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AIR CONDITIONING & HEAT PUMP SYSTEMS
A/C - HEAT PUMP CONTROLS & SWITCHES
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR FILTER EFFICIENCY
AIR FILTERS, FIBERGLASS PARTICLES
AIR FLOW MEASUREMENT CFM
APPLIANCE DIAGNOSIS & REPAIR
APPLIANCE EFFICIENCY RATINGS
BLOWER DOORS & AIR INFILTRATION
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING
BOOKSTORE - Air Conditioning "How To" Books
CAPACITORS for HARD STARTING MOTORS
CLEANING & Legionella BACTERIA
CHINESE DRYWALL HAZARDS
CONDENSATION or SWEATING PIPES, TANKS
DEFINITION of HEATING & COOLING TERMS
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DUCTS - Asbestos
DUCT INSULATION, Asbestos Paper
DUCT INSULATION for SOUNDPROOFING
DUCT SYSTEM & DUCT DEFECTS
DUCT SYSTEM NOISES
DUCTS, Asbestos Transite Pipe
DUST, HVAC CONTAMINATION STUDY
ELECTRIC MOTOR OVERLOAD RESET SWITCH
EVAPORATIVE COOLING SYSTEMS
FAN LIMIT SWITCH
FAN NOISES, HVAC
GAS EXPOSURE EFFECTS, TOXIC
GAS DETECTION INSTRUMENTS
HEAT LOSS (or GAIN) in buildings
HEAT LOSS (or GAIN) INDICATORS
HEAT LOSS R U & K VALUE CALCULATION
HEATING SMALL LOADS
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS, A/C SYSTEMS
LEED GREEN BUILDING CERTIFICATION
LOST COOLING CAPACITY
LOW VOLTAGE TRANSFORMER TEST
MOTOR OVERLOAD RESET SWITCH
MOLD in AIR HANDLERS & DUCT WORK
OPERATING COST, AIR CONDITIONER
OPERATING DEFECTS, AIR CONDITIONING
REPAIR GUIDES A/C / HEAT PUMP
REPAIR & DIAGNOSTIC FAQs for A/C
THERMOSTATS, HEATING / COOLING
THERMOSTATIC EXPANSION VALVES
WATER COOLED AIR CONDITIONERS
WINDOW / WALL AIR CONDITIONERS
WINDOW / WALL A/C SUPPORTS
Air filters on HVAC systems:
This article series answers almost any question you might ask about air filters for central 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" are generally ineffective in buildings where a problem particle reservoir is present, and they are not capable of removing an indoor air quality problem.
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In our photo at left you can see a blue and white electrostatic air cleaner on the air handler.
But notice at the upper left of the photo just below the brown metal of the air handler body: see that silver sheet metal handle?
Removing the two screws on either end of the pull-out will permit you to expose another air filter that is in this location - the handle is a tip-off that the air handler is meant to be opened at this location in order to remove/replace an air filter.
Are these the only two air filters on this system? Nope. Our discussion of cascaded air filters found
In sum, finding a filter on a duct system or air handler is no promise that it's the only air filter installed. Inspect the system thoroughly. If more than one air filter is provided, document the location of all of the filters installed.
Next inspect the air conditioning filter type and condition. What about filters that are missing completely or are very dirty? What problems can a dirty or blocked air conditioner filter cause for the air conditioning system and how do we fix these snafus? That's what we'll cover in this article.
Placement of filters and air handler access doors in very hard-to-access locations such as at the far end of a minuscule attic behind a forest of trusses means that the system is very unlikely to receive the periodic inspection and maintenance it needs.
We prefer to see A/C and heating filters placed at the building side of the air return register or grille, so as to protect the return duct from debris accumulation. The more common filter placement on many systems is right at or in the air handler.
After reading the text just below, if you still cannot find your heating or air conditioning system air filter read our detailed instructions on how to find air filters in our article: Air Filter Location. Just below we summarize some common locations where air filters are found.
