Supply Air Heating & Cooling Register & Duct System Defects in Air Registers & HVAC Zones
SUPPLY DUCTS & REGISTERS - CONTENTS: 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
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HVAC supply air ducts, registers, & controls:
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 at the top of this page 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.
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
The air flow rate supplied by an HVAC duct and air handler system is a function of several variables that can explain why you may not measure an air flow in cubic feet per minute or CFM that matches neither your expectations nor the equipment's CFM rating. These include at least
Pressure losses due to friction within the 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.
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 those listed below.
Air 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.
Restrictions in the duct system caused by bends, changes in diameter, or even crimps such as the hung and crimped flex-duct shown in our photo earlier on this page. Here is a catalog of common restrictions we find in both supply and return air ducts:
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
Ductwork diameter warning: with conventional-velocity heating and air conditioning systems if you find 6-inch round ductwork it is likely to be inadequate. A 6-inch diameter round duct moves notably less air in cubic feet per minute (CFM) than larger duct diameters. An 8-inch round HVAC duct moves twice as much air as a 6-inch diameter duct.
Fire damper that has become stuck in the closed or partly-closed position interferes with proper airflow through the system FIRE & SMOKE DAMPERS, AUTOMATIC
Flex duct defects: Because of the airflow reduction caused by its internal ribbed surface, flex-duct does not move building air as effectively as smooth-sided metal ductwork. Flexduct that is compressed or that has extra and unnecesary turns and bends further restricts airflow. To maximize the air flow rate through flexible ductwork,keep the actual flex-duct length cut as short as possible and stretch it out completely between connections.
See AIR FLOW IMPROVEMENT, HVAC.
Collapsed sections of flex duct block or stop airflow in either supply or return air systems can virtually stop airflow.
See DUCT ROUTING & SUPPORT
Restrictions at the HVAC duct system supply (or return) registers: a supply air register that is smaller in area than the duct that supplies it will reduce the air flow significantly. For example a 4-inch x 7-inch (28 sq.in) rectangular supply register, though it is fed by a 10-inch round air duct (79 sq.in. of supply duct cross section) only delivers air as if it were a 6-inch round duct (28 sq.in).
Watch out: and if the supply air register is shut or partially shut air flow may be reduced almost completely. Dirty supply register louvers or screens further reduce air flow through the heating or air conditioning system.
Restrictions in the HVAC duct system: any restriction in the duct system reduces the air flow to the smaller size found at the restriction. It does not help much to have 8-inch diameter air ducts in a building if one or more of the duct runs has been compressed or smashed.
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.
See RETURN AIR REGISTERS & DUCTS
Supply or reutrn duct elbows & tees: significantly reduce air flow through the system. Minimize the number of turns, bends and especially sharp bends or 90's, especially right at the supply plenum.
Splits & joints in ductwork: splitting an air supply duct into two subordinate runs using a "Y" or "T" (worse) connection does not improve warm or cool air delivery. For example if we split an 8-inch diameter round air duct into two sub-runs each of 8-inch diameter, we're only going to see about the equivalent of a 4-inch duct air flow through each of the subordinate runs of ductwork. For this reason, when adding supply ducts in a building, take each new supply duct run off of the supply air plenum, not off of an existing air duct line. 
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
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:
The following tables are adapted from "How to Evaluate Furnace Duct Work &
Cure Short Cycling or Inadequate Ductwork Problems" published by the Vermont Department for Children and Families ; similar tables are widespread through HVAC literature and resources cited at REFERENCES
Gas Furnace Heating Duct Requirements
Square Inch Duct Size Needed
for both supply & Return Duct
Reference: adapted from Wilcox, "Evaluating Duct Work, How to Evaluate Furnace Duct Work &
Cure Short Cycling or Inadequate Ductwork Problems" Vermont Department for Children and Families, Office of Economic Opportunity, - dcf.vermont.gov/sites/dcf/files/pdf/oeo/WAPManual/AppendixI.pdf retrieved 12/5/2013 
Oil-fired Warm Air Furnace Duct Sizing Requirements
Square Inch Duct Size Needed
for both supply & Return Duct
Min. CFM Airflow
45,000 to 55,000
60,000 to 70,000
75,000 to 85,000
95,000 to 106,000
106,000 to 115,000
125,000 to 150,000
Reference: op cit.
Relationship of Round Duct to Square Inches to CFM Air Flow to Heating
Duct Square Inches
CFM Airflow Supported
Heating BTUs Capacity
7,400 BTUs output
Reference: op cit.
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 Jeff Wilcox, "Evaluating Duct Work, How to Evaluate Furnace Duct Work & Cure Short Cycling or Inadequate Ductwork Problems" Vermont Department for Children and Families, Office of Economic Opportunity, - dcf.vermont.gov/sites/dcf/files/pdf/oeo/WAPManual/AppendixI.pdf retrieved 12/5/2013. Geoff Wilcox
Vermont Office of Economic Opportunity
Weatherization Assistance Program
Reference Material for the original article came from:
1. Saturn Mechanical Systems Field Guide
2. Bacharach (Rudy Leatherman)
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
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
"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
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