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Air leaks or obstructions in return air ducts of an HVAC system:
This article describes the inspection of the defects in return air registers & ducts for heating or cooling systems (HVAC) 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.
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.
The master document, of which this is a chapter, describes the inspection of residential air conditioning systems (A/C systems
Return Duct Air Leaks & What They Mean
When the air conditioning or heating system is in operation, return ducts are under negative pressure
(since the blower in the air handler is drawing air in to the cooling or heating unit). At Soffit Cool Air Blast we describe a more egregious case of duct work that was missing entirely.
Take a look at the flooding crawl space and rusty leaky ducts on dirt in our field photo (left). This crawl area suffered from flooding, sewer backups, and mold. This is not a good place from which to draw air into the building.
Missing return ducts mean
The system is drawing return air from spaces other than the conditioned space
The air conditioning or heating system operating costs are increased
There is the potential for drawing un-wanted dust, mold, pathogens, or even combustion gases
or flue gases into the system and therefore moving these into the living space
HVAC Return Air Duct Leaks & Obstructions that Reduce Return Air Flow Rates
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). We discuss this formula and also provide supply a table of supply and return duct sizing at SUPPLY DUCTS & REGISTERS.
Air filters that are debris clogged or an air filter that has collapsed can cause 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
Flex duct defects: In addition to punctures or loss of insulation that then lead to tears in the flex duct walls, collapsed sections of flex duct block or stop airflow in either supply or return air system
See this example: GOODMAN GRAY FLEXDUCT
Leaks in the supply air duct system are a very common HVAC duct defect that results in poor heating or cooling air flow.
See 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 for more information
Transite air ducts: Crushed or collapsed transite duct
Read TRANSITE PIPE AIR DUCTS that may contain asbestos
and are often found buried in slabs, collapsed or allowing leakage or entry of duct system contaminants
See SLAB DUCTWORK
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.
See ZONE DAMPER CONTROLS
Continue reading at LEAKY DUCT CONNECTIONS or select a topic from the More Reading links or topic ARTICLE INDEX shown below.
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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 - email@example.com - 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: firstname.lastname@example.org 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
"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]
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