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 &: Repairs.
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Article Series Contents
Here we provide air handler unit or blower assembly troubleshooting by expanded annotated information from the US EPA who provided suggestions for investigating the air handling unit during an indoor air quality investigation.
Also see our complete HVAC cooling and heat pump diagnostic guide beginning
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.
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.
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
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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Frequently Asked Questions (FAQs)
Question: air handler not running
(Feb 5, 2014) GENE said:
My air handler unit won't come on
See AIR CONDITIONER WON'T START How to Diagnose & Repair an Air Conditioner that Won't Start
for help with the problem you cite - IF you are talking about a cooling system or heat pump
If you are asking about a heating system or if the heater runs but the blower won't start,
for diagnosis and repair help.
Question: air handler control board fuses blown
(May 18, 2014) Gary Matthias said:
Blown fuses (3A) on the air handler control board? Carrier Model CD3BXA036000AAAA made in 1995 repeatedly blows the fuse on the control board. I replaced the thermostat, disconnected one of the wires to the blower motor, but still have the problem.
The Red LED light on the board begins to flash after the fuse has blown. Any suggestions would be appreciated, I have limited electrical knowledge, but I do have a voltage meter and can generally troubleshoot or component replace. Thanks
Blown fuses on a control board often take you to a faulty control or control board itself; check the obvious: loose or damaged wires; hard-starting motors, burnt relays, contacts.
Question: loud thumping sound after the a.c. turns off
(July 24, 2014) H.Mulligan said:
My blower unit makes a loud thumping sound after the a.c. turns off. It sounds like something hitting against something. It sounds like air on a fan. Fan unitbwasvreplaced last year. Technicians can't find the problem.
Did you check for loose fan, bad fan bearing, or motor mounts that jiggle as the spinning components in the air handler slow down?
Question: buzzing noise coming from the Return Air Handler
(Sept 16, 2014) barbara holton said:
there is a buzzing noise coming from the Return Air Handler. I live in an apartment and they keep putting screws on the metal plate and the noise stops but then the buzzing returns later. If I switch the unit off at the circuit breaker the noise stops. Do you think it may be the transformer or relay switch. Thank you
You're doing a better diagnostic job than your repair people. I agree that one should look for the buzzing component: often a failing low voltage transformer or a control relay.
Take a look at LOW VOLTAGE TRANSFORMER TEST for details.
Of course other vibration noises might be tracked to loose duct components or blower components, even an access panel. Those noises would also stop when the equipment is shut off.
Question: strong air flow buckled air handler door - makes noises
(Sept 26, 2014) Don Brown said:
Had a new ac unit installed and have great air flow that causes the air handler door to buckle and retract when blower starts and stops making an annoying sound. Any suggestions?
Don it sounds as if there is inadequate return air ducting - the air handler is air-starved. I'd look into that problem rather than just stiffening the door. Just improving the door to stop the noise is treating the symptom not the problem.
Question: The heating coil has ruptured
(Oct 1, 2014) Jimbo 10/1/14 said:
I have a small air handler with both cooling and heating coils inside. They are very close to one another and the air must pass over both of them regardless of whether heating or cooling. The heating coil has ruptured twice in a three year period.
I am being told that this will happen if the air conditioner is running and the heating coils have cold water in them. I guess I am being told that the water in the heating coil is freezing and expanding. This does not make sense to me. Must I drain the water out of the heating coils at the end of each heating season?
(Oct 5, 2014) DanJoeFriedman (mod) said:
I'm with you Jimbo - I can't make sense out of what you're being told. I don't know what sort of equipment you've got installed.
Question: how long does it take for mold to grow on the air-conditioning unit?
(Oct 18, 2014) Donielle said:
How long does it take for mold to grow on the air-conditioning unit?
When conditions of moisture and temperature are favorable for molds commonly found in buildings and mechanical systems mold can grow in 24-48 hours.
Question: air handler door safety switch voltage
1 Nov 2014 Anonymous said:
on the air handler door safety switch , would like to know what volts is suppling the switch and where thanks.
Reply: proper blower door safety switch voltage, installation, replacement, repair
Usually a furnace or air handler blower door safety switch carries 120 volts. The switch is connected in series between a relay in the air handler that operates the blower fan and a terminal on a control circuit board in the unit. The switch is not powered independently. It's power comes through the circuit board.
You'll be able to see this by following wires from the blower door safety switch back into the air handler.
WATCH OUT for dangerous shock hazards. Be sure all power to equipment is off before touching any switches, wiring, etc.
Replacement blower door switches are easily obtained for your air handler by brand and model via your local HVAC supplier, from the manufacturer, or often online. Some safety switches will work properly on several air handler brands and models.
I add that the blower door switch is an important safety feature that helps avoid chock or chopped-off fingers or worse. Be sure that your replacement switch is properly mounted so as to be reliably operated by the blower compartment door(s) - the switch opens- cuts off power - when the door is opened, and is depressed in to close and permit power to the blower when the door is properly closed.
Questions & answers or comments 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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Technical Reviewers & References
Since the failure of an air conditioner to turn on, loss of air conditioner cooling capacity, reduced air conditioning output temperatures, loss of cool air supply, or even loss of air flow entirely can be due to a variety of problems with one or more components of an air conditioner or air conditioning system, after reviewing the lost air conditioner cooling diagnosis procedures described in this article, be sure to also review the diagnostic procedures at each of the individual air conditioning diagnosis and repair major topics listed just below. To return to our air conditioning and refrigeration home page go to AIR CONDITIONING & HEAT PUMP SYSTEMS.
If your air conditioning or heat pump system has lost its cooling capacity or won't start select one or more of the diagnostic articles listed below.