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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
HVAC return air improvement guide: how to increase HVAC system return air to increase heating or cool air output by improving the flow of return air to the air handler. This article describes problems with return air inlet size, location, and ductwork.
Inadequate return air seriously limits both air flow rates and also the degree to which building air is cooled (or heated) by the HVAC system. The photograph above shows a return air inlet grille for a commercial office space after the air conditioning return register and ducts were increased in size as part of improvements in the building cooling system.
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Add More Return Air Inlets & Ducting
Adding additional return air inlets and ducts to increase airflow to the air handler is an effective way to improve air conditioning or or warm air heating system performance, provided that the system is in fact running "air starved".
There are several easy and amateur ways to check for an air conditioning or warm air heating system that is not getting enough return air.
Watch out: don't leave the cover off of an air handler - it's potentially very dangerous, as we explain
Watch out: it may be necessary to temporarily tape or bypass a blower door compartment interlock switch to try this subjective test. Don't leave the door interlock switch bypassed or taped - doing so is dangerous. Details about this switch are
A simple test for air movement at the return air inlet is illustrated in our sketch.
Just hold a tissue or piece of toilet paper near the inlet grille face. If air is moving into the grille the tissue will be pulled against the opening.
For true air flow measurements that provide quantitative results such as air flow measured in CFM,
In that article at Typical Manufacturer's Air Flow Rate CFM Specifications for HVAC equipment we describe typical HVAC air flow rates measured in CFM.
When the cooling ability of an air conditioning system is inadequate, particularly when the volume of air being delivered in the building seems too low, we often see evidence of an attempt to boost heating or cooling air delivery in this "stopgap" manner.
We find extra return air openings having been cut in the return plenum right at the air handler unit at a combination air conditioning and hot air heating furnace or at an attic or basement air conditioning-only air handler.
Indeed this boosts the air coming out of the system if the air handler was "air starved" due to insufficient return ducts in the first place. An example of this poor practice is shown in the photograph.
But this is a very inefficient way to operate the system since a significant portion of the air volume is moving only "one way" from an attic or basement into the cooling unit and out to a remote living area.
This is an expensive way to run an air conditioning system: keep taking "new" air, cool it, and blow it where it's wanted. Proper design re circulates air from the occupied space which permits it to be cooled and filtered.
Watch out: Worse than inefficient, the approach of taking return air from a basement or crawl space utility area where gas or oil fired heating equipment is located can be dangerous, in particular if by the location of the "new" return air opening draws flue gases from a nearby draft hood or barometric damper, or if the heating equipment is located in a small enclosed space where drawing return air can interfere with the provision of adequate combustion air for the heating equipment.
Flue gases: may be drawn into the duct system if these "improvement" openings are cut too close to heating equipment, particularly gas-fired furnaces, boilers, and water heaters. We say more about this at UNSAFE OPENINGS below (see link at left).
Blower Fan too Slow for Cooling Season
An HVAC system that is simply not capable of moving enough cubic feet of air per minute will not be able to adequately cool or warm the occupied space. Higher air speeds are needed during the cooling season.
Carson Dunlop Associates' sketch (left) points out that the (typical) desirable rate of cool air flow in an air conditioning system is around 400 to 450 cubic feet per minute.
The illustration also points out that if air flow is too slow across the cooling coil, that component may become ice or frost-blocked.
See FROST BUILD-UP on AIR CONDITIONER COILS for details.
What slows down the air speed in an air conditioning or warm air heating system?
Here we provide a list of causes of inadequate air flow, including conditions that slow the speed of movement of air through the duct system as well as other HVAC duct system defects. For our complete list of HVAC duct system inspection, diagnosis, and repair topics
Definitions & Measurements for Air Flow in Buildings
How do we measure air flow in CFM (cubic feet per minute) in an air conditioner or furnace
How is CFM measured? - Anon.
Air flow rates for HVAC systems are expressed as a volume of air being delivered at some rate, typically cubic feet per minute (CFM) or m/sec (meters per second), ft/sec (feet per second), or ft/min (feet per minute).
