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.