Photograph of an inadequate, mis-located single return air duct for a building apartment. Return Air Registers & Ducts
HVAC Return Air Duct Install, Inspect, Troubleshoot

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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

Photograph of one-way return air on basement AHU also may draw flue gases from nearby gas fired equipmentThe 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.

Photograph of one-way return air on basement AHU also may draw flue gases from nearby gas fired equipment

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.

Also see our safety warnings

Attic Air Handler Air Conditioner Return Duct Defects

Photograph of one-way return air on basement AHU also may draw flue gases from nearby gas fired equipment

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.

In addition, 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

Disconnected ductwork (C) D FriedmanPressure 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.

Blocked air return duct (C) D Friedman M Burton

Suspended Ceilings & Drop Ceilings Used as Return Air Plenums

Drop ceiling return plenum (C) D Friedman

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

Mouse in suspended ceiling (C) D Friedman


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.

Or see RETURN AIR REGISTER & DUCT FAQs - Q&A posted originally at this pagbe

Or see these

HVAC Return Air Improvement Articles

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