1 February 1999
Do not add the air exhausted from the separate room or local exhaust to the return air or
transfer it to any other areas.
2-5. FILTRATION. For administrative facilities, commercial facilities, and similar occupancies
where indoor air quality is of primary concern, filter the combined supply air, including return and
outside air, using a combination of 25- to 30-percent efficient prefilter(s) and 80- to 85- percent
efficient final filter(s) as determined by the dust spot test specified in ASHRAE Standard 52.
Where practical, provide separate filtration or other means to clean the outdoor air, typically
equivalent to that used for the combined air stream, prior to mixing it with the return air.
Separate filtration for the outdoor air will reduce the contaminants in the outdoors from entering
the primary air stream. Even in areas where the outdoor air is seemingly clean, low levels of
auto emissions, pollen, dust, etc., can accumulate on the interior of ductwork and plenums and
later cause inadequate air quality problems. Due to the decrease in system airflow as the
pressure drop across the filter increases, size fans for the "dirty" filter condition. This will ensure
that each fan has adequate capacity to deliver the design airflow as the filter becomes loaded.
2-6. DUCT DESIGN.
a. Use either the Static Regain or the T-Method method to design ducts for VAV systems.
Use round or oval prefabricated duct where feasible. Round/oval prefabricated duct reduces
leakage and friction losses, which correspondingly reduces the amount of conditioning and fan
energy used. The additional material cost for round/oval prefabricated duct is often offset by
reduced installation cost and time.
b. Ensure that duct design incorporates all features necessary to accommodate testing,
adjusting, and balancing (TAB). For example, provide adequate length of duct, both upstream
and downstream of fans and coils. Show the necessary fittings, transitions, test ports, etc.
required for successful TAB, for duct inspection and cleaning, and for damper access and
c. Do not use the following types of construction where the potential for subterranean
termite infestation is high:
(1) Sub-slab or intra-slab HVAC ducts.
(2) Plenum-type, subfloor HVAC systems, as currently defined in Federal Housing
Administration minimum acceptable construction criteria guidance.
(3) HVAC ducts in enclosed crawl spaces that are exposed to the ground.
(4) HVAC systems where any part of the ducting is in contact with or exposed to the
2-7. RADON. Include the following features in designs for all new facilities to reduce the
potential radon exposure to the facility occupants:
a. Seal all penetrations through foundation wall/slab in accordance with TM 5-805-6 or
with prefabricated seals designed for the prevention of radon entry.