Photograph of  a low MERV filterGasketed Air Filters, Dirt in HVAC Systems,
Washable Air Filters

  • GASKETED AIR FILTERS - CONTENTS: Benefits of gasketed air filters on HVAC systems. Significance of dirt and debris in the return air plenum or ducts. Washable air filters for HVAC systems
  • POST a QUESTION or READ FAQs about using gasketed & washable air filters and their effects of dirt & debris in HVAC duct systems.
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Gasketed air conditioner or heating filters, leaky air filters, washable air conditioning or heater air filters: This article explains and explain leaky air filters and the value of gasketed air filters, followed by comments on the effectiveness of washable air filters. This website 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. In these articles we are referring to filters installed on central air conditioning or central heating systems that move air through air handlers and duct systems.

Standalone or portable "air cleaners" or "air purifiers" are generally ineffective in buildings. We include photographs to assist readers in recognizing cooling system defects.

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The Benefits of Gasketed Air Filters on HVAC Systems

Photograph of a defective HVAC air filter

The page top photograph is of a low-MERV HVAC filter in an air handler.

A gasketed heating or air conditioning system air filter improves the filter effectiveness. The benefits of a high efficiency air filter material when used in an air handler and duct system are not fully realized if air bypasses the filter around its installation frame due to poor fit or poor installation.

[The filter shown here is not a high quality gasket-type air filter.] That's why some manufacturers emphasize the value of air-sealing gaskets on their products.

Simple observations made in the field will make for agreement with the manufacturers that filter bypass is an issue at some heating or air conditioning installations. When changing an air filter at a return air inlet or at an air handler, when the old filter has been removed, take a look inside the return duct at the return register, or inside the return plenum at the air handler, to see just how much debris is passing the filter.

[The photograph above shows how a badly-fit HVAC air filter can not only leak bypass air into the blower compartment but also it can collapse right into the blower fan - this is a potential fire hazard!]

Implications of dust and debris in the return air plenum

Photograph of dirt collecting in a return air plenum

On an old heating or air conditioning system where filters have been ineffective, not properly installed, leaky, or not maintained and changed on schedule, we often find quite a bit of debris entering the air handler and duct system.

This is in fact so common that most HVAC installers and technicians consider dirty ducts to be "normal". And they're quite correct: typical house dust that has collected inside of a duct system is not normally a toxic substance.


1) a high level of dust and debris inside of an air handler or duct system indicates that system filtration has not been effective,

2) the dust level indicates that the system has been delivering dust and debris to the occupants providing poorly-filtered air, and

3) there is an increased risk of mold in the duct or air handler system if moisture enters the system from a leak or from air conditioning condensate mishandling.

How Good are Washable air filters for heating and air conditioning systems

Washable re-usable air filters are very appealing from a cost viewpoint. I would look for data to indicate whether or not the washable filter runs at a lower MERV or filtering efficiency.

Photograph of dirt collecting in a return air plenum Room air conditioner filter cleanout (C) Daniel Friedman

In our cascaded filter proposal (discussed at this website) we included and continued to use a washable filter that came with the equipment since it was not reducing the airflow CFM, but we preceded it with other filters for different purposes.

Inserting a cleaned washable air filter into the air conditioner unit (C) Daniel Friedman

The photograph at above left shows a typical washable metal air filter used in central air conditioning systems, and at above right is a washable foam air conditioning filter used in a portable room air conditioner. Other types of washable air filters are available as well.

At left we illustrate a plastic and nylon washable air filter used in both window air conditioners and in wall-mounted split system air conditioner units.

Unlike the foam type air filter shown above, these plastic and nylon air filters withstand repeated cleaning without disintegrating.

The efficiency of these filters increases as debris begins to settle on the filter surface, but eventually the improvement in air filtration efficiency is offset by reduced airflow - so clean the air filters in accordance with the cooling unit manufacturer's instructions - typically monthly during constant use.

Also see Spit System Air Conditioner Air Filter Maintenance - How to Clean a Split System Air Conditioner Filter.

A metallic air filter (shown ) is not very efficient (effective) at trapping airborne particles when it is clean, though efficiency improves as it gets dirty. Other pleated, fabric or paper air filters that are also washable are more efficient at trapping airborne debris.

Be sure to read PARTICLE SIZES & IAQ for a description of the sizes and behaviors of some of the common particles that are an indoor air quality concern.


Continue reading at MISSING / LEAKY AIR FILTERS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


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