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Guide to air volume controls on water tanks:
This article describes water tank air volume controls (AVCs) used to keep a proper air charge in a water pressure tank and thus avoid well pump turning on and off to frequently.
We describe what AVCs look like, we explain the types of air volume controls used on jet pumps and on submersible pumps, and we describe where to find them, and how these devices work, and how they can be repaired, replaced or just abandoned.
In water pressure tanks that do not use an internal bladder, over time, the air in the tank will be absorbed into the water and the tank will become ‘waterlogged’. This means that the tank is full or nearly full of water.
The pump will come on and off very quickly (short cycling or rapid cycling water pump).
This short cycling is hard on the pump, and air is added to the tank to correct the situation. - Adapted with permission from Carson Dunlop Associates Home Reference Book.
[Click to enlarge any image]
The job of the AVC is to keep air in the water tank to avoid waterlogged water tanks or well pump short cycling.
The air volume control or "AVC" device mounted (usually) on older steel water tanks (ones that do not use an internal bladder to keep water and air separate) is designed to automatically add air to the water pressure tank when it's needed.
If a water tank loses its air charge it stops working properly and usually results in the water pump turning on and off rapidly - "short cycling" of the water pump. This condition, in turn, can damage or even burn up the water pump.
An "automatic" air volume control (AVC) device may be installed on the water pressure tank if it's an older, bladderless steel tank.
The AVC is intended to automatically put a little bit of replacement air into the tank from time to time as water pressure cycles up and down, that is, each time that the water pump runs.
If you see a round steel disc of about 4" diameter and about 1" thick on the water tank, mounted perhaps at the middle of the tank height or at the water outlet to the tank, or perhaps on the side of the tank with a plastic or copper tube connecting the disc to a fitting on the tank or nearby piping, this is the AVC..
Used on jet pump water supply systems, on each pump "on" cycle the AVC draws in a small volume of air that is then pushed onwards into the water pressure tank.
The tube connecting the AVC to the pumping chamber suction side provides the pressure drop that pulls on a diaphragm on the AVC that in turn causes the AVC to draw in its air charge.
The air volume control might be found on the side of the water tank (above left) and is typically connected to the water pump itself (above right) by a flexible copper tube.
The AVC may also be located right on top of a one line or two line jet pump such as is visible at the top center in our Meyers™ well pump photo at left.
The alternative air volume control shown below does not include a disc-shaped device and has no connection to the water system's pump or other piping. The hole seen in the end of the brass screw is the air inlet for this air volume control.
The deep-well AVC below does not use copper tubing, all it's parts are self-contained. This is a valve that you may hear hissing if it's working.
Watch out: As we discuss
at SNIFTER VALVES below,
in general these air volume control valves function only to release excess air in the water pressure tank;
in that case air is obtained from other components (such as the snifter valve). They let air out. Air is let into the system using a separate device, usually a snifter valve discussed below.
At WATER TANK AIR VOLUME CONTROL DIAGNOSTIC FAQs we provide more details (and photos) explaining the difference between the US Gauge Type 300L or Type 6 AVC and the US Gauge Type WJ or Type 310WJ air volume control.
Using a US Gauge Type WJ or similar air volume control to manage air in the water pressure tank is a different approach that is unfamiliar to so many homeowners that we've seen some odd advice like "take off the part and throw it away" - which is usually not a good idea.
The air volume control approach used with these valves is also called a drainback system. When water level falls inside the pressure tank to a level below the valve and its float, the valve opens, bleeding excess air out of the pressure tank.
Separately, a snifter valve (see below in this article) in the well piping (usually hidden inside the well) is the device that forces air into the water piping from the well and thus into the pressure tank at each pump on-cycle.
Our photo (below left and right) shows a U.S. Gauge Type WJ - so we know this installation is for a deep well.
As U.S. Gauge points out,
U.S. Gauge Air Volume Controls are designed for domestic water supply systems which deliver a quantity of air to the pressure tank with each cycle of pump operation.
Insufficient air in the pressure tank causes frequent operation of the pump.
Too much air in the pressure tank will permit large bubbles to be carried into the piping system. This causes a disagreeable noise and sputtering at the faucets.
It is the function of U.S. Gauge air volume controls to maintain the correct relationship between the volume of air and the quantity of water in the pressure tank.