A nice clear technical answer of how we measure flow rate is provided by Flow Kinetics:
Here's a simplistic example: If I held up a one-foot square sensor in front of an air source (say an air supply register) and the sensor measured air velocity at 12 inches per minute, I'd be measuring 1 CFM of airflow. (One cubic foot = 12 x 12 x 12 inches). Or if we measured an air velocity at an air supply register of one foot per minute and we knew that the duct work was a 12-inch square duct, we'd figure we were seeing one cubic foot per minute of air supply at that location.
Actually here are more than one answer to your question about how airflow is measured in an HVAC system because there is a range of air flow measurement instruments on the market.
The measuring devices vary in price, accuracy, and in operating principle, and there are also of course multiple sources of CFM data: manufacturers specifications, theoretical numbers, and actual measurements. We are most interested in the last category.
Continue reading at AIR FLOW MEASUREMENT CFM or select a topic from the More Reading links shown below.
Or see AIR FLOW IMPROVEMENT, HVAC
Suggested citation for this web page
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
Beginning at COMPRESSOR & CONDENSING COIL, A/C see the articles at the "More Reading" links at the bottom of this article for detailed HVAC compressor/condenser troubleshooting and repair procedures.
We include more general compressor/condenser return air system questions just below.
Reader Comments & Kudos: (questions & replies follow)
31 Oct 2014 Grace De Bavay said:
This is really a great and very informative post. We normally install HVAC and don't pay attention over such small things which causes big problems. I too had ventilation system installed in my home and it is working well and good so far.
Reply: Thanks for the nice note, Grace. We also welcome questions, criticism, and content suggestions and especially would appreciate noting differences in requirements, terms, or HVAC issues special to Australia.
Question: compressor just runs for a few seconds, then cuts off; short cycling A/C compressor problems
My condenser starts for a few seconds then cuts off cycle continues - Anon.
my compressor fun has started to short cycle, it run for 2-3 min then shuts off for one second then restarts runs again then stops etc. with no funny sounds or noises - Don
It's not clear from your question where in the entire cooling system your unit's troubles begin, so I'd look at diagnostic tips that may explain your compressor/condenser short cycling are found at LOST COOLING CAPACITY. Certainly a compressor that is having trouble starting can be diagnosed at HARD STARTING COMPRESSOR MOTORS, if you're sure the trouble is outside.
Question: ice on the refrigerant line, no cooling in the house
My line set going to outdoor unit has ice on it. Lost cooling in house. What is the problem and is there anything I can do to fix the issue - Pat
Reply: check the indoor air handler for an ice-blocked coil
A problem with a refrigerant metering device (thermostatic expansion valve, capillary tube, etc), or even a dirty air filter can lead to improper metering of refrigerant into the indoor cooling coil, leading to icing; but other causes of ice formation include an improper refrigerant charge or even something as simple as missing refrigerant line insulation. But when you also lose cooling inside, I suspect an ice blocked coil. See FROST BUILD-UP on AIR CONDITIONER COILS
Question: ice build up inside the ductwork
I have ice build up on the inside of the furnace duct work. This will plug up the lines and the fans will not circulate thus stop blowing cold air
If you find ice build-up inside the ductwork itself that's pretty unusual except in winter in freezing climates when moist air is rising into the duct system. Usually ice forms at the cooling coil in an air conditioner air handler or blower unit.
Question: My A/C unit kept blowing a fuse, now nothing runs
My Center A/C unit had a tuned up. I clean the evaporator, I put the blower down and put the new motor and clean the blades deeply, I have a clean filter, I clean the condenser deeply, and I bought a new thermostat. OK. The unit star running normally and you fill the normal warm air in the fan (outside) But 3 to 5 minutes later the heat in the condenser air increased suddenly the condenser stop with noise, and I need to turn it off. what is this? Please give me a clue? - Anon 6/28/22
I have a pretty old Tappan unit, and when the power goes out, if the A/C is running it will blow the fuse, usually we just replace the fuse, and now nothing will run on the A/C unit. any ideas? - Shantel 6/20/11
Question: My hi line is cold, low line is warm and the condenser blows hot air
In my situation the high pressure line is colder than the low pressure line. The condensor coil blows mainly hot air, but the end closest to the compressor blows hotter air than the end furthest from the compressor line. Any suggestions? - Jim T. 7/13/11
Mundo suggested: hey jim t check for restriction at the evaporator
Question: water comes out of my system when I turn on the A/C
When i turn on the (AC Outside unit Lennox ) there is an overflow of water in my basement at the furnace which is a gas furnace- Don 7/24/11
Don if you see water in your basement at the A/C unit when it's on, your condensate drain or pump is not working.
