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Photograph of  a 1930's supply duct in active use. 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
  • POST a QUESTION or READ FAQs about HVAC supply air ducts & registers
  • REFERENCES
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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.



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Common Duct Defects that Impact Air Flow Rates

Article Contents

How to Diagnose Stains at Ceiling Air Supply Registers

Photograph of supply register with leak stains at a ceiling. Photograph of supply register with soot or debris stains at a ceiling.

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:

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

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).

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.

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:

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 at REFERENCES

Gas Furnace Heating Duct Requirements
Furnace Input BTUs Square Inch Duct Size Needed
for both supply & Return Duct
40,000
80 sq.in.
60,000
120
80,000
160
100,000
200
120,000
240
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]
Oil-fired Warm Air Furnace Duct Sizing Requirements
Output BTU’s Square Inch Duct Size Needed
for both supply & Return Duct
Min. CFM Airflow
45,000 to 55,000
100 sq.in.
500
60,000 to 70,000
140
700
75,000 to 85,000
170
800
95,000 to 106,000
190
900
106,000 to 115,000
220
1100
125,000 to 150,000
280
1400
Reference: op cit.
Relationship of Round Duct to Square Inches to CFM Air Flow to Heating
BTU’s
Duct Diameter Duct Square Inches CFM Airflow Supported Heating BTUs Capacity
6-inch
28 sq.in.
100 cfm
7,400 BTUs output
7
38
145
10,700
8
50
210
15,600
9
64
290
21,500
10
64
390
28,900
12
113
620
45,900
Reference: op cit.

Continue reading at SUPPLY DUCT AIR LEAKS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see LEAKY DUCT CONNECTIONS

Or see AIR LEAK NOISES

Or see AIR HANDLER / BLOWER UNITS - home

Or see AIR CONDITIONING & HEAT PUMP SYSTEMS - home

Or see FURNACES, HEATING - home

Suggested citation for this web page

SUPPLY DUCTS & REGISTERS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

INDEX to RELATED ARTICLES: ARTICLE INDEX to ARTICLE INDEX to HVAC DUCT SYSTEMS

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