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.

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.

This article series answers almost any question you might ask about air filters for heating or air conditioning systems.

Page top photo: this much-too-big air filter at the return air plenum for this furnace/air conditioner means bypass leaks around the air filter that in turn increase system operating cost and risk clogging the blower fan itself.

Worse, if this is a gas furnace the blower fan, drawing air in around this leaky air filter, could interfere with the gas burner's operation and the system could then be unsafe.

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.

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.

Also see DEFINITION of BYPASS AIR

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 above 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.

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.

Effects of a Missing or Leaky Air Filter on an Air Conditioner or Heating Furnace

• Reduced air flow due to dirty air handler blower fan - at DIRTY A/C BLOWERS we explain that a dirty blower fan reduces the blower assembly air flow rate, increasing heating and cooling costs,
  
  See also BLOWER FAN ASSEMBLY CLEANING
• Blocked evaporator coil from dirt and debris or cooling coil, described at DIRTY COOLING COIL / EVAPORATOR COIL
• Dirty HVAC air handler and HVAC duct interiors, possibly contributing to mold growth inside the ductwork or air handler unit.
  
  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.
• Increased heating and cooling costs as a result of reduced air flow due to clogging

Leaving off the air filter entirely in an attic air handler and leaving the filter slot opening will raw 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.

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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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

3-Inch gap at top and sides of air filter. Do you agree that that's not right?

I'm worried about the air filter placement in a new system.

My new AC/Furnace unit has worked great for the last year, but when I went to check the filter (located just before the blower), it's perfectly clean. I believe it isn't catching any dust because there is a 3-inch gap between the top of the filter and the side of the duct where it sits.

This allows dirty air to get sucked around the filter and into the blower. Is that correct? How tightly should the filter fit in its rack? - On 2016-08-26 by cvd6262

Reply by mod: a 3-inch wide air bypass leak at the filter is a serious error

@CVD6262: we agree with you completely. Leaving inches of open gap around an air filter is pretty much equivalent to having no air filter in place at all in an HVAC system.

The results of this mistake include

• Poor IAQ: Failure to filter building air entering the HVAC system, thus lower indoor air quality
• Equipment damage & safety worries: Clogging of the blower fan blades, reducing air flow, increasing operating cost, and in extreme cases, risking dangerous overheating of the heat exchanger<

Luckily it's probably the case that five minutes with a measuring tape and a trip to your local hardware store or building supplier can provide you with an air filter that's the right size.

Do that ASAP and let me know if you need more help.

Oh, and a photo of the air filter system you described could confirm what we're saying and would sure be helpful; Daniel.

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Continue reading at OPTIMUM INDOOR AIR FILTERS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• AIR FILTERS for HVAC SYSTEMS
  • AIR FILTER EFFICIENCY
  • AIR FILTERS, FIBERGLASS PARTICLES
  • AIR FILTER LOCATION
  • INSULATION BLOCKED COOLING COIL
• AIR FILTER OPTIMUM INDOOR
• AIR FLOW IMPROVEMENT, HVAC
• GASKETED AIR FILTERS

Suggested citation for this web page

MISSING / LEAKY AIR FILTERS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING INDOOR AIR QUALITY IAQ

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Citations & References

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