RESIDENTIAL DUCTWORK SPECIFICATIONS

THE FOLLOWING SPECIFICATIONS ARE BASED ON (ACCA) AIR CONDITIONING CONTRACTORS OF AMERICA, RECOMMENDATIONS

Proper ductwork is critical to the efficiency of a forced air heating and cooling system. If the ductwork is not made and installed properly you will have uneven temperatures, noise from air movement and an inefficient system regardless of the efficiency of the furnace or air conditioner  that is installed with it.

Ductwork shall be made from good quality galvanized duct.  The minimum thickness gauge shall be 26.  Ductwork with a perimeter of 64 inches or larger shall be 24 gauge.  The aspect ratio shall not be more than 3 to 1.  

The ductwork shall be sealed with an approved duct sealer at all joints and all seams.  It will be necessary to seal snap-lock seams in ductwork.  If the ductwork has Pittsburgh seams these seams will not have to be sealed.  A good quality foil tape may be used on square or rectangular duct if approved by the local code authority and the owner.  Approved duct sealer is to be used for round to square duct connections.  Duct leakage shall be 2% or less.  Owner reserves the right to have ductwork leakage test performed.

If ductwork is located in an unconditioned space (attic, crawl space, etc.) it shall be insulated with two-inch thick Foil Scrim Kraft duct wrap.  The duct wrap shall be sealed vapor tight with approved foil tape.

If an air handler supplies different floors there shall be dampers in the main trunk line duct to control airflow to the different floors or zones.  All branch supply ducts shall have double bearing dampers to control airflow.

SUPPLY DUCTWORK VELOCITIES AND PRESSURE

The main duct supply velocity shall not exceed 900 feet per minute.  Branch duct velocity shall not exceed 600 feet per minute.  Supply register velocity shall not exceed 500 feet per minute.  In no case shall the static pressure of the supply ductwork exceed .10 inches of water column pressure.

The main return air supply velocity shall not exceed 700 feet per minute.  Branch return air velocity shall not exceed 400 feet per minute.  Return air grill velocity shall not exceed 400 feet per minute.  Velocity across a media filter shall not exceed 400 feet per minute.  In no case shall the static pressure of the return air ductwork exceed .08 inches of water column pressure.

NOISE REDUCTION (OPTIONAL)

Ductwork shall be lined within 6 feet of the air-moving device (blower coil, furnace).  The duct liner shall be ½ inch thick liner and shall be fastened with welded pins and the appropriate mastic.

RESIDENTIAL DUCTWORK DESIGN

Heat load calculations (heating and cooling loads) shall be performed using Air Conditioning Contractors of America (ACCA) “MANUAL J” (preferably version 8) load calculation system.  This load calculation system is the standard for residential heating and cooling. Information for this load calculation system is derived from American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE) information.

Heating and cooling loads shall be performed on a “room by room” basis so the heating and cooling loads for each room are calculated.

Ductwork shall be sized for each room based on the cooling and heating requirement for each room.  Airflow for cooling requirements should be based on 400 CFM per 12,000 BTU per hour.  A room that needs 3,000 BTU per hour of cooling should have 100 CFM and a room that needs cooling of 9,000 BTU per hour should have 300 CFM of airflow.  Airflow for heating of each room should be proportional to the total of all of the rooms.  Airflow for heat pump systems should be based on 450 CFM per 12,000 BTU per hour.

If supply registers are placed in the floor it is recommended that return air registers be placed high and low on the wall.  Dampers on the high return registers should open in the summer and closed in the winter.  Dampers on the low return registers should be open in the winter and closed in the summer. 

As a minimal requirement return air grills should be placed high for one story or ranch homes,  on multi-level homes the return air grills should be placed high on the upper levels and low on the lower levels as a general rule.  There are exceptions with multi-level homes, depending upon room layout.

Return air grills are not installed in kitchen and bathroom areas so the return air system adjacent to those rooms needs to be increased to handle the additional airflow from those areas.

On tight homes, it is recommended that a heat recovery ventilation (HRV) system be installed. energy recovery ventilation (ERV) is not recommended in this area because the heating season is longer than the cooling season. Combustion air should be brought directly to the furnace and or boiler from outside of the house.