Supply Air Heating & Cooling Register & Duct System Defects: Air Registers & Zones

Heating & Cooling Ducts - Supply Registers & Zones
- How to get more cool air flow from floor-level cool air supply registers
- Air Conditioning (or Heating) Supply Duct & Air Register & Duct ZOne Defects
Questions & answers aboutHVAC supply air ducts & registers
References

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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.

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

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 SUPPLY DUCT AIR LEAKS and see LEAKY DUCT CONNECTIONS.

How to Diagnose Stains at Ceiling Air Supply Registers

Photograph of supply register with leak stains at a ceiling.

Photograph of supply register with soot or debris stains at a ceiling.

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 and SUPPLY DUCT AIR LEAKS discuss air duct leaks.
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.

How to Get More 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).

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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Technical Reviewers & References

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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
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides:
- Commercial Building Inspection Courses - protocol ASTM Standard E 2018-08 for Property Condition Assessments
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  Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference Book, a reference & inspection report product for building owners & inspectors.
  Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
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  Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter inspectaehrb in the order payment page "Promo/Redemption" space.
- The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors.
  Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
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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.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.

Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.

Building inspection education & report writing systems from Carson, Dunlop & Associates Ltd

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
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
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
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
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