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This article describes the inspection of heating and air conditioning (HVAC) duct air supply and return registers & duct zones for defects such as missing air conditioning cool air supply or return air registers, undersized air conditioning duct openings, improper cooling duct routing, cooling (or heating) air duct corrosion, leaky air duct connections, defective heating or cooling ductwork materials. We include a discussion of how to increase the supply of cool air or air conditioner output in a building.

The photograph above shows a 1930's heating and cooling air supply register still in active use, but with leakage around the register which transmitted odors and mold from the building basement. Also see LEAKY DUCT CONNECTIONS.

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The master document, of which this is a chapter, describes the inspection of residential air conditioning systems (A/C systems) to inform home buyers, owners, and home inspectors of common cooling system defects. Asbestos or mold in or on HVAC ducts are a possible hazards for which we provide links separate documents - see "Asbestos HVAC Ducts" at below left.

SUPPLY REGISTERS, & ZONES - Cooling Distribution Ducts, Registers, & Zones

How to Diagnose Stains at Ceiling Air Supply Registers

The ceiling air supply register shown in the photo at left is typical of modern residential installations except for those leak stains around the register.

Further investigation was needed to discover whether the stains were due to a roof leak above this point or an air conditioning leak into and around the supply duct or as we've seen in some northern climates, accumulation of condensation and even ice in ceiling air conditioning ducts caused by movement of moist air backwards through the duct system by natural convection when the HVAC system was turned off.

The ceiling register in the second photo at right has blown soot and debris onto the ceiling itself. Further investigation was needed to determine if this soiling was from failure to maintain filters in the duct system, mold or debris in the duct system, a failed (and dangerous) oil-fired or gas-fired furnace heat exchanger, or other causes.

Heating & Cooling source in each room:

The minimum air conditioning system inspection standard (or heating inspection standard) includes the observation of the presence of a cooling source in each habitable room in the building. Ductwork to each room (at least connected to visible supply registers) & supply registers themselves should be observed in every room, delivering conditioned air to each habitable room (we can exclude closets and utility rooms and in some locales, baths).

Beware of "dummy" supply registers that are not connected to anything.

Beware of supply registers that are connected to ductwork but have no air flow due to duct routing errors, pinched, or disconnected duct work, or similar faults. This defect can only be observed if conditions permit operating the system.

Placement of Supply & Return Ducts & Registers for Heating versus Cooling

In a home inspection report, cooling system duct work defects may be reported under "Heating System" for cases where same ductwork is used for both heating and cooling. However, optimal supply and return placement for cooling is different from the optimum placement for cooling supply and return air. A heating system may deliver warm air low on walls or at floor level (warm air rises).

A cooling system may prefer to deliver cool air from a supply diffuser high on the wall or in ceilings (cool air falls) and draw cool air to the system return duct from a separate high-location in a ceiling (warm air rises to the high return duct for air conditioning). This topic is discussed in more detail at LOCATION OF DUCTS below.

Heating or Cooling Zone Control for Furnaces & Central Air Conditioning Systems

Zone control for air handling systems can be provided by:

• Dampers at individual air supply registers
• Manual dampers in the duct work to permit balancing the system
• Motorized zone dampers that open or close sections of the ductwork to airflow provided by a single air handler unit
• Installation of individual air handlers to serve different building areas, typically in residential properties, upper and lower floors.

See ZONE DAMPER CONTROLS for details about how to achieve individual zone control for warm air heating for for air conditioning systems.

HVAC Supply Air Duct Leaks & Obstructions - Theory & Practice

Leaks in HVAC cooling or heating air supply ducts mean that cooling or heating costs are increased, since the HVAC system needs to run longer to reach the desired indoor temperature and humidity. Where more severe supply air duct leaks are occurring, some areas of the building or even the entire building may receive no heated or cooled air at all, even though the equipment is running.

Technical note: The D'Arcy-Weisbach equation for pressure and head loss can be used to calculate the actual pressure loss due to friction in a building piping or air duct system. The Engineering Toolbox provides the D'Arcy-Weisbach formula: Δp = λ (l / d_h) (ρ v² / 2)  where Δp = pressure loss (Pa, N/m²), and l = length of duct or pipe (m) and d_h = hydraulic diameter (m) and finally, ρ = density (kg/m³).

