Photograph of a water pressure tank air volume controlAir Volume Control (AVC) Replacement
How to replace a water tank air volume control

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Air volume control (AVC) replacement: how to replace the air volume control at a water tank or water pressure tank. What alternative AVCs or air volume control types work best on different water tank & pump combinations?

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How to Replace a Water Tank AVC - Air Volume Control Valve

Reader Question: Can I Replace a Round Diaphragm AVC with a Rectangular US Gauge Type 300L or 310WJ?

I was wondering where I could get a Rectangular Air Volume control with no copper tube – D.W. Abingdon VA


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 unit shown above and 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.

Also the mounting location of the gauge has to be correct - if your tank tapping is in the wrong place on the water tank, the float may not function properly. Check your water tank to see if there is a tapping in the right location. And see the AVC product instructions and check with the manufacturer's customer service before trying this swap.

US Gauge air volume control valveWatch 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.

The shallow-well U.S. Gauge 300L AVC, for example, does not eliminate the tubing connecting the device to the water pump.

On this gauge, (photo at left, U.S. Gauge product literature), includes a brass fitting at the gauge under-side. You can see it in the bottom left of the photo at left.

That brass fitting is an air inlet valve that connects through a plastic (polyethylene) tube to a snifter valve that is mounted on the well pump itself.

Depending on the AVC float position and operation, the snifter is a one-way valve that allows air into the pump and thus into the incoming water supply and into the water pressure tank. When enough air is in the water tank the float responds and closes the air inlet valve. Quoting from US Gauge's Type 300L AVC installation instructions:

Shallow Well Operation - Type 300L: When the water level is high, the float, secured to a float rod extending through a flexible dividing wall, opens an air inlet valve in the body of the control. The air inlet valve is connected through 48” long polyethylene tubing to a snifter valve on the pump and has no direct connection into the tank.

The snifter valve admits air into the pump but prevents the water in the pump from escaping back through it. Air is drawn into the pump through the air inlet valve, connecting tubing and snifter valve and carried out with the water into the tank.

This operation continues until the volume of air increases to the proper amount, at which time the float closes the air inlet valve, shutting off the supply of air to the pump. The optimum relationship between the volume of air and water in the tank is thus maintained.[1][2]

The deep-well U.S. Gauge Type 310WJ Air Volume Control, as we described and illustrated above at Rectangular Air Volume Controls, does not include this tubing connection for a snifter valve and its operation is a bit different. The Type 310WJ AVC for deep wells also uses a float inserted into the water tank interior. But there is no external fitting for a tubing connection.

A chamber inside the AVC uses an adjustable pressure relief valve to allow excess air to escape from the valve when the tank water level (and thus the float level) drops. That process explains the air volume control device "hiss" that we discuss at What is that Hissing Sound.

So if pressure in the tank exceeds the AVC pressure setting air is vented from the water tank. The device is adjustable between 15 and 40 psi, and is factory set to 25 psi. Quoting from US Gauge's Type 310WJ AVC installation instructions:

Deep Well Operation - Type 310WJ When there is an excess of air and the water level is low, the float opens the main valve of the control, permitting air to be vented to a chamber within the control. This chamber contains an adjustable pressure relief valve which exhausts the excess air to the atmosphere, providing the pressure in the tank is higher than the relief valve setting.

As the float rises with the increase in water level the main valve closes, trapping the remaining volume of air in the tank. The optimum relationship between the volume of air and water in the tank is thus maintained.[1][2]

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