As we illustrate below, this air volume control device uses a float that moves as the water level inside the water tank changes. The float movement allows air into or out of the water pressure tank as needed.
Adjustment of the US Gauge type WJ AVCs is in a separate article found
At DRAIN BACK / SNIFTER VALVE TROUBLESHOOTING we detail how the AVC is tested on a drain-back system or bleed-back system is tested to see if it is in fact releasing excess air as it should.
An excerpt is given just below
Air Volume Control Valves on drain-back pump and water systems are one component of a three-part system designed to protect well or lake water supply pipes from freezing and to maintain the air charge in a water pressure tank on some submersible pump systems where a bladderless water tank is installed.
A snifter valve is installed at a check valve at the water tank (photo at left) and works with a drain-back valve located in the well or lake below the frost line to insert air into the water piping system for freeze protection.
Because the snifter valve & drain back valve will result in a large charge of air pushed back up the water piping and into the water tank at each water pump on-cycle, the air volume control used on the pressure tank must release the excess air to avoid over-charging the water tank with air.
At DRAIN BACK / SNIFTER VALVE TROUBLESHOOTING we detail how these drain-back system or bleed-back system components are tested, including the AVC on the water pressure tank.
An excerpt is just below:
If the air volume control is working on a drain-back system you will at least occasionally hear air hissing out of the excesss air release port on the AVC (blue arrow in the photo at below left).
If the air volume control is not working on a drain-back system, because each pump on cycle is pushing a lot of air into the water tank you will ultimately see air spurting out of the building's plumbing fixtures such as faucets. That's because the AVC is not releasing the excess air in the system.
If the AVC is not working properly on a non drain-down water systems you will find the opposite problem: a water-logged pressure tank causing well pump is short cycling.
Watch out: short cycling can still occur on a drain-down (bleed-back) system too, but not because the AVC isn't working.
Rather, if the snifter valve stops admitting air into the piping system OR if the drain-down valve becomes clogged and stops draining water out of the water piping, then because air isn't entering the piping no air is being pushed back up into the bladderless pressure when the pump runs.
Eventually the water tank will become water logged and the well pump or lake pump will cycle on and off rapidly when water is run in the building.
Watch out: On a drain-down / bleed-back water system with either of these two problems, because the well piping is not able to drain and not able to fill itself with air at the end of each pump-on cycle, there is also a risk of frozen well or lake water piping.
If your drain-down system pump is short cycling and your water piping between well (or lake) and the water tank is not buried below the frost line, you may find you have two problems: you had a warning of trouble when the well pump was short cycling, and now you've got frozen pipes too.
Watch out: if on a submersible well pump system that uses a snifter valve for air volume control you later convert a bladderless water pressure tank to a tank using an internal bladder, you should remember to remove both the snifter valve located on the check valve near the water pressure tank and the bleeder orifice or drain-back valve located on the well piping.
See SNIFTER & DRAIN BACK VALVES for a complete discussion of drain back systems, snifter valves, drain back valves and their related components.
Definition: Schrader valves (American Valves) and Dill Valves are used principally on vehicle tires to insert (or release) air to a specified pressure.
The valves are also used, often in modified form with an internal spring with strength matched to the application where air inlet or outlet or water or air pressure management require adjustment on water pressure tanks and similar systems.
The Schrader valve, Brady Valve, Dill valve ( different companies) consists of an externally and internally threaded metal stem.
The external valve base and stem connect (using appropriate seals or washers) to the vehicle wheel rim, or in plumbing to a water pressure tank or water piping at an appropriate location.
The internal stem of the valve accepts a replaceable part, the actual control valve that opens (when a center pin is pressed) to admit air or to release air. In reverse, pressure inside the system pushing against the valve stem core closes the valve when its center pin is not depressed. 
Similar control valves but of different diameters are used in other countries than those comprising North America.
An example of a special class of these air valves is the snifter valve discussed just above and used on drain-down water systems. This valve includes a low-pressure valve stem core that will open to admit air into the valve at about 10 psi. Special versions of snifter valves can operate with as little as 5 psi.
Details are at SNIFTER & DRAIN BACK VALVES
A different range of air inlet valve is used at other (non-drain-down) water systems at the water pressure tank to add or adjust the air charge in the pressure tank. Photos of a typical air inlet valve located at a water tank tee is shown above along with a typical valve stem core. These may be found on both bladderless and internal-bladder type water tanks.
