Photograph of  this ugly duct routing risking water entry, mold, rodents, high operating cost.Duct System Defects
Heating and Cooling Duct Work Inspections for Adequacy, Safety, Defective Materials, Asbestos, Mold

  • DUCT SYSTEM & DUCT DEFECTS - CONTENTS: Diagnostic List of Heating & Cooling Air Duct Defects: Things to Inspect, Check, Repair, or Replace - Air Conditioning or Heating Air Duct Defects: how to inspect, find, diagnose, and repair problems with air ducts on HVAC systems. HVAC Supply & Return Air Duct Leaks & Obstructions - Theory vs Practice. Theory: The D'Arcy-Weisbach formula. Practice: List of Common Air Flow Obstructions in HVAC Systems & Ductwork.Heating & Cooling air Duct installation defects, safety hazards. Return air adequacy or lack of return air can prevent adequate warm or cool air flow in a home
  • POST a QUESTION or READ FAQs about Air Duct & Air Flow Problems on Heating & Air Conditioning Systems

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Defects in HVAC duct systems:

This article explains how to inspect and diagnose trouble with heating or cooling air flow, air ducts, air registers and zone components of heating and air conditioning (HVAC) duct systems.

This diagnostic article describes common defects found in HVAC duct work such as incomplete supply or return ducts, poor choice of location or size for supply and return registers, locating ducts in problem areas, leaks into and corrosion of duct work, leaky duct connections, and special notes on product failures by two manufacturers of flexible duct work.

The article includes important safety warnings about dangerous openings in HVAC duct work.

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Diagnostic List of Heating & Cooling Air Duct Defects: Things to Inspect, Check, Repair, or Replace

We explain how to find and correct air duct problems 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 such as Goodman gray flex-duct, some Owens Corning Flex-duct, and asbestos-containing air conditioning or heating duct work.

Article Contents

Air conditioning duct system defects include a remarkably wide range of errors, from failure to supply cool air or failure to even circulate air in the building, to health hazards such as use of asbestos material in or on duct work, to very dangerous conditions such as drawing heating equipment combustion gases into the building cooling (or heating) 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. Asbestos in or on HVAC ducts is a possible hazard for which we provide links to a separate document - see "Asbestos HVAC Ducts" at below left.

Below we list some common sources of poor air flow from air conditioning or heating duct systems: check these first before ordering a more costly survey or repair job.

HVAC Supply & Return Air Duct Leaks & Obstructions - Theory vs 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 D'Arcy-Weisbach formula

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)

ρ = density (kg/m3).

List of Common Air Flow Obstructions in HVAC Systems & Ductwork

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,

    See DIRTY AIR FILTER PROBLEMS - 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.
  • Fire damper that has become stuck in the closed or partly-closed position interferes with proper airflow through the system
  • Flex duct defects: Collapsed sections of flex duct block or stop airflow in either supply or return air system -
  • Insulation loose in air ducts: Collapsed duct interior insulation,
  • Leaks in the supply air duct system are a very common HVAC duct defect that results in poor heating or cooling air flow.


    Illustrations of a leaky cast iron sewer line that sent sewer gases into the building's transite (asbestos cement) heating duct system can be seen at TRANSITE PIPE AIR DUCTS

  • 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.
  • Transite air ducts: Crushed or collapsed transite duct
  • 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.



Continue reading at AIR FILTERS for HVAC SYSTEMS or select a topic from the More Reading links shown below.

Also see our explanation of problems with reduced air flow, dirty air handler blowers, blocked cooling coils, blower leaks and mold, frost and ice build up in ducts, and blower sizing problems of air conditioning retrofit projects which we discuss at: AIR HANDLER / BLOWER UNITS

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DUCT SYSTEM & DUCT DEFECTS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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