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How to correct weak air flow at air conditioner or heat pump or warm air heating air supply registers:
Air conditioning or heating duct air flow improvements. This article describes the causes of inadequate cool or warm air from air conditioning or heating ducts and suggests how to increase air flow to improve system operation. Sketch provided courtesy Carson Dunlop Associates.
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.
[Click to enlarge any image]
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.
Minimum Evaporator Blower Air Speed of 350 CFM per Ton of Capacity
While recommended air speeds will vary by manufacturer and air conditioner or heat pump model, typically, as Whirlpool recommends that all air conditioning or heat pump models should run no less than 350 CFM per ton of cooling capacity. 
Other OEM Sources of Variations in Air Handler Blower Air Delivery Rates in CFM
Evaporator unit blower air speeds vary considerably by manufacturer and model. For example Whirlpool's WGPH45 heat pump system product literature describes varying blower air flow speeds in CFM depending on which motor speed electrical tap is connected, giving CFM (cubic feet per minute) air speeds ranging from 545 CFM up to 2200 CFM. 
Air flow rates across the evaporator coil in the air handler are generally given in "dry coil" speeds and without considering the reduction in air flow caused by the air filter (about 0.8" H2O). When the cooling coil is wet with condensate the air speed drops further.
Wet coil pressure drop is approximately 0.1" H2O, for two-row indoor coil; 0.2” H2O, for three-row indoor coil; and 0.3” H2O, for
four-row indoor coil.
Weak Cool or Warm Air Supply: Causes of Reductions in the Air Flow, Air Quantity, or Air Speed in HVAC Ductwork
Here are causes of inadequate air flow rate through the HVAC duct system, including conditions that slow the speed of movement of air through the duct system as well as other HVAC duct system defects.
Air filter dirty, damaged, collapsed, blocked: an air filter or any other item that has been sucked into the duct system will block air flow and can risk a fire if drawn into the blower assembly fan, DIRTY AIR FILTER PROBLEMS are perhaps the most common cause of unsatisfactory airflow in an HVAC system.
Air leaks from unconditioned space into the air supply system mean that cool air is diluted in summer or warm air is diluted in winter.
The sketch at left illustrates a common diluting air leak that can reduce the effectiveness of air conditioning during the cooling season: a humidifier intended for winter use that short-circuits return air right over into the supply air duct without passing it through the cooling coil.
Air Registers Located Outside the Room (return air) mean that if the room door is closed and not under-cut, both heating and cooling capacity in that room will be reduced.
To understand the effect of a room that has only air supply registers and no return registers when the room door is shut, just imagine the air conditioning or warm air heating system having to blow air into a pressurized space.
Lower-cost new central air conditioning systems and retrofit air conditioning systems often provide supply ducts to the various rooms served but install just one or two central cool air returns.
Try leaving a door ajar to see if you can observe stronger air flow out of the supply registers in the room. If this makes a noticeable difference, other than leaving the door open you can improve heating or cooling by one of these means:
undercut the door bottom by an inch or 1.5 inches to allow more air flow through that opening
install a through-wall register higher on the wall that lets warm room air flow out to the central return register during the cooling season, or both high and low through wall registers to improve air flow during both heating and cooling seasons.
Close the high register during heating and close the low register during cooling. Details are provided by more sketches later in this article.
Balance of air flow among different building areas may have been subverted by occupants in one or two areas. Particularly in commercial spaces or buildings that use the area above a suspended ceiling as a giant return air plenum, people who have moved or removed a suspended ceiling panel to improve air conditioning or heating over their immediate area have un-knowingly subverted the air flow balance in the entire system.
If you see ceiling panels out of place or are surprised to see some air supply registers closed, check for these problems.
Blower Fan: dirty blades on a squirrel cage blower assembly fan significantly reduce the blower fan's ability to move air into the HVAC system from the return-air side as well as reducing its ability to push conditioned air into the occupied space.