Change your air filters every month when the air conditioning system is in operation. Make sure you find all of the filters as some systems have multiple filters and even multiple types of filters installed, such as a fiberglass or pleated paper filter, a washable filter, and an electrostatic air cleaner. These last two are cleaned, not replaced, when they're dirty.
See our detailed article about clogged air filters found
Excerpts are below.Dirty Air Filters: are a source of increased operating costs and poor cooling system operation. Dirty air filters can:
The filters on an air conditioning or hot air heating system should be changed monthly when the system is in use. Discuss with your heating/cooling service professional the possible need to clean the blower fan and duct work.
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.
Watch out: 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! For an example
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.
Question: How do I know which way my air filter should face when it is installed?
I am in the process of changing the filters in my new home for the first time - there are three located in various areas, easily accessible. I have purchased the correct sizes. Although marked by "air flow" arrows, I'm still confused as to which way they should be installed - does the "webbed grill" side face in or out? I know this may be a "no brainer" but it is a first for me and I want to make sure I place them correctly. - Beverly Upperman
Think about the problem we need to avoid: if an air filter gets sucked into a duct or worse, into a blower fan assembly, it can jam the blower fan, cause a motor overheat, and even cause a fire. So we want the air filter to be placed so that its reinforced side will prevent it from being drawn into the duct or blower system.
Air filters have an arrow on their edges to give us a clue about how they should be placed. Insert the air filter so that the arrow is facing in the same direction as the direction of air flow through the filter. So if you are placing a filter on an HVAC system central air return grille, air is "returning" or "entering" the grille from the building, so air is flowing from the building into the duct system at that location. So the arrow would point the same way - into the duct system.
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?
See our detailed article at Missing or Leaky Air Filters. Excerpts are below.
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 Your Air Conditioner or Warm Air Heater Doesn't Have Air Filters, Install One or More Filter
If an air filter is not present on your air conditioner or warm air heating furnace system, have one installed. Installing an air filter is normally a minor expense involving placement of a rack to hold the air filter at each return air register, and installation of the air filter itself.
The alternative to installing and maintaining good air filtration on any air conditioner or warm air heating system is dirty ductwork and a dirty air handler. 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.
Continue reading at AIR FILTER EFFECTIVENESS or select a topic from the More Reading links shown below.
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Frequently Asked Questions (FAQs)
Question: filters for a commercial AC unit never need changing? Says who?
(May 2, 2014) Raquel said:
Really? I would very much appreciate it if whoever is quoting regulations would be kind enough to tell us the regulation authority, publication name, date, and where the regulation could be read. I'd be glad to study that information further.
It is possible that HVAC filtration is being performed in a rooftop unit, that's a common design. I'd prefer that all of the return ducts were protected by filters but that is certainly not a requirement and many systems filter only at the air handler.
What you might want to know is the maintenance schedule for the rooftop filtration system. Often remote filters suffer from being forgotten.
Yes it is a concern if you add too many layers of filtration as if you restrict the air flow beyond what the system is designed to handle it will not work properly and could fail entirely.
Question: dirty air filters and indoor air quality
(June 2, 2014) DIRTY AC FILTERS said:
I moved into a older one bedroom Florida condo about 6 weeks ago...as part of my rental agreement I agreed to change the AC filter every 30 days. I am IMMACULATE. After about a week of living there I noticed the filter (the AC unit is located in a utility closet) was totally black....I immediately replaced it...two days later, again it is black....started to read about causes- had the coils cleaned (they weren't very dirty FYI) and paid out of pocket for a duct cleaning
mind you, the unit is only 730 SF so it only had one duct attached to the AC unit with a vent opening in the living area and the bedroom (FYI I also bought a mold test from Home Depot and taped it to the air blowing out as it directed to see if any mold spores grew-I did NOT take a sample from the duct itself-it looks like soot, not mold).
