Floor air supply registers for cool air (C) Carson Dunlop Associates Supply Duct Air Flow Increase
How to find & fix HVAC duct leaks & restrictions

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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.

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Causes of Poor Air Flow in Air Conditioning or Heating Systems

Air conditioner air flow rate notes (C) Carson Dunlop Associates

Article Contents

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. [17]

See AIR FLOW MEASUREMENT CFM for details on how to measure air flow.

The illustration also points out that if air flow is too slow across the cooling coil, that component may become ice or frost-blocked.


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. [17]

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. [17]

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.

Loose blower assembly pulley or belt reduces airflow (C) Carson Dunlop AssociatesAir 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.

Sketch at left courtesy Carson Dunlop Associates.

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.

Loose blower assembly pulley or belt reduces airflow (C) Carson Dunlop Associates 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:

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.

See DIRTY A/C BLOWERS for details.

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.

Loose blower assembly pulley or belt reduces airflow (C) Carson Dunlop AssociatesAlso see BLOWER FAN OPERATION & TESTING.

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.

Sketch at left courtesy Carson Dunlop Associates.

Floor air supply registers for cool air (C) Carson Dunlop AssociatesFire 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.

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.

Sketches provided courtesy Carson Dunlop Associates.

Floor air supply registers for cool air (C) Carson Dunlop Associates

Flex duct defects: Collapsed sections of flex duct block or stop airflow in either supply or return air systems
See 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.

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.


Also see these defective duct materials that may lose insulation and also may develop leaks:

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.




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

Floor air supply registers for cool air (C) Carson Dunlop Associates

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:

Loose blower assembly pulley or belt reduces airflow (C) Carson Dunlop Associates

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).

Sketch at above left courtesy Carson Dunlop Associates.

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.

Loose blower assembly pulley or belt reduces airflow (C) Carson Dunlop AssociatesAlso 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.

See ASBESTOS DUCTS, Transite Pipe for details.

Sketch at left courtesy Carson Dunlop Associates.



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).

This Q&A has been moved to a full article on this topic. Please

See WARM AIR SUPPLY TEMPERATURE & IMPROVEMENT for suggestions to improve the air flow and temperature at forced warm air furnace or heating systems.


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HVAC Duct Air Flow Article Series

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