Watch out: it is not obvious to a novice, but even a small amount of dust and dirt that begins to fill the curvature of the blades of a squirrel cage type blower fan will reduce the air flow rate. The fan spins and looks fine, but the loss of that curve means that the blades are not picking up and moving air.
Blower fan speed control operating problems on a dual-speed fan can cause the air flow to be weak during the cooling season. On combination heating and air conditioning systems we often install a two-speed fan, intending to operate the blower fan at a higher speed during the cooling season. But if the fan control does not switch to the higher speed when the air conditioner is on, the air flow may be inadequate.
Ductwork too small or duct sizes mismatched between the air handler, supply plenum, return air plenum, blower assembly, cooling coil. See the sketch at left: the cross sectional areas of the supply ducts and return ducts at the furnace or air handler should be about the same size.
Ductwork sizes not properly matched on a retrofit add-on of air conditioning to an existing warm air heating system, or during A-coil replacement in an existing air conditioning system can result in improper or poor air flow.
See ADDING A/C: RETROFIT SIZING.
Fire damper that has become stuck in the closed or partly-closed position interferes with proper airflow through the system
FIRE & SMOKE DAMPERS, AUTOMATIC.
Also check for a stuck or inoperative automatic zone damper if your HVAC duct system uses those devices to control air flow among building areas.
See ZONE DAMPER CONTROLS.
Floor air supply registers for cool air in an older-style duct work air conditioning system reduce cooling capacity if there is not proper return air flow.
Cool air supply ducts located in the floors can result in a build-up of warm air in that area of the building and difficulty in providing adequate cooling capacity.
As our sketch (left) illustrates, it is more difficult to push cool air upwards into the upper floors of a hot building than it is to cool the lower floors, since the more dense, cool air, by weight, tends by convection (or "gravity" if you prefer) downwards rather than upwards in a building.
Flex duct defects: Collapsed sections of flex duct block or stop airflow in either supply or return air systems
DUCT ROUTING & SUPPORT (see our photo, above-left).
Frosted or Iced-over cooling coil in the air conditioner air handler is a common cause of reduced air flow or complete loss of air flow during the cooling season. A dirty air filter or low refrigerant charge could be at fault.
Watch out: this same problem occurs "in reverse" - that is, besides air flow blockage due to frost or ice formation on the cooling coil due to a refrigerant, refrigerant flow, or similar operating problem, low air flow through the duct system for any reason (dirty filter, crushed ducts, dirty blower fan, for example) can itself cause coil frost or ice formation.
Watch out: if the supply air flow at air conditioning or heating supply registers seems weak, most experts suggest first checking for a dirty air filter, dirty blower fan, or leaky, crushed, disconnected, or damaged ductwork. But if the supply air has always been weak throughout the system, don't forget that inadequate return air can be the root cause of the problem.
More Causes of Inadequate Supply Air Flow from Air Conditioning or Heating Air Duct Systems
Combination Heating & Cooling Air Duct Systems:
How to adjust for summer cooling vs. winter heating:
As Carson Dunlop Associates' sketch (left) explains, the high air returns on building walls help cool a room during summer, but in winter we prefer to close these off and instead open low air returns near the building floor.
[Click to enlarge any image]
In summer we want to draw warm air from high in the room back into the cooling system.
In winter we want to draw chilly air that collects lower in the room near the floor back into the heating system.
Watch out: this return air register adjustment technique only works in rooms for which return air duct inlets have been installed both near the ceiling and near the floor.
Loose blower fan belt or pulley:
For air handlers whose blower fan is operated by a motor, belt, and pulley system, a loose pulley or worn, slipping drive belt can significantly reduce the air flow in the system (and may also make horrible squealing noises).
Return air inlets: Return air inlet grilles that are obstructed with dirt, debris, or furniture or that are improperly located or are just too small mean that because the heating or cooling system is "starved for air", the supply air flow into occupied spaces will also be reduced.
RETURN AIR REGISTERS & DUCTS
Transite air ducts: Crushed or collapsed transite duct
Zone dampers that are stuck partly closed obstruct air supply into that building area, or if stuck "open" when the zone damper should be closed, airflow to other building areas will be reduced.
