ETL 1110-3-438
15 SEP 93
to create negative pressure fields under the floor. Provide a suction
stack connection, using non-perforated PVC pipe, stubbed through the
floor slab and capped. The suction stack should be near the middle of
the under slab pipe run and be located where it can be extended
through the building roof with minimum changes of direction.
Requirements for letter codes B, C, and D are identical except for the
spacing for under slab pipe runs. In structures with basements or
other below grade spaces, connect the sub-slab piping to the
foundation drainage system so that the negative pressure field is
extended to the earth side of below grade walls. Where the foundation
drainage has gravity outfall, provide an interior suction stack and a
water trap as shown in Details 31 and 32. Where foundation drains
discharge to a sump, seal the sump cover and provide a suction stack
connection to the sump as shown in Detail 33; large buildings will
require additional suction stacks remote from the sump. Building
sites with seasonally high ground water may require a high water
sensor at the suction stack connection to shut off the suction fan.
Radon gas is water soluble. It is reasonable to assume that a
properly designed sub-soil drainage system will remove radon from
sites with severe ground water problems.
(3) Passive Suction Stack, Letter Code E. Extend non-perforated
PVC pipe suction stack through the roof with as few changes in
direction as possible. Passive stacks are more effective in colder
climates and during winter months than during warm weather. During
warm weather they will be most effective in naturally ventilated, non
air conditioned buildings.
(4) Active Suction Stack, Letter Code F. Extend the suction
stack as described above for the passive stack and install an exhaust
fan near the stack discharge. preferred location for the exhaust fan
is above the roof. Condensation of moisture in soil gases will be
minimized if the fan is located in a heated space, however, any air
leakage that may occur at the fan discharge connection will blow
concentrated radon into the building. Any air leakage at the fan
intake connection will reduce the suction in the stack.
(5) Design of Sub-Slab Suction System. Pipe sizing for the
suction system should be designed similar to any exhaust system
utilizing round duct. The under slab perforated pipe should be no
larger than 4 inch diameter. In very large structures, do not
increase the pipe diameter in order to increase the length of pipe
runs, this may reduce uniformity in the pressure field. The
quantity of air flow in a sub-slab suction system is effected by a
number of variables. The most significant are porosity of the soil,
permeability of the capillary water barrier, and air leakage from
the building into the soil. In most locations, the air flow per
1,000 sq. ft. of slab area will be 20 cfm or less. Multiple under
slab pipe runs may be cross connected below the floor using non-
perforated pipe of sufficient diameter to maintain uniform pressure
in all pipe runs. Multiple stacks may be connected to a common
exhaust fan. Pipes for stacks and any manifolds will be sized
4
A-6
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