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.

Common Duct Defects that Impact Air Flow Rates

Article Contents

• SUPPLY DUCTS & REGISTERS
  • STAINS at SUPPLY REGISTERS
  • LOCATION of REGISTERS for COOLING vs HEATING
  • OBSTRUCTIONS & PRESSURE LOSSES in DUCTWORK
  • INCREASE AIR FLOW from SUPPLY REGISTERS
  • HVAC DUCT SIZE REQUIREMENTS TABLES

 

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.

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 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.
    
    Also
    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
  • Insulation loose in air ducts: Collapsed duct interior insulation
    
    See 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.
    
    See LEAKY DUCT CONNECTIONS
    and
    
    See SUPPLY DUCT AIR LEAKS
  • 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. [1]
  • Transite air ducts: Crushed or collapsed transite duct
    
    See TRANSITE PIPE AIR DUCT ASBESTOS RISKS
  • 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
  • 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 RETURN AIR, INCREASE.
• Consider adding a small booster fan at key floor registers to improve air flow
• See DIAGNOSTIC GUIDE A/C or HEAT PUMP for more air conditioner problem diagnosis steps
• Also see RETURN AIR REGISTERS & DUCTS and the suggestions listed
  
  at RETURN DUCT AIR LEAKS.

How to Increase Cool Air Flow From an Air Conditioning System

Please see COOL AIR SUPPLY IMPROVEMENT

How to Get More Warm Air Supply from Heating Systems

Please see WARM AIR SUPPLY IMPROVEMENTS

Ductwork Requirements Rules of Thumb & Tables

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 [1]; similar tables are widespread through HVAC literature and resources cited atReferences or Citations

Notes to the table above

• 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 [1]

Notes to the table above

• Reference: op cit.

Notes to the table above:

• Reference: op cit.

...

Continue reading at SUPPLY DUCT AIR LEAKS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• LEAKY DUCT CONNECTIONS
• AIR LEAK NOISES
• AIR HANDLER / BLOWER UNITS - home
• AIR CONDITIONING & HEAT PUMP SYSTEMS - home
• FURNACES, HEATING - home

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