Return Air Registers & Ducts for Heating / Cooling Systems InspectAPedia® -
Defects in return air ducts & registers for heating or cooling systems
Return air adequacy
Air Conditioning (or Heating) Return Air & Duct Defects
Questions & answers about HVAC system return air or "makeup air" and its effect on warm or cool air delivery in the building
This HVAC ductwork article describes the
inspection of the defects in return air registers & ducts for heating or cooling systems (HVAC) to help detect duct defects like 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.
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The photograph above shows a single central return air duct located outside of the
apartment which the duct system intends to serve. This installation prevents recirculating of air through the air handler for heating or cooling and
results in poor air flow and increased heating and cooling costs for the apartment owners.
Also see our safety warnings at INCREASING RETURN AIR.
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.
Steps to Inspect, Diagnose, & Improve Heating or Cooling Return Air Flow
Basement Air Handler, No Return Air Ducts, All Return Air Taken at Air Handler
One-Way Cooling or Heating Systems: Some retrofit HVAC installers simply cannot figure out where
to place return ducts, so they are just omitted such as shown in the two photographs here. For example a basement or crawl space air handler may be found with
no return duct connections at all.
Rather you'll see that the return plenum is simply open to
the basement or crawl space. The second photograph confirms that wet, possibly moldy debris enters the HVAC system at this single point basement
air return to the blower unit. In the foreground of the photograph you can see our Burkard (TM) air sampling machine being used to take a look
at what the basement is sending into the home's ductwork and living space.
This forms a "one way" cooling or heating system - 100% of the air is drawn
from the area around the air handler, cooled (or heated) and blown "one way" into the occupied space. This
is the most inefficient design possible as well as possibly a dangerous one (see "Flue Gases" above).
Preferably return air is drawn from the heated or cooled
space. Taking "new" makeup air from an unheated space, heating it, and blowing
it "one way" into the heated area has to be a more costly way to heat a
building. For better indoor air quality and as recommended by ASHRAE, commercial and large residential heating and cooling systems may include apportion of outdoor fresh air input to
the duct system as well.
Attic Air Handler Air Conditioner Return Duct Defects
This photograph shows a typical black, large-diameter return flex-duct moving building air to the air conditioning system air handler unit located in an attic.
Notice the loose fiberglass insulation around the base of the large-diameter black flex-duct carrying return air? Poor connections
at this location were pulling loose fiberglass insulation fragments and attic dust and debris into the air handler system.
In addition,
this poor return duct connection, by leaking attic air into the duct system, increased the air conditioning operating cost by feeding
it hot attic air instead of building living space air.
This photograph has a couple of other interesting details. Notice that the return duct is partly crimped and thus obstructed as it passes
through the site-built building truss?
Also, what's that electric motor doing lying askew in the insulation next to the flex-duct?
Perhaps the central air return, located in the ceiling of the space below, was placed where previously there had been a ceiling-mounted
whole house fan, for which we see the motor, abandoned in the attic floor.
Understanding the history of changes made to a building
can help interpret the meaning of clues about the building condition. It would be a safer practice to remove the still-connected but
un-used fan motor.
Either remove the un-used fan circuit wiring completely, or enclose the termination of the un-used wires
in a junction box.
HVAC Return Air Duct Leaks & Obstructions - Theory & Practice
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 / dh) (ρ v2 / 2) where Δp= pressure loss (Pa, N/m2), and l= length of duct or pipe (m) and dh= 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 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.
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
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
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
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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 - info@flexibleduct.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: editor.engineeringtoolbox@gmail.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."
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, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book 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. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
"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]
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.).
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