Question: No water is coming out of the condensate drain
water is not going through the drain hose - Anon 7/20/12
Question: Is the problem with the fan motor or the compressor motor in my condenser unit?
I've a Lennox outside a/c unit. The fan/motor and compressor are part of the unit. The fan was not spinning and the A/c is not running. I've isolated the problem down to the fan/motor or the compressor, but don't know how to check or find out which it is. Any idea on how to check if the fan/motor is 'bad' or the compressor is 'bad'? - Fan or ... 7/23/11
Fan or compressor: you could also have a bad control board, or relay switch in the compressor/condenser unit. If the fan is bad and the compressor is OK you'll hear the compressor turn on at the start of a cooling cycle, though a bad fan and system overheating could later cause some compressors to turn themselves back off. Similarly, if the compressor is bad your fan motor would still run. So I'm suspecting a control or switch problem./
Question: Our new AC won't keep running
uhm my A/C compressor broke about two weeks ago, we got a new one but when the fan starts the compressor works for a while and then it shuts down, and its turning on and off and i dont know whats wrong - Help. 7/25/11
Help: if your new A/c won't keep running after it has just been installed, surely you should call the installer and expect it to be repaired under warranty - it's just been two weeks.
Question: My AC won't work if I set the temperature down below 25C
my a/c is working properly when it is adjusted at 25 degree temprature, but if it is lower than 25, the compressor is disconnected by the overload protection. what is the problem with my a/c. - Mohamed 8/5/11
Mohamed an A/C compressor may cut off on thermal overload due to overheating from local conditions around the compressor/condenser unit (no shade, very hot weather), an older motor that is failing, a start capacitor problem, or even a refrigerant metering device problem. If there's nothing obvious to you, I'd get a service tech to take a look. Keep us posted.
Question: Icing at the compressor; compressor not turning on?
the outside unit of the heat pump is icing up on the compressor - Steve 8/7/11
Hi my fan in my house goes on but my compressor unit does not all the electric seems to be fine . could it be my thermostat? - Sam 8/7/11
the accu very heat,although it is new gen. clean,2Tr LS-60psi & HS-100psi, - Anon 8/6/11
Steve: icing at the compressor/condenser sounds like a refrigerant charger or refrigerant metering problem.
Question: My A/C compressor just won't stop running
I recently noticed my AC condenser unit still running when the system inside shutoff by itself. and also, does the condenser unit have to be leveled. I notice the slap which holding the unit is slopping. Any suggestions? Thank you. - Rick 8/16/11
A/C compressor/condenser won't stop running:
Question: can the condenser get oil logged
can a condenser become oil logged, how and why? - Dominic 8/22/11
Dom, it is normal for some refrigerant oil to circulate through the HVAC refrigeration system; Oil logging can occur in some installations if the refrigerant piping is not properly routed or due to an improper charge. The more usual problem is not at the condenser coil but at smaller orifices such as a cap tube or the TEV.
Question: our A/C compressor tries to start but then shuts right back off
I turned my AC on and noticed it wasn't getting cooler inside, I went out to the condensing unit and the compressor tries to kick on but then shuts right back off. Can not having enough refrigerant cause this? If not what's the deal? - Derek 8/22/11
Derek, as another reader commented, your compressor/condenser may be having trouble starting. A start capacitor can sometimes fix this problem and give some added life to the unit.
