TM 5-852-4/AFM 88-19, Chap. 4
flatter roof pitch or greater overhang, or other means, it
the necessary guidance is available when needed. If
may be necessary to employ plenum chambers and
dampers are installed they should be placed on the
200
upwind side . Eliminating downwind dampers allows
stacks or chimneys as shown in figures 4-26, 4-27 and 4-
convection currents to remove warm air from ducts even
28. If other considerations should make it essential to
with the upwind damper closed. Buildings with ventilated
rely on ventilation flow parallel with the predominant wind
foundations have greater potential rates of heat loss in
direction and it is not feasible to elevate the intake and
winter than structures resting directly on the ground
exhaust sufficiently, fences or shrubbery may be installed
because they are exposed to the cold air on all their
upwind of the structure to induce drifting at that position
surfaces. Special care is therefore required in insulation
and thus to minimize drifting close to the building itself.
and heating. This is further discussed in d below.
Sometimes snow ridges pushed by snow plowing
(l) If snow drifting is not a problem or if
operations may be counted on to induce drifting in
the intake and exhaust can be elevated so that snow
desired locations. Extreme distance of drifting behind a
drifting will not interfere with air flow, structure orientation
snow fence is about 25 times the height of fence.
should be such that maximum velocity and effectiveness
Principles of snow drift control have been outlined by
77
Mellor . The use of completely closed skirting around
of air circulation will be obtained, combining both thermal
foundations constructed with open air spaces must be
and wind-induced effects. If wind is of significant
avoided. Open picket-type skirting around foundations
strength and consistency in direction during the freezing
has been successfully used to permit ventilation, while
season it is then desirable to orient air intakes into the
preventing significant snowdrifting under the building,
wind and to configure the exhaust end of the system to
keeping children and animals out of the air space, and
maximize wind-induced suction, whether wind-induced
beneficially
modifying
the
overall
architectural
draft is taken into account in design computations or not.
appearance of the building. If used, such skirting should
In many areas, however, this may not be practical
be elevated sufficiently above the ground to avoid
because winter winds are too light or too variable in
damage due to frost heaving. Wire mesh may also be
direction and velocity; in such cases possible benefits
employed over openings to foundation air spaces but the
from wind-induced drafts should be ignored in
mesh openings should not be smaller than about 2
developing the system design.
Where wind-blown
inches.
precipitation comes from variable directions, rotating
(k) For maximum effectiveness a
wind-oriented ventilator units may be employed at tops of
foundation cooling system utilizing natural low winter
exhaust stacks to minimize snow ingestion and
temperatures should be shut off in the spring when air
maximize draft. Exhausts should terminate at positions
temperatures reach such a level that circulation of the
on the lee side of the building.
ambient air through the system would add to the summer
(m) Thermal analysis of simple
heat input into the foundation. Turning off the system in
ventilated foundation. The depth of thaw under an open
the spring and turning it on again in the fall is necessary
air space type ventilated foundation may be
for systems using forced circulation, or stack or chimney
approximated from figure 4-4a for certain homogeneous
systems. However, experience shows that when such a
soil and moisture content conditions. For situations not
system is dependent upon manual opening or closing of
covered by figure 4-4a, the depth of thaw should be
ports or dampers, or turning electrical switches on or off,
calculated by means of the modified Berggren equation
and these operations are required only twice a year, the
for either a homogeneous or multilayered system as
necessary actions may be forgotten or may be carried
applicable, using procedures outlined in TM 5-852-
14
6/AFM 88-19, Chapter 6 .
An n-factor of 1.0 is
out incorrectly. Experience also shows that system
applicable for determining the surface thawing index of
operating manuals are easily misplaced. Therefore,
the shaded area under the building, under either
whenever possible, designs should be selected which
approach.
are automatic in operation and do not require specific
(n) Thermal analysis of ducted
manual actions. Fail-safe differential thermostat control
foundation. No simple mathematical expression exists to
systems can reduce these problems, but still require
analyze the heat flow in the case of a ventilated floor
dependable power supply, checking for proper operation
system consisting of a duct or pipe system installed
of controls, and resetting of circuit breakers. Reliability
within or at some depth beneath the floor, with air
becomes less in relatively complex systems which
circulation induced by stack effect. The depth to which
involve numerous dampers, blowers or other elements.
freezing temperatures will penetrate is computed by
Therefore, simple ventilated foundations or through-duct
means of the modified Berggren equation except that the
systems of the type illustrated in figure 4-25 which are
air-freezing index at the outlet governs. The index is
entirely free of control mechanisms requiring setting are
influenced by a number of design variables, i.e., average
far preferable. Where required, operating directions for
daily air
dampers, switches, etc. should be stenciled directly
adjacent to the particular control element to ensure that
4-40
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