How to Add Air to a Water Pressure Tank - Air Volume Control
WaterAPedia ^©

	Enter your search terms Submit search form
	Search This Website - InspectAPedia.com

This article describes how to add air to a building water pressure tank and it reviews the function and repair of water tank air volume controls in a building water supply system where a private well is the water source. Readers of this document should also see Water pump and pressure tank repair diagnosis & cost an specific case which offers an example of diagnosis of loss of water pressure, loss of water, and analyzes the actual repair cost. The illustration at page top is courtesy of Carson Dunlop, Inc. in Toronto. The round air volume control photographs shown here and just below are courtesy of Ben Langston.

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.

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. Makeup air is needed in these water tanks because over time the air charge in the tank is absorbed into the tank's water.

The most common versions of air volume controls look like a disk of about three inches in diameter, as we show in the photographs here. A copper or plastic tube that extends from the AVC to a fitting on the water pump itself, as we showed on the photograph above. Look at the fitting screwed into the side of the water tank about 2/3 of the way up from the tank bottom and on the right side of the water tank in the photograph above.

In that picture as well as in this closeup of an AVC you can also see the small diameter copper tube which connects the air volume control valve to a fitting (usually) on the water pump itself so that the valve can respond to the cycling on and off of the water pump (and changes in water pressure).

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. 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.

Does the air volume control valve work? Can it be removed? In 40 years of messing with plumbing I've never found one of these that works reliably for long. An air volume control on a water pressure tank can be left in place but don't count on it to work. If the air volume control valve itself is leaking or fittings on its tubing leak, you may decide, as do many plumbers, to simply remove it. Just screw in a pipe plug where it was mounted on the water tank, and a smaller (usually brass) plug may be needed at the point of connection of the other end of the copper tube if one was installed on your system.

(You'll have to turn off the pump and release water pressure and drain some water from the tank before replacing an AVC with a new one or with a pipe plug.)

If your AVC is not working or has been removed, and if your water tank is a non-bladder type system, you'll need to use one of the other methods discussed here for adding air to your water tank when it's needed. Forget this method for maintaining the proper air charge in a water tank, except for the Leaky Air Volume Control tip below.