Question: compressor starts, fan doesn't start, I hear a hissing
My compressor turns on but the fan does not turn(Ican turn the blades by hand) within 10 t0 15 seconds I get a long a loud hissssssssssssssssssssss.
I HAVE SOME MORE INFO ON MY A/C WHEN THE A/C IS TURNED ON BY DROPPING THE TEMP IN THE HOUSE THE COMPRESSOR TURNS ON BUT THE FAN DOES NOT TURN, IF YOU START TO TURN THE FAN BY HAND IT STARTS SPINNING. WHEN I TURN IT OFF BY PICKING UP THE TEMP IT STOPS. WHEN I DROP THE TEMP AGAIN NOTHING GOES ON. IF I WAIT A FEW MINUITES AND TRY IT AGAIN THE COMPRESSOR STARTS BUT THE FAN STILL HAS TO BE STARTED BY HAND. THANKS FOR ANY HELP YOU CAN GIVE...
Henry I suspect a bad fan start-run capacitor.
Question: no air is coming out of our supply registers
I have a coleman unit. Had heating probs 2 years ago leading to complete computer panel replaced, evaporator coils cleaner, gas regulator replaced. Two days ago, my A/C decided to stop working. I hear the freon flowing into the coil region, the fan outside is turning, but no air is coming out of registers. What can it be? - Cheryl 9/9/11
Cheryl if you have no air coming out of supply registers the blower fan is not working or the duct system has become disconnected or blocked. Coil ice-over in the air handler can also block airflow. Start by determining that the blower is running and that the air filter is clean.
Question: water at the air return vent
what makes the water from an airconditioner build up in the return vent? - Connie Woyan 9/15/11
Connie: if you see water at the return vent it may be condensation on a cool surface, or it could be condensate leaking down into the return vent from your air handler. You'll need to make a closer inspection of the air handler (blower unit) itself.
Question: what type of copper is used in HVAC refrigeration piping or tubing?
What is the content of copper tubes used in AC? (i.e I would like to know the contents of pipe which is being connected between the Fan and compressor of split AC) - Pratik Shah 10/1/11
Question: no sweat on the suction line
what could be the problem when the sunction line of an air conditioner is not sweating? - Diana 2/3/12
Question: hard starting compressor
jon- ac system is less than a year old, when I turned on ac to check, condensor comes on then shuts back off, then 30-45 seconds after comes on again for 2-5 seconds. Whats wrong? - Jon Anon 8/18/12
Compressor on Lennox centeal air not turning even if the thermostat is activated.. It is blowing warm air.
Anon: regarding your hard-starting compressor/condenser unit: we can't with confidence say what's wrong with so little information - your service tech can diagnose the problem, probably fairly quickly. But a common cause of the symptoms you describe is a bad start/run capacitor or worse, a failing compressor motor.
Todd said to Roger:
Rpger...not a professional myself but I had the same issue. The motor on my outside unit had worn out to the point that it needed to be replaced. Forturantely for me it was under warranty from a unit that I purchase 4-5 years ago. This might be strange for most units but we've had some above average summers here in Texas the last couple of years.
Question: does it help to spray the compressor/condenser coils when it's hot outside?
Does it help to spray water on the compressor coils when the outside air temp. is 100+? The inside air is going up in the afternoon to 82. - Greg Warner 7/7/12
Some readers have reported that their compressor was overheating and ran ok when water cooled - this is perhaps a diagnostic step but not a reasonable repair. And don't soak electrical components.
Question: oil in the capillary tube? - recurrent cap tube clogging
i have replaced my compressor from company and when i started it was giving cool air but the copper pipe out side (large) was not cool enough and after 40 mints its timer light flashes and after 10 more mints its down automatically. it happens again and again i sent back it to company technician is looking into it. He said oil was in capillary and it needs flushing so he will replace its capilary since after cleaning its not fixed. do u think there is no fault in compressor? or what possibly is the problem.. - Wagar 7/11/12
You'll want to check the installation manual to find the control light decoding for your brand and model of cooling equipment. Could be a control or control board problem too. It's just about impossible to flush a cap tube so replacement is normal. But if there is debris in the system, or excessive oil, the new cap can clog as well; good practice would be to empty the system, pull a vacuum, flush the whole system, install a filter dryer, replace the cap tube, and recharge.
