the filter, soiling the blower fan, slowing air flow, and leading to more costly cleaning and service later.

Furthermore when you bend the filter as this owner did, you interrupt the structural integrity of the filter's frame, risking filter collapse. A collapsed air filter can be drawn right into the blower fan, causing damage to the fan motor or even leading to a fire!

The photo at right shows how a college HVAC maintenance crew kept the A/C system running when the school did not have the proper filter size in stock. This filter installation also will have severe bypass leakage around the filter where the pleated section contacts the edges of the filter slot.

Installing a filter that is the wrong size for the heating or air conditioning air handler defeats the purpose of air filters because of leakage and it may be unsafe. Install a properly-sized filter in locations like this as soon as possible and watch out for unsafe filter collapsing.

How to Construct or Obtain Large or Special-dimension HVAC System Air Filters

The same college HVAC maintenance staff who was struggling with improperly-fit air filters we discussed earlier was also faced with the task of coming up with a much larger air filter for the air conditioner air handler over their computer center. The neatly-taped "built-up" air filter shown in this photo was nicely constructed but we don't recommend this practice:

The filter may come apart and send fragments into the blower assembly, damaging the blower or leading to overheating and a fire.
The filter is not delivering the total cross-section of filtering area that was anticipated by the HVAC design engineer who specified the dimensions of the return air plenum at which this filter was installed - we're seeing less total CFM of airflow.

Air filter suppliers and manufacturers have no trouble providing air filters of special dimensions. Furthermore if the filter is built by a manufacturer it's more likely that they'll understand the structural and strength requirements of the filter as well as the required airflow characteristics and filtering ability. We list some suppliers of air filters at SOURCES FOR AIR FILTERS

Incidentally, except unusual cases with special requirements, wouldn't it have made sense for the HVAC or duct system designer to have specified a filter that is one of the many standard sizes?

Missing Air Conditioner Filters: effect on HVAC operations

Lost or missing HVAC system air filters mean increased system operating costs, energy wastage, and ultimately increased service and repair costs.

Photograph of a missing air filter opening.

Look closely at this photograph. On the right we can see a tan "Air Filter Cover" plate which marks the intended location of the HVAC air filter. But there is an open slot to the left of the air filter cover, possibly where another filter was previously being installed. When the new air filter slot was constructed and nicely covered (so as not to leak) the old slot was simply left open.

You can see my piece of adhesive tape bending into the opening, demonstrating (not too scientifically) that there was airflow into the unit from this location. This is a great way to draw attic insulation fiberglass into the air handler and to blow it into the living area. And of course any other unwanted attic dust and debris is also being invited into the air handling system and blown into the occupied space.

Failure to properly filter dust from the return air supply will load the fan and cooling coil, dirty the duct system, and lead to the problems listed above. As the ductwork debris level increases you increase the risk of forming an allergen or mold reservoir, especially if there are water or condensate leaks into the duct system interior.

If an air filter is not present on your air conditioning or warm air heating system, have one installed. Installing a filter is normally a minor expense. Duct cleaning or duct replacement can be a significant expense. Cleaning up a moldy HVAC system, where mold may have been caused by coil icing which was caused by a dirty coil or filter is still more costly.

Air Conditioning & Warm Air Heating Operating Cost Savings by Proper Air Filtration

Should we Filter or Clean Air Before it Enters the Air Handler Unit?

If we clean the air before it enters the air handling unit will that reduce the workload and hence the electric consumption of the air handling unit fan , and if so by how much ? In addition I would like to ask where else can there be savings in electricity. Is there a study made by ASHRAE or someone else that I can buy ? - Ziad I. Ghandour

Reply: Proper Air Filtration Saves Energy and Reduces HVAC System Operating & Service Costs

Cleaning the air before it enters the air handler, if you mean by using an air filter upstream of the air handler unit (AHU) is good practice. In the short run, airborne dust and debris don't have much impact on the blower assembly itself. But over time the following problems occur:

Dust collects on the squirrel cage blower assembly, filling in fan blade curvature and significantly reducing the air output of the system

Dust collects on the evaporator coil, ultimately blocking it, reducing air flow, and often causing coil icing

Those conditions increase the operating cost of the system and can eventually lead to costly service calls These are practical HVAC maintenance conditions, not something for which we have seen a peer-reviewed ASHRAE study.

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Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

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Wikipedia provided background information about the definition of HEPA and airborne particle interception.
Thanks to reader Ziad I. Ghandour for discussing air filtering benefits and HVAC operating cost - August 2010
Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson, John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages
Air Conditioning SEER - New DOE Air Conditioner and Heat Pump Efficiency Standard
Asbestos HVAC Ducts and Flues field identification photos and guide
Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation

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