the cleaning service said they could only think that the insulation inside the one duct (about 8 feet long) was covered in old cigarette smoke that was causing the filter to become filthy. Since the condo has been somewhat updated there is no odor or visible signs of smoking EXCEPT perhaps within the duct (unit were built in the 1970s) after they cleaned the duct I again immediately replaced the filter (this would be the fourth filter in 5 weeks) and within 12 hours it was pitch black again
. I am at a loss-I cannot get any indoor air quality testing done thru the state/county since its a residential unit and I am now at odds with my landlord (who plainly does not want to put even $1 into unit upkeep) So what does that leave me with?? Paying for air testing out of my own pocket or breaking my lease ??(which I would rather avoid due to moving expenses,etc) I am not a hypochondriac but if the AC filter is catching the dirty air and it is black within 12 hours that can't be good?!? I'm breathing that air. I really don't know what to do. .I live in Palm Beach COunty, Florida. .thank you for any assistance. Lee
Air testing as younsuggestmwould be an unreliable approach in the first place and not diagnostic in the second.mif you want to do some useful testing I suggest sending a tape sample of the air filter debris to a lab for particle identification. That filter and the HVAC system have already made a huge air sample for you.
(June 2, 2014) Anonymous said:
Thank you- not one person that has looked at the situation can figure out what it is...I am sure the ductwork needs to be replaced as the duct cleaners stated bc the insulation is "contaminated" but in order to "force" my landlord to do so I have to prove what is actually causing the filters to turn black...I wish I could send an attachment for readers to view...are there any labs that you suggest? I have never dealt with anything like this
Just to let you know, I looked into lab testing and most quotes were in the thousand+ dollar range...wayyy out of my reach
Typically dominant particle identification of a tape sample of dust on an air filter surfac is $50 - $100. U.S. I sent you some lab names by email.
Keep in mind that even harmless fabric fibers could look quite dark on an air filter.
(July 22, 2014) a said:
Question: clicking air cleaner sounds
(July 29, 2014) kathleenmccallartist said:
Look for another dust source nearby.
Question: Smart Vent unit is not so smart at cooling the house - collapsed air duct
(Aug 26, 2014) Bobbie in Roseville said:
IF an air filter were very sorely blocked by accumulated dust and debris and if the return duct were made of flex-duct then yes that flex duct might collapse under the negative pressure created when the air handler was running.
Worse, if a section of duct or filter is sucked into the air handler the blower can jam and we might even risk a fire.
But a dirty air filter would not cause the collapse of a supply duct.
So we need to know which vent was collapsed and what it was made-of.
Question: where do I get the air filter size?
(Sept 20, 2014) Anonymous said:
Where do I get the right air filter when shopping at Loews - Annie 5Jan 2015
What kind of filter: air? oil? Air filter dimensions are determined by its mounting location.
You can probably read the filter dimensions in the owners manual for your heat pump air handler or blower unit. Then you can buy an air filter of the right size or dimensions at any HVAC supplier or at construction supply stores such as Home Depot or Loews or at your local building supplier.
Question: inaccessible or hard to access air filter
29 January 2015 Harry said:
A step ladder is de-rigeur. You need to be able to un-latch the cover as well as remove and replace the filter. Having just tried some overhead tools to change a recessed light bulb at a Chicago condominium I can testify that working overhead with a pole trying to do anything precise is difficult and often of marginal success. The bulb broke off in its socket. We were able to un-screw the broken bulb base from the socket using a rubber-tipped extendable pole but having to press hard enough to turn the broken base pushed the light fixture up higher into its recess, making bulb replacement impossible. Ultimately we had to go get a ladder. In my OPINION the same issue faces you.
- relocate the return plenum or grille to a more accessible location (costly, probably not reasonable)
- switch to a longer-lasting air filter, a deeper plated unit (possible but may require removal and replacement of the ceiling register itself with a different design that accepts a different filter. Be sure the new filter does not impact air flow rate or you'll foul up the HVAC system operation.
Questions about air filters: buying, installing, using, and problems with HVAC air filters .
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