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Photograph of a filthy air conditioning air handler filter Missing or Wrong Sized Air Filters

  • MISSING / LEAKY AIR FILTERS - CONTENTS: 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
  • POST a QUESTION or READ FAQs about air 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:

Tthis 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.



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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 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

Photograph of a home made air filter. 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:

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

Photograph of a missing air filter opening.Lost or missing HVAC system air filters mean increased system operating costs, energy wastage, and ultimately increased service and repair costs.

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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