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Heating or air conditioning return air supply ducts & registers:
Discussed here: HVAC return air duct installation, inspection, specifications, troubleshooting guide. Defects in return air ducts & registers for heating or cooling systems. Return air adequacy. Air Conditioning (or Heating) Return Air & Duct Defects.
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.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
Steps to Inspect, Diagnose, & Improve Heating or Cooling Return Air Flow
The photograph at page top 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.
At our photo shown here is a "one-way" cooling system: all of the return air is being taken from a basement. This is a bad HVAC design that is the most-costly to operate, and it can also be unsafe as we explain below.
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 photograph below 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.
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
Pressure losses due to friction in the air duct system
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)
Δp= pressure loss (Pa, N/m2)
l= length of duct or pipe (m)
dh= hydraulic diameter (m) and finally, ρ = density (kg/m3)
These Practical Considerations May Overcome Calculated HVAC Duct Pressures
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 the duct defects listed below.
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.
Disconnected HVAC ducts (photo at left) can cause very great losses in supply air and contamination in return air without discovery when the errors occur in hard-to-access spaces such as this tight crawl space that we inspected.
Fire damper that has become stuck in the closed or partly-closed position interferes with proper airflow through the system
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.
Our photo (left) of a severely blocked return air inlet register was provided by Mark Berton.
Suspended Ceilings & Drop Ceilings Used as Return Air Plenums
A suspended ceiling is often used as a huge return-air plenum in commercial HVAC installations as well as in some older homes in which HVAC ducts were added as a retrofit project and/or where high ceilings were "lowered" in an effort to reduce heating or cooling costs and to provide a passageway for supply air ducts.
But concerns arise in the suspended-ceiling return air plenum design, including
Forgotten or unattended openings between building areas may be hidden in the suspended ceiling space. Our photo (above left) shows our discovery of an open fire/door passage between two building sections of a New York City office space.
The client's space (foreground) had its own air conditioning system installed but their system was pulling return air from a neighboring office suite - not only fouling up everyone's air conditioning delivery balance, but risking odors and contaminants from other areas in the building.
Worse, the neighboring space included ceiling openings over a stairwell, sending air from even lower building floors into this upper ceiling air plenum.
Dirt, debris, falling plaster, asbestos, lead paint, mold, rodents and similar hazards often lurk in hidden spaces above suspended ceilings or in other building areas from which return air may be pulled into a heating or air conditioning system.
When return air is routed through these areas it may become contaminated.
The mouse and mouse droppings shown above were among the debris found on the upper side of a suspended ceiling that enclosed a space being used to serve as a return air plenum over offices we inspected.
Probably more serious are such spaces where we encounter asbestos heating pipe insulation in poor, damaged, friable condition, or large mold contamination reservoirs.
Debris or contaminants in the suspended ceiling cavity, particularly when building heating or cooling air flow through that space, are readily transported to other building areas and into the occupied space.
Watch out: at STAINS HUMAN OCCUPANTwe report a case of "brown stains" and "brown dust" blamed on previous occupants who were "heavy smokers". The stains were traced to an unsafe gas-fueled furnace that took its return air from the dirt floor of a crawl area. Subsequently dangerous carbon monoxide hazards were also found to be present.
Continue reading at AIR LEAKS in RETURN DUCTS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES 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.
 Thanks to Mark A Berton, an editor & photographer who has provided InspectApedia with photographs of old house conditions from a 1907 Pennsylvania home.
 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]
Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: email@example.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 extensive home inspection education and report writing material.
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.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates 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.
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.
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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.
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