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AIR CONDITIONING & HEAT PUMP SYSTEMS
AIR FILTERS for HVAC SYSTEMS
AIR HANDLER / BLOWER UNITS
BACKUP HEAT for HEAT PUMPS
BANGING HEATING PIPES RADIATORS
BLOWER FAN OPERATION & TESTING
CARBON MONOXIDE - CO
CHIMNEY INSPECTION DIAGNOSIS REPAIR
CIRCULATOR PUMPS & RELAYS
COMPRESSOR / CONDENSER REPAIR
CONDENSATE HANDLING, A/C
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DUCT SYSTEM & DUCT DEFECTS
ELECTRIC MOTOR DIAGNOSTIC GUIDE
EVAPORATIVE COOLING SYSTEMS
FAN, AIR HANDLER BLOWER UNIT
FAN, COMPRESSOR/CONDENSER UNIT
FILTERS, AIR for HVAC SYSTEMS
FILTERS, OIL on HEATING EQUIPMENT
GAS PIPING, VALVES, CONTROLS
GEOTHERMAL HEATING SYSTEMS
HEAT PUMPS, DIAGNOSIS, REPAIR
HEATING COST SAVINGS METHODS
HOT WATER HEATERS
MANUALS & PARTS GUIDES - HVAC
NO HEAT - BOILER
NO HEAT - FURNACE
NOISE AIR CONDITIONER / HEAT PUMP
NOISE, HEATING SYSTEMS
NOISE, WATER HEATER
ODORS in AIR HANDLERS & DUCT WORK
ODORS FROM HEATING SYSTEMS
PORTABLE ROOM AIR CONDITIONERS
REFRIGERANTS & PIPING
REPAIR GUIDES A/C / HEAT PUMP
REFRIGERANTS & PIPING
STAIN DIAGNOSIS on BUILDING INTERIORS
STEAM HEATING SYSTEMS
THERMOSTATS, HEATING / COOLING
THERMOSTATS, WATER HEATER
THERMOSTATIC EXPANSION VALVES
VIDEO GUIDES: HEATING SYSTEMS
WATER COOLED AIR CONDITIONERS
WINDOW / WALL AIR CONDITIONERS
WATER HEATERS for HOME HEATING USE?
WOOD-OIL COMBINATION HEATERS
WOOD STOVE OPERATION & SAFETY
ZONE VALVES, HEATING
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.
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Target blower fan air speed: 400-450 cu ft/min.
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. 
See AIR FLOW MEASUREMENT CFM for details on how to measure air flow.
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.
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. For our complete list of HVAC duct system inspection, diagnosis, and repair topics see DUCT SYSTEM & DUCT 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.
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.
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.
See BALANCING AIR DUCT FLOW for details.
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.
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.
Cooling coil dirty, dirt-clogged, or damaged: if the cooling coil (evaporator coil) in the air handler is dirt-clogged air flow through the system will be reduced.
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.
Fire damper that has become stuck in the closed or partly-closed position interferes with proper airflow through the system
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.
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.
Sketches provided courtesy Carson Dunlop Associates.
Flex duct defects: Collapsed sections of flex duct block or stop airflow in either supply or return air systems
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.
So cause and effect can be confused without further diagnosis.
Insulation loose in air ducts: Collapsed duct interior insulation blocks air flow and can cause air handler overheating, risking a fire.
Leaks in the supply air duct system are a very common HVAC duct defect that results in poor heating or cooling air flow.
Return air flow inadequate: insufficient number, size, or less than optimum location of return air inlets and ducts significantly reduces the air conditioning or heating air flow in HVAC systems.
Details are at INCREASING RETURN AIR
and at RETURN AIR REGISTERS & DUCTS.
Also see UNDERSIZED RETURN DUCTS.
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.
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.
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.
Also see UNDERSIZED RETURN DUCTS.
HVAC air ducts located inside concrete slab floors invite flooding, mold, insects, and where transite - cement asbestos - ductwork was used, asbestos particle contamination or collapsed ductwork.
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).
This Q&A has been moved to a full article on this topic. Please
See WARM AIR SUPPLY IMPROVEMENT for suggestions to improve the air flow and temperature at forced warm air furnace or heating systems.
Continue reading at AIR FLOW TOO WEAK or select a topic from the More Reading links shown below.
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