Most air ducts here in Texas are located in the attic where the temperatures of even properly vented attics can reach 135 degrees.  it is for this reason we really don't like attic installations or

like to do duct repair after 12:00 noon during summer months; however, sometimes we must.

 

     New HVAC codes (2003) now require an insulation factor of R-6 for all flex ducts and an insulation wrap of 1 1/4 inch thickness for hard ducts.  the City of Fort Worth Codes permit this R-6 duct for 14 SEER and better new equipment installations, R-8 is required for any new system below 14 SEER.  This code also requires all ducts must be sealed with UL Approved tape and Mastic.

 

     The pictures here are of flex duct and depicts the proper sealing method with a substance called 'Mastic'.  This sealing method prevents air leakage from the supply ducts into the unconditioned space and more importantly prevents unconditioned and contaminated dirty air from being drawn into the return air stream which is then distributed throughout the home.  Prior to this code requirement flex ducts were secured with plastic or metal wire tie wraps and duct tape, (also known as temporary tape.)

 

     Any duct modifications and repairs should also include straightening the duct runs as much as possible, and verifying that they are the correct size.  Eliminating as many bends and turns in the ducts will reduce the static pressure created by the air as it flows through it.  Hard metal duct is the most efficient for air flow distribution, but due to it's installation costs, most homeowners and home builders select flex duct.  Flex duct properly installed will provide the correct amount of air distribution in any home and is an accepted practice.  Static pressure (measured in inches of    water column) is the resistance of air flow through the entire system that is created by  the ducts and all the other components of the system, this pressure measurement should be kept as low       as possible, ideally 0.5Hg. 

 

     Sealing the return air compartment in any system is crucial for both correct system operation and for eliminating dust, dirt, debris and unconditioned air from entering the return air stream. This is especially true under up flow systems (those located in a closet). - - - In fact it is now a Code requirement - - - "Return air systems must be sealed to prevent unconditioned air from entering the conditioned air stream". 

 

     An air conditioning system is designed with a basic 400 CFM (Cubic Feet per Minute) of discharge air flow (air into the home) per ton of cooling. No physics required to calculate 400 CFM out requires 400 CFM in, but there are simple rules of physics that do apply.  Every 2 CFM of air flow requires 1 square inch of opening for the return air to enter.  Example: a 3 ton system will produce 400 x 3 = 1200 CFM, 1200 CFM divided by 2 = 600 square inches of return air opening required.

See: Am I getting the SEER I paid for?( coming soon) 

 

     Without a properly sealed return air system (shown here is the compartment below a heater in    a closet) the blower will find the air it needs from anywhere and everywhere; between sheet rock walls, under the floor, even pulling hot attic air down between the sheet rock walls and mixing with the cool conditioned air.

 

    Heaters located in the attic can create similar problems, if the return air compartment and ducts are not sealed the blower will pull 135 degree attic air, attic insulation 'fines' and any other air born debris into your system and distribute it throughout your home, standard air filters will not catch all

of it.

 

      Regardless of where your heater (with the evaporator coil and blower) is located, it is important to have properly sealed ducts, return air systems and the proper amount of return air in order to have your air conditioning and heating system operate at it's most efficient capacity.

 

     To view a short video on return air duct sealing, visit HGTV PRO web site.