HVAC Air Distribution
Location of Air Intakes
Air intakes shall be located to prevent contamination supply air from any pollutants or nuisance odors. Special care must be taken to prevent the introduction of building, fume hood, generator, automobile, or general exhaust, sources of moisture or odors from landscape materials, dumpsters, etc. Intake openings shall be protected with ½” x ½” corrosion resistant hardware cloth installed inside the louver and when necessary, painted to match.
The designer shall indicate on the duct plan drawings areas of room pressures with the use of arrows and CFM values at doorways.
Incorporate necessary attenuation strategies to minimize noise in occupied spaces. ASHRAE’s noise guidelines are considered to be the maximum acceptable noise levels.
Building HVAC systems must be able to operate under the established requirements of the centralized chilled water, steam, and heating hot water systems.
- Bends in the flex duct shall be no less than one duct diameter centerline radius. Flex ducts shall extend a few inches past sheet metal prior to bending.
- Provide hard elbows or three straight duct diameters at diffuser connections.
- Flex ducts shall be supported at least every 5 feet.
- Flex ducts shall sag less than ½” per foot.
- Flex ducts shall be installed fully extended and not in the compressed state (including pinched between other building components).
- The hanger material in contact with flexible ducting shall be a minimum of 1.5″ wide.
A separate minimum OA damper is required when the minimum OA flow less is than 25% of supply air flow. The minimum OA damper shall be located above the maximum OA damper.
Duct fittings shall not have excessive pressure drops. Fan system effects shall be avoided. Excessive pressure drops shall be approved by UNC Engineering Services.
Duct fittings shall not have angles of divergence greater than 30 degrees.
Duct-board is not permitted.
Outside Air Economizer
Outside air economizers are required.
Assure water will not pond on horizontal surfaces. Round or oval duct is generally preferred.
For clothes dryers, provide smooth aluminum or stainless steel exhaust duct with long radius elbows.
All ductwork shall conform to SMACNA HVAC duct construction standards, metal and flexible, latest edition.
Ductwork Leakage Test
- Duct Leakage Testing: 100% leakage testing shall be provided for all duct 4 i.w.c. or greater. For laboratories, provide 100% leakage testing of 2 i.w.c. or greater ducting.
- Duct Seal Class: Provide SMACNA Seal Class A on all metal duct 2 i.w.c. or greater. In no case shall the ductwork sealant be less than SMACNA Seal Class B.
- Testing shall be completed before the installation of duct insulation. If duct is insulated before leakage testing is completed and leaks are found, it shall be the responsibility of the contractor to remove insulation from the entire section of the leaking duct, repair the leaks and replace the insulation.
- Perform the following field tests and inspections according to SMACNA’s “HVAC Air Duct Leakage Test Manual” and prepare test reports.
- Do not pressurize systems above maximum design operating pressure.
- Provide the designer, UNC Engineering Services and UNC Building Services at least seven days’ advance notice of testing.
- Maximum Allowable Leakage: Maximum leakage shall be 1% of total cfm delivered by the air moving device(s).
- Remake leaking joints and retest at contractor’s expense until leakage is equal to or less than maximum allowable.
Comply with NFPA 90A, 4.3.4 ‘Air Duct Access and Inspection’. In addition require the contractor to provide a minimum 144 sq. inches of access to the damper on ductwork larger than 14″ on one side. On smaller ductwork specify a 2 ft. long removable section of duct for access.
A safe means of maintenance access shall be provided for all duct damper actuators and damper access doors. A clear line of site shall be provided for inspection of all damper actuators.
Provide a schedule on the drawings specifying the maximum leakage of access doors as follows:
|Duct Size||Minimum Door Size||Maximum Leakage|
|< 18″||12″ x 6″||0.064 cfm|
|18″ to 28″||18″ x 10″||0.133 cfm|
|> 28″||21″ x 14″||0.206 cfm|
|Body access||25″ x 17″||0.286 cfm|
Access doors shall be shown on the drawings.
Provide a safety chain for doors accessed by ladder. Provide grab handles for doors 18″ x 10″ and larger when there is a positive pressure greater than 3 i.w.c.
Specify long-life closed-cell gaskets.
Long-life gaskets and blank-off panels shall be provided to prevent the by-pass of air around filters.
Filters shall be standard sizes of 2′ x 2′ or 1′ x 2″. Filter face velocities shall be 500 fpm or less.
Provide a minimum of MERV 8 filtration upstream of air-flow measurement stations and energy recovery coils.
Furnish three complete sets of filters (pre-filters and final filters) for all AHUs. Prior to any testing or start-up of AHUs; the first set shall be installed. The second set shall be installed after testing and balancing, but prior to acceptance of the building. The thirds sets shall be delivered to UNC prior to acceptance. The first set may remain in the equipment during TAB and acceptance if the filters are less than 50% loaded and accepted as is by UNC.