Photograph of attic air conditioning air handler, condensate drips on floor Air Handler Diagnosis & Repair
Blower Assembly, Blower Fan, Cooling Coil / Evaporator Coil & Air Filter: inspection, diagnosis, & repair guide

  • AIR HANDLER / BLOWER UNITS - CONTENTS: Air conditioning air handlers - Fan Coil Unit Inspection, Diagnosis, Repair, Replacement - Quick troubleshooting guide for air handling units or blower assemblies in air conditioners, heat pumps, some furnaces. Dirty, contaminated air conditioner blower fans, reduced air output, fan cleaning advice. Air Conditioning air handler unit (AHU) leaks. Causes & Photographs of Mold Growth in Air Conditioning Blower Fans. How to Keep the HVAC Blower Assembly Clean
  • POST a QUESTION or READ FAQs about how to diagnose & repair problems with the air handler or blower unit / fan assembly in a warm air furnace or air conditioning system

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Indoor air handler or blower unit for A/C or heat pump or furnace systems: how to inspect & repair the indoor components of air conditioners and heat pumps: the air conditioning system indoor air handler units, blower units, or AHU's, (also called fan coil units) including the air conditioner blower fan, the indoor cooling coil or "evaporator coil", air conditioning system filters, and the causes, cures, and prevention of air handler leaks that lead to rust, damage, and mold contamination in the air conditioning system.

Air Conditioning Blower Fan, Cooling Coil, Fan Coil Unit Leaks, Mold, and Filter Problems &amp: Repairs.

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Air Handling Unit Troubleshooting - Quick Checks to fix no air flow or weak air flow

Photograph of toxic mold growth on a squirrel cage air conditioner blower fan unit[Click to enlarge any image]

Article Series Contents

Basic air handler unit or blower unit troubleshooting checks

Here we provide air handler unit or blower assembly troubleshooting by expanded annotated information from the US EPA [5]who provided suggestions for investigating the air handling unit during an indoor air quality investigation.

  • Is the system turned on? Check thermostat to see that it is calling for heat or cooling, service switches at the equipment, check fuses or circuit breakers serving the equipment; for cooling systems check for a condensate drip tray overflow switch that may have turned off the system.
  • If your air conditioning or heat pump system has lost its cooling capacity or won't start
    If all of the controls are calling for system operation but the blower motor won't run,

    See How to determine the cooling capacity of air conditioning equipment if the system seems to be working but is inadequate to cool your building.
  • Is air flowing from the air supply registers?
    see DUCT SYSTEM & DUCT DEFECTS - also check for a clogged or dirty filter or blocked air return. Initial, simple diagnostic checks of the air handler system are also described
    at Ducts & Air Handler diagnosis: Basic checks of the indoor air handler (blower), air ducts, and filter systems.
  • Is the blower assembly fan operating? You should hear the fan motor running and the fan assembly should be spinning. If the blower fan motor won't run
  • Are the air filters installed, proper size, clean?
  • Are the duct dampers working? Some HVAC systems use manual or automatic duct dampers
    or ZONE DAMPER CONTROLS to open or close airflow through individual duct system sections. Commercial and large building duct systems may include fire dampers that could be closed.
  • Is the air handler or its condensate drip tray dirty? Are there moisture, debris, visible mold, or other contaminants (rodents, birds) in or around the air handler or blower assembly?
    See DIRTY A/C BLOWERS below in this article.
  • Are the coils dirty or iced or blocked? Are the evaporator coils (cooling coils or heating coils) clean?

    see COOLING COIL or EVAPORATOR COIL for a description of diagnosis and repair of cooling coil problems since this component is normally located within the air handler chassis as well. A cooling coil which is blocked by debris or ice and frost can obstruct air flow and reduce air conditioning system output
  • Is the condensate pan clean and is condensate draining out of the pan?
  • Is the heating combustion equipment venting properly and does it have adequate combustion air supply? See CHIMNEY INSPECTION DIAGNOSIS REPAIR

Outdoor components: For a discussion of the outdoor components of an air conditioning or heat pump system see COMPRESSOR / CONDENSER REPAIR.

Also see our complete HVAC cooling and heat pump diagnostic guide beginning

Dirty A/C or Heat Pump Blower Units - Dirt-blocked air conditioner air handler fans

Photograph of  a typical squirrel cage blower fan interior

We also discuss how to improve indoor air quality by installing a cascaded air filter system at the air conditioning (or heating system) air handler to provide optimal air particle filtration for people with asthma, allergies, or indoor mold or other IAQ concerns.