Watch out: when replacing an air inlet valve stem core be sure to select a valve core whose opening pressure (to admit air) is properly-matched to the application. Installing an automotive tire valve stem core into the stem of a snifter valve will make you sorry.
On some water storage tanks such as the antique 450-gallon tank shown at left (courtesy of reader Craig Revill), the air volume control may not be so obvious (photo below right).
The device shown is (we should say "was") an air volume control produced by Penn Electric Switch Co., Des Moines Iowa.
You could guess at the function of this device even if the manufacturer hadn't generously given an identification tag: notice the small diameter brass tube connecting the water tank to the well pump.
The patent for this air volume control switch was published in 1943 and patented by Burton E. Shaw. At left we illustrate (from the patent application) just how this air volume control switch worked.
Here is the air volume control patent text [PDF] and an explanation of how this AVC worked.
Shaw explained this air volume control or AVC was intended as a deep well device - by this he probably meant not shallow or "dug" wells.
The device was intended to be mounted in the wall of a water storage tank (as above) and adjacent to the water level in the tank so that the valve could respond to both air pressure and the water level in the tank itself.
The valve relieved air from the tank when the water level was low as a result of excess air from the pneumatic head of the tank, but it would prevent the release of air from the tank when the working pressure inside the tank was below the required amount.
Details about how to inspect & test the AVC to determine if it is working to keep a proper air charge in the water tank are
Excerpts are below.
If the air volume control valve is working properly, it uses the pressure changes caused by the cycling on and off of the water pump to automatically add air to the water tank when it's needed.
If the air charge in your water pressure tank is not being maintained, either there is a leak in the tank or the AVC is not working. Usually the case is the latter.
If an auto-venting AVC is installed (a type that expels excess air charge in the water tank) and if the AVC is working you will occasionally hear air hissing out of the fitting, as we discuss
Details about AVC or air volume control adjustment are now
Note: On a water tank that uses an internal bladder (keeping water and air separated), the air charge is not normally lost and the air volume does not normally need adjustment.
Our complete article about repair procedures for AVCs - Air Volume Controls - is now
at WATER TANK AIR VOLUME CONTROL REPAIR. An excerpt is below.
Recapping, both types of these air volume controls are used only on bladderless water pressure tanks - tanks that do not use an internal bladder.
This control is used on shallow wells and jet pumps.me controls are
Details about replacement options for AVCs are now found
at AIR VOLUME CONTROLS, REPLACE. Excerpts are below.
U.S. Gauge makes rectangular type AVCs, as we illustrate above. So do some other companies.
Watch out: rectangular AVCs like the U.S. Gauge AVC + Gauge unit illustrated here from the company's product literature, incorporates a float inside the water tank. If you look closely at our photos you 'll see that the gauge mounts through a 1 1/4" diameter ANPT threaded pipe connection into the water tank.
Watch out: also to be sure to order the proper air volume control model. For example the U.S. Gauge AVC Type 300L is designed for shallow well operation, and the U.S. Gauge Type 310WJ Air Volume Control is designed for deep well operation.
These devices do not work in an identical fashion, so buying the wrong model for your well would be a mistake.
Details about getting rid of the AVC as well as some advuce about making that change a success are
If you convert from a non-bladder type water pressure tank to a water tank using an internal bladder, part of that installation will include the removal of any air volume control valves on the system, including an AVC that may be mounted on the well pump (above-ground jet pumps) or a hidden AVC that is found inside the well piping (submersible well pumps only).
I've looked all over my water pressure tank for that round disk thingie that you show in the photos in this AVC article but I just can't find it, nor do I see that rectangular version that is sometimes on the tank side. Where is it?
Bladder type water tanks do not use an air volume control valve: Air volume control valves are present only on steel water tanks which do not include an internal bladder to keep water and air separated inside the water tank.
In other words, if your water tank is one of the newer models which uses an internal bladder, you won't find an AVC installed.
See details at WATER TANK BLADDERS & CAPTIVE AIR
A bladder-type water tank keeps the air charge separated from the water. The air is in the tank and the water is inside the bladder inside the tank. Thus the air charge does not become lost by absorption into the water.
Hidden AVCs that may be found inside the well are discussed
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