See ZONE DAMPER CONTROLS
How to Improve Cool Air Flow from Floor-Level Air Conditioning Supply Registers
How can I improve cold air delivery from my air conditioner?
Older Florida home with air handler under house in crawl space. Air is ducted to floor
registers. Not very efficient as cold air doesn't rise much. House has a flat roof no attic
space to get up into. Any thoughts on how to improve?
- D. (Anon).
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 Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects"
section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and
home inspection educator.
 Thanks to Jon Bolton, an ASHI, FABI, and otherwise certified Florida home inspector who provided photos of failing Goodman gray flex duct in a hot attic.
 Air Diffusion Council, 1901 N. Roselle Road, Suite 800, Schaumburg, Illinois 60195, Tel: (847) 706-6750, Fax: (847) 706-6751 - firstname.lastname@example.org - www.flexibleduct.org/ - "The ADC has produced the 4th Edition of the Flexible Duct Performance & Installation Standards (a 28-page manual) for use and reference by designers, architects, engineers, contractors, installers and users for evaluating, selecting, specifying and properly installing flexible duct in heating and air conditioning systems.
Features covered in depth include: descriptions of typical styles, characteristics and requirements, testing, listing, reporting, certifying, packaging and product marking.
Guidelines for proper installation are treated and illustrated in depth, featuring connections, splices and proper support methods for flexible duct. A single and uniform method of making end connections and splices is graphically presented for both non-metallic and metallic with plain ends."
The printed manual is available in English only. Downloadable PDF is available in English and Spanish.
 Engineering toolbox properties of water - http://www.engineeringtoolbox.com/water-thermal-properties-d_162.html and email: email@example.com web search 09/16/2010
 Owens Corning Duct Solutions - www.owenscorning.com/ductsolutions/ - provides current HVAC ductwork and duct insulating product descriptions and a dealer locator. Owens Corning Insulating Systems, LLC, One Owens Corning Parkway, Toledo, OH 43659 1-800-GET-PINK™
 "Flexible Duct Media Fiberglas™ Insulation, Product Data Sheet", Owens Corning - see owenscorning.com/quietzone/pdfs/QZFlexible_DataSheet.pdf "Owens Corning Flexible Duct Media Insulation is a lightweight, flexible, resilient thermal and acoustical insulation made of
inorganic glass fibers bonded with a thermosetting resin."
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Bryan, Texas 77801 USA, Tel: (979) 680-0659, Email: firstname.lastname@example.org, Website: www.flowkinetics.com, "FKS 1DP-PBM Multi-Function Meter
Pressure, Velocity & Flow
User’s Manual", web search 07/16/2012, original source: http://www.flowkinetics.com/FKS_1DP_PBM_Manual.pdf [copy on file] and "FKT Series Flow Measurement And
Pressure Acquisition System
User's Manual" http://www.flowkinetics.com/FKTSeriesManual.pdf [copy on file]
 Histoire de l'Académie royale des sciences avec les mémoires de mathématique et de physique tirés des registres de cette Académie: 363–376. Retrieved 2009-06-19.- Pitot Tubes, Henri Pitot (1732)
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"Pressure sensor", retrieved 7/16/2012
 "GE Zoneline® Owners Manual and Installation Instructions, Heat/Cool Model 2900, Heat Pump Model 3900", General Electric Corporation, [copy on file].
 "GE Zoneline® Owners Manual and Installation Instructions, Heat Pump Model 5800", General Electric Corporation, [copy on file].
 N Lu, YL Xie, Z Huang, "Air Conditioner Compressor Performance Model", U.S. Department of Energy, August 2008, [copy on file as PNNL-17796.pdf] Available to the public from the National Technical Information Service,
U.S. Department of Commerce, 5285 Port Royal Rd., Springfield, VA 22161
ph: (800) 553-6847, fax: (703) 605-6900
online ordering: http://www.ntis.gov/ordering.htm
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