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WATER PUMPS, TANKS, TESTS, WELLS, REPAIRS
WATER CONSERVATION MEASURES
WATER CONTAMINANT LEVELS
WATER FILTERS, HOME USE
WATER HAMMER NOISE DIAGNOSE & CURE
WATER ODORS, CAUSE CURE
WATER PUMP REPAIR GUIDE
WATER PRESSURE LOSS DIAGNOSIS & REPAIR
WATER PUMP SHORT CYCLING
WATER SOFTENERS & CONDITIONERS
WATER TANK REPAIR PROCEDURES
WATER TANK: USES, TROUBLESHOOTING
WATER TESTS, CONTAMINANTS, TREATMENT
WATER TREATMENT EQUIPMENT CHOICES
WELLS CISTERNS & SPRINGS
WELL CHLORINATION & DISINFECTION
WELL FLOW RATE
WELL WATER PRESSURE DIAGNOSIS
WELL YIELD IMPROVEMENT
WINTERIZE A BUILDING
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.
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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.
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.
US Gauge & Similar Type 310WJ, Type 300SL, or Type 6 Rectangular Air Volume Controls = Excess Air Vents
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 our FAQs section below 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.
How does a US Gauge Type WJ or similar air volume control work? Drainback 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,
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 below at AIR VOLUME CONTROL TEST
Details about snifter valves have moved to SNIFTER VALVES since this article on AVCs was becoming long and unwieldy.
A summary excerpt is just below. Snifter valves are a two-part air volume control system designed to maintain the air charge in a well water pressure tank on some submersible pump systems where a bladderless water tank is installed.
Snifter valves, used only on submersible well pump systems (the pump is inside the well), are a type of automatic air volume control system that adds a charge of air into the water pressure tank during each well pump on-cycle.
Snifter valves provide an air inlet, typically located on a check valve on the water line near the bladderless water pressure tank. The air inlet work in concert with a water drain-back valve (a bleeder orifice) that you cannot see - located on well piping inside the well where a submersible pump is used.
If the well pump is a submersible unit (located in the well) and if also the water pressure tank is a non-bladder type, there could also be one or even two air inlet valves installed on vertical section(s) of well piping inside the well.
Our photo at below left (Rasmussen Well Drilling, Inc., & DJ Friedman) shows the tee where the well service company worker has removed the air inlet vent detailed in our photographs found below.
The vent(s) are installed at a tee just above the top level of the static head of water in the well so that air can enter the vent during pumping cycles.
Our well piping air volume control valve photographs below show the simple design of this well piping air inlet control valve. When the submersible well pump is off, pressure against the ball inside the air inlet valve is released, the ball can move away from the valve opening, and air can enter the valve. When the submersible well pump is on (pumping water), pressure against the ball inside the valve body closes the air inlet valve.
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.
Definition of Schrader Valves & Dill Valves
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 or Dill valve (two 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.
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.
Here is the air volume control patent text 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 at AIR VOLUME CONTROL TEST.
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 at AIR VOLUME CONTROLS, HISSING.
Details about AVC or air volume control adjustment are now at AIR VOLUME CONTROL ADJUSTMENT
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.
How to Repair a Leaky US Gauge Type WJ or Type 300L Drainback System or Snifter Valve System Pressure Tank Air Volume Control
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. At above left a photo of a US Gauge a Type 6 Air Volume Control showing its operating float (that you cannot normally see as it is inside the water tank).
This control is used on shallow wells and jet pumps.me controls are at AIR VOLUME CONTROLS, HISSING
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 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 at AIR VOLUME CONTROLS, GET RID OF
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).
Continue reading at WATER TANK AIR VOLUME CONTROL REPAIR or select a topic from the More Reading links shown below.
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Frequently Asked Questions (FAQs)
Question: Why Can't I Find the Air Volume Control on My Water Pump or Water Tank?
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?
Reply: your well water system may not have an AVC, or the AVC may be hidden on the well piping inside the well
Bladder type water tanks (WATER TANK BLADDERS & CAPTIVE AIR) 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.
Hidden AVCs that may be found inside the well are discussed at Air volume control valves located inside the well
Question: does it matter where the AVC is installed?
Does it matter where the AVC is installed? i have a 1/4" opening 3/4 up from the base,probably used for pressure gauge,rather then use the 1 1/4" opening 2/3 the way up. - Schnitzl
Reply: Yes - here are some details about proper AVC location
Yes the AVC has to be installed at the proper location on the tank relative to the tank's moving water level - take a look at the instructions that come with your unit and you'll see that information, usually in the form of a sketch.
An opening up 3/4 of the way from tank base is probably OK but I'm not sure - it may depend on your particular type and model of Air volume control device. They vary.
In sum, the location of an AVC depends also on the type of device. For example on some submersible pump wells air volume in the pressