Cistern Vent & Sight Assembly
The Cistern Vent and Sight Assembly is a patented design performing
four important functions:
1 Provide an inlet for air, as water is being drafted.
2. Provide an exit for air, as the tank is being filled.
3. Provide a visual “drive by” indication of readiness.
4. Provide freeze protection.
Design Features:
1. Drafting: As water is being removed from
the tank it must be replaced with air. Failure to provide adequate
air intake, restricts proper water flow and/or may cause the tank
to implode due to negative vacuum. Air intake holes under the
bonnet, provides the needed air flow, thus permitting maximum
water flow while preventing damage to the tank walls.
2. Filling: After use, the tank must be refilled.
Air in the tank is displaced by water. The same vent area, used
for “suction”, is now used in reverse for “filling”.
Vent holes prevent pressure buildup, which could occur due to
rapid fill rates. The overall design of the underground tank system,
also allows both suction and filling operations to occur at the
same time.
3. Sight windows: Two clear windows about 12
inches long located opposite each other near the top of the unit.
These windows provide visual inspection as to the readiness of
the water supply. Visible in the window is a colored cylinder
attached to a float ball on the lower end of a rod. When an “all
green” is visible, the tank is “full” and ready
for use. If an “all red” is visible in the tank window,
then the tank was either not refilled after use or the tank has
a leak and the water level has receded below limits. A visual
combination of “red & green” on the colored cylinder,
would indicate a water level between the previous two points.
The amount of water required to “top off” the tank,
can be estimated by the distance shown in the window. Depending
on the neck design of the tank and the depth that the float ball
is installed, this window measures only the top 1 to 2 ft of water
in the tank. It is not designed as a full water depth gage but
as an indicator the tank is full and ready for use.
4. Freeze protection: It is essential plumbing
connected to underground tanks have some mechanism to prevent
freezing. It is very easy to overfill the tank, as water is replenished
after use. Overfilling, would cause water rise to the top of the
lowest extension and during freezing conditions, could possibly
prevent the use of the water supply or cause a rupture of the
plumbing.
As both the “fill” pipe and “suction”
pipe must be sealed to the tank, no avenue for drainage can be
established at these locations. The vent however, is designed
to provide drainage for the entire cistern. Four ¼”
diameter holes located around the bottom of the vent, provide
drainage should the system require it. Installation instructions,
call for a bed of gravel to be located around these holes. This
gravel, provides a “drain back” reservoir for overfill.
As water seeks its own level, this “drain back” reservoir
allows water, which may remain in pipes above the tank, to drain
down into the ground preventing freezing.
Flow Impact:
The 8" Cistern Vent is designed to work with large size suction
heads (4 1/2", 5" or 6”) having an open waterway
up to 6". The largest of these being a 6" male, having
a cross section opening of 28.27 sq. in.
An 8” Cistern Vent is recommended when using large size suctions,
with flows greater than 750 gpm. The larger opening will prevent
tank collapse, caused from excessive negative vacuum being generated
inside your tank. For smaller (2 ½” – 4”)
suction systems with flows less than 750 gpm, a 6” Cistern
Vent is permissible.
As water is suctioned from the tank it must be replaced by air.
Intake air holes are located under the bonnet in the very top of
the vent. This area contains 396 holes, each drilled .44" in
diameter having a combined cross section of 60.2 sq. in. Thus the
intake air ratio of the vent as compared to a discharge of a 6"
male head would be 2.13:1. In other words the “air intake
area” is more than twice the size of the “suction”
point. Should you use smaller diameter suction this ratio would
be even better. With better than a 2:1 intake ratio no restriction
in flow should occur.
A word of caution for areas where heavy snow may be
experienced. Additional height may be required to
assure the vent area remains above the snow cover. Prior to use,
a quick visual inspection should be made to assure no restriction
exist on the vent surface. Hazards to watch for are; snow, vines,
trash, vandalism, damage, and insect nest.
Suction Considerations:
Information on use of female dry hydrant heads.
A "Female" Dry Hydrant does not have the same waterway
diameter as does a “Male” Dry Hydrant. Due to the designed
wall thickness required for the female swivel, the water way is
reduced. For a 6” Female Dry Hydrant the actual water way
only measures 5” I.D. In order to get a full 6" waterway,
it will be necessary to use a "Male" Dry Hydrant.
While the smaller waterway on the female head would decrease flow,
as long as a 6” Suction Hose was used and 6” or larger
suction pipe was used, the overall flow loss will be minimal.
For additional information on Male vs Female suction, see “Dry
Hydrant / Information / Male vs Female” located on the
ETT web site.
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