Filters protect the blower assembly: The typical A/C system circulates air through the building duct work using a "squirrel cage" blower fan. It is very important for you to check and change air filters at least monthly when the system is in use to protect the blower fan from dirt clogging.

Cupped fan blades can become blocked by debris: The blades of a squirrel-cage fan are cupped in order to cause the spinning cage to move air.

Dirt accumulation on the blades fills-in this cupped area, ultimately changing the "cup" to a simple flat area. The fan will spin just fine. I've seen the cubic feet per minute of air conditioning air-flow literally double when a very dirty squirrel cage fan fan of this type was cleaned or replaced.

Cleaning an air conditioner squirrel cage fan with compressed air? If the blower fan blades have significant dirt accumulation, you should have the system professionally cleaned. While this is a fairly costly service call (requiring blower disassembly and removal for cleaning) it can make a dramatic improvement in system performance.

Do not permit a simple "blow out" of the blower by compressed air if the air handler/blower are in the living area. Some HVAC service companies use a foaming cleaner for removing debris from an evaporator coil. That same material might assist in cleaning a squirrel cage fan.

Otherwise we recommend removing the fan and cleaning it thoroughly outside. Clean the remainder of the fan cabinet and housing before returning the air handler to service.

Leaks, Dirt, Rodents Getting Into HVAC Blowers, Blower Compartments, Air Plenums

Photograph of rodent and trash in a return air plenum.

Very dirty, wet, leaked-into, or rodent-infested blower compartments risk indoor air quality and health issues in buildings. The air handler unit in the photo at left needs cleaning and a check for unsafe wiring due to leaks.

Leaks into the blower compartment of an HVAC air handler invite mold problems too.

The return air plenum in the photo at right has both a dead mouse and some other sort of trash that probably fell down a return air floor grille.

Mice in HVAC systems are a potential viral or bacterial hazard as well as an indicator of poor housekeeping. These conditions suggested that there had not been regular inspections of this equipment.




Causes & Photographs of Mold Growth in Air Conditioning Blower Fans

Photograph of toxic mold growth on a squirrel cage air conditioner blower fan unit

Notice the multiple colors and textures of mold growing on this air conditioner squirrel cage blower fan. Mold, depending on its genera/species, moisture conditions, and growth substrate material, can grow on many different surfaces where some organic material is available for "food".

We might find mold growing in an air conditioner on plastic, on air cabinet or duct insulation facing, hidden in fiberglass insulation, or on other materials.

Most likely the mold growing on this blower fan found a home in some of the organic constituents of common house dust: skin cells and dust mite fecals.

Very often the mold we sample and identify in air conditioner blower cabinets is a very common genera, Cladosporium sp. - a mold so common it's called "the king of molds." Kicking up leaves outdoors exposes you to more Cladosporium sp. than the mold on this blower fan.

But we also find some genera/species of more toxic molds that will grow right on top of other molds that are already present. So while air cabinet conditions may start by producing one mold of not too much concern, other more highly allergenic or even toxic molds could be present.

The fact that the mold in this photograph includes multiple colors and textures of material suggests that more than one genera or species of mold is present.

How to Handle Small Areas of HVAC System Mold Contamination

If the total area of moldy material is small (say less than 30 sq.ft. of contiguous mold) we would not bother to take a mold sample back to our lab for identification. Instead we'd suggest that the moldy surfaces be cleaned or non-cleanable material like insulation, replaced, and the cause for its growth corrected. We might, however, sample even a small amount of visible mold as part of a larger building diagnostic program if there were illness or air quality complaints associated with the building.

Examples of condensate spill-over inside of an air handler blower compartment, a condition creating a risk of mold growth inside the HVAC system are

Information about the potential of toxic mold growth in fiberglass ducts or fiberglass HVAC duct insulation as well as other fiberglass insulation products and more example photographs of that condition are
at Mold in Fiberglass in Insulation.

How to Keep the HVAC Blower Assembly Clean

BLOWER OPERATION: For effective operation of this system it is essential that it never be operated without air filters in place.

If you permit the filters to become very dirty you may need expensive special cleaning or other repairs.

COOLING MAINTENANCE TIP: The HVAC blower assembly should be serviced every two years. Without proper service the system may have no more than a five year lifetime.

ENERGY CONSERVATION: Keeping the fireplace flue closed (if you have one) when not in use will improve both heating and cooling efficiency in all buildings. or select a topic from the More Reading links shown below.


Continue reading at BLOWER LEAKS, RUST & MOLD or select a topic from the More Reading links shown below.

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AIR HANDLER / BLOWER UNITS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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