Question: A/C not cooling, condenser fan not running
Several days ago I noticed that even though my air handeler was rinning the AC was not cooling, I foudn that the condensor was not turning, I flipped the breaker (that apprered to be in the on position) and it worked for awhile. 15 minutes later I noticed that the condenser was not operating again so I flipped the breaker again it corrected the problem.
I then went on the replace the breaker which seemed to correct the problem, the AC worked normally for several days. Now I find the condenser is not working again and flipping the breaker fails the correct the problem. When the condenser was working it was providing good normal cool air.
The system is 2 years old. I assume I have an electrical problem in the condenser. What is likely to cause this problem and how can I test for it (I have a test meter) but I want to be careful because I know that a coil can retain a dangerous electrical charge even when all power is off and disconnected. - Rick 7/13/12
typically we see if the fan spins, if it has power, and if the controls and fan contactor are bringing power to the fan when the condenser unit starts up.
My problem turned out to be a bad contacter (damaged by ants) - Rick 7/13/12
Question: loud cricket chirping sounds at the compressor/condenser unit, and slow starting
outside unit making loud cricket sounding noise when running. Takes long time to come on when temp set low. - JT 7/276/12
JT NOISE AIR CONDITIONER / HEAT PUMP provides noise diagnostics.
As long as the SET temperature is lower than the room temperature the cooling system will turn on.
Question: short circuits in air flow - supply air flows right into the return air duct
(May 19, 2014) fgh said:
Should the return vent and the vent that the air flows out of be located side by side. It seems that the return is sucking up the cool air and the air-conditioner doesn't cut off on a hot day.
In the airflow and it will limit the effectiveness of the system and increase operating costs.
Question: Debris-clogged evaporator coil reduces air flow across the coil
(June 29, 2014) john said:
can a dirty (or old) residential a/c evaporator coil effect and/or reduce the forced air flow(coming from the vents)thus effecting the units overall cooling efficiency, even if the evaporator coil is not freezing over and the vent air temperature is adequate?
Certainly yes. Debris-clogged evaporator coil reduces air flow across the coil and also reduces the transfer of heat from the air to the coil even if the coil is not iced or frosted over
Question: ratio between air return grille size (opening) and filter size?
(Aug 15, 2014) Anonymous said:
Is there a ratio between air return grille size (opening) and filter size? If I have "clear air" (all doors are open to all rooms being supplied by the a/c handler) provided the return and filter are the same number of square inches, is that sufficient to ensure my system is not air starved? Thank you, Ed
Anon, usually the air filter length and with dimensions are matched to the size of the return air grille itself.
It is the total return air capacity, determined by the number, location, and size of return air inlets that must be matched to the air handler's blower capacity and supply duct design.
There are some crude rules of thumb and an elementary test that can give a good guess at return air adequacy:
If the return air inlet square inches is smaller than the cross section of the blower or air handler's return air inlet in cross section then there is probably not enough return air.
Also we propose a crude return air test: with the blower fan running (and if there is no door safety interlock switch in play) if we open the door and release it and it SLAMS shut with vigor I figure that the blower compartment was under high vacuum and there is probably not enough return air. (Admittedly this is crude).
Keep in mind that even if good quantity of return air inlet by location, size, and ducting are all provided, if we are considering a system with central return air and room doors are shut (or not under-cut), the ability of the air system to force conditioned air INTO the rooms will be limited.
Question: water in return air compartment
(Sept 6, 2014) lee said:
had new a coil installed,getting water in return air compartment
Lee this is baffling but I suspect that there is so much moisture in your return air that there is condensation in the return air plenum OR *more likely* depending on the type and position of your system's air handler, there may simply be leakage from the condensate handling system (or on occasion from a poorly-located or leaky humidifier on combined heating/cooling systems).
Questions & answers or comments about falling air conditioners & heat pumps or split system compressor/condenser inverter units.
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