Watch out: But keep in mind that even this apparently accurate calculation of the effect of piping on air pressure and airflow loss will not include the effects of leaks or obstructions in the building return air or supply air duct system such as

• Air filter or 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. See AIR FILTERS for HVAC SYSTEMS for complete 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. DIRTY A/C BLOWERS
• Fire damper that has become stuck in the closed or partly-closed position interferes with proper airflow through the system FIRE DAMPERS in DUCTWORK
• Flex duct defects: Collapsed sections of flex duct block or stop airflow in either supply or return air systems DUCT ROUTING & SUPPORT
• Insulation loose in air ducts: Collapsed duct interior insulation, (FIBERGLASS DUCT, RIGID CONSTRUCTION
• Leaks in the supply air duct system are a very common HVAC duct defect that results in poor heating or cooling air flow. LEAKY DUCT CONNECTIONS
• 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 TRANSITE PIPE AIR DUCTS
• 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. ZONE DAMPER CONTROLS
• Also see UNDERSIZED RETURN DUCTS.

Scroll down or use the links at left to continue reading about duct defects with the next section: registers and zones.

How to Get More Cool Air Flow from Floor-Level Air Conditioning Supply Registers

Question: 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).

Reply: Checklist of Air Conditioner Airflow Improvements

A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem. That said, here are some things to consider in improving cool air flow in your home:

We agree completely that cool air works better delivered from ceilings than from floors since cool air tends naturally to fall through the occupied space. It takes more energy to blow cool air "up" than to drop it into a room from supply registers mounted high on walls or in the ceilings. We discuss this further at LOCATION OF REGISTERS & DUCTS.

But the cost of changing ductwork is rather high and the work disruptive so that's not your first choice.

Here are some A/C cool air flow improvement steps:

• Review the discussion about supply ducts and registers beginning at SUPPLY DUCTS & REGISTERS
• Be sure air filters are clean and not leaky - see AIR FILTERS for HVAC SYSTEMS and see Dirty Air Filter Problems
• Be sure no ducts are leaky, blocked, disconnected - see LEAKY DUCT CONNECTIONS and also see SUPPLY DUCTS & REGISTERS.
• Be sure the blower fan is completely clean - a small amount of dirt greatly reduces air flow - see AIR HANDLER / BLOWER UNITS
• Check that return air to the system is adequate and that all return air is being taken from inside the occupied space. See our suggestions beginning at INCREASING RETURN AIR.
• Consider adding a small booster fan at key floor registers to improve air flow
• See AIR CONDITIONER NOT WORKING for more air conditioner problem diagnosis steps
• Also see RETURN AIR REGISTERS & DUCTS and the suggestions listed at RETURN DUCT AIR LEAKS.

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AIR CONDITIONING & HEAT PUMP SYSTEMS
AIR CONDITIONER COMPONENT PARTS
A/C - HEAT PUMP CONTROLS & SWITCHES
A/C - HEAT PUMP CRITICAL DEFECTS
A/C DIAGNOSTIC FAQs
A/C TYPES, ENERGY SOURCES
A/C REFRIGERANTS

AIR CONDITIONER BTU CHART
SEER RATINGS & OTHER DEFINITIONS
OPERATING COST
SYSTEM OPERATION
OPERATING TEMPERATURES
OPERATING DEFECTS
LOST COOLING CAPACITY
CHINESE DRYWALL HAZARDS
COMBUSTION GASES & PARTICLE HAZARDS
CONDENSATE HANDLING, A/C
CONDENSATION or SWEATING PIPES, TANKS
COMPRESSOR & CONDENSING COIL, A/C
AIR FILTERS for HVAC SYSTEMS

AIR HANDLER / BLOWER UNITS
ANIMAL ALLERGENS / PET DANDER
ANIMAL ENTRY POINTS in buildings
ANIMAL ODORS IN buildings
APPLIANCE EFFICIENCY RATINGS
ASBESTOS IDENTIFICATION IN buildings

BACKUP HEAT for HEAT PUMPS

BLOWER DOORS & AIR INFILTRATION
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING

BOOKSTORE - Air Conditioning "How To" Books

CAPACITORS for HARD STARTING MOTORS
CAPILLARY TUBES
CLEANING & Legionella BACTERIA
CHINESE DRYWALL HAZARDS
COMBUSTION GASES & PARTICLE HAZARDS

COMPRESSOR & CONDENSING COIL, A/C

CONDENSATE HANDLING, A/C
BACKUP HEAT for HEAT PUMPS

BLOWER DOORS & AIR INFILTRATION
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING

BOOKSTORE - Air Conditioning "How To" Books

CAPACITORS for HARD STARTING MOTORS
CAPILLARY TUBES
CLEANING & Legionella BACTERIA
DUCT SYSTEM & DUCT DEFECTS
  AIR FILTERS for HVAC SYSTEMS
  ALLOY SYSTEMS FLEXDUCT
  ASBESTOS HVAC DUCTS
  ASBESTOS PAPER on DUCTWORK
  ASBESTOS TRANSITE DUCTWORK
  BALANCING AIR DUCT FLOW
  DUCT & AIR HANDLER ODORS
  DUCT DAMAGE, MECHANICAL
  DUCT INSULATION - Asbestos Paper
  DUCT ROUTING & SUPPORT
  DUST CONTAMINATION FROM HVAC?
  FIBERGLASS DUCT, RIGID CONSTRUCTION
  FIBERGLASS HVAC DUCTS
  FIRE DAMPERS in DUCTWORK
  FLOOD DAMAGE in DUCT WORK
  GOODMAN GRAY FLEXDUCT
  INCREASING RETURN AIR
  LEAKY DUCT CONNECTIONS
  LOCATION OF REGISTERS & DUCTS
  MOLD in AIR HANDLERS & DUCT WORK
  NOISES in DUCT SYSTEM
  ODORS in AIR HANDLERS & DUCT WORK
  OWENS CORNING FLEXDUCT
  OWL FLEXDUCT
  RETURN AIR REGISTERS & DUCTS
  SOUNDPROOFING for DUCTWORK
  SUPPLY DUCTS & REGISTERS
  TRANSITE PIPE AIR DUCTS
  UNDERSIZED RETURN DUCTS
  UNSAFE DUCT OPENINGS
  VIBRATION DAMPENERS
  WATER & ICE IN DUCT WORK
  WET CORRODED DUCT WORK
  ZONE DAMPER CONTROLS

ENERGY SAVINGS in buildings
EVAPORATOR COIL or COOLING COIL
EVAPORATIVE COOLING SYSTEMS
EVAPORATOR COIL or COOLING COIL
EXPANSION VALVES, REFRIGERANT

FAN, AIR HANDLER BLOWER UNIT
FAN AUTO ON Thermostat Switch
A/C REFRIGERANTS
A/C - HEAT PUMP CRITICAL DEFECTS
A/C DIAGNOSTIC FAQs
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS

FURNACES WARM AIR HEATING SYSTEMS
INDOOR AIR QUALITY IMPROVEMENT GUIDE

• Engineering toolbox properties of water - http://www.engineeringtoolbox.com/water-thermal-properties-d_162.html and email: editor.engineeringtoolbox@gmail.com web search 09/16/2010
• Thanks to reader Dennis for discussing how to improve cool air supply in a building where air conditioner output is through floor registers - May 2010 , January 2011

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
• Complete List of Air Conditioning & Heat Pump Design, Inspection, Repair Books at the InspectAPedia Bookstore.

• Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
• Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979
• "Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
• Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson, John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages
• Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
• Air Conditioning Inspection, Diagnosis, Repair, Efficiency all the basics for home owners, inspectors, new repairmen
• Air Conditioning SEER - New DOE Air Conditioner and Heat Pump Efficiency Standard
• Asbestos HVAC Ducts and Flues field identification photos and guide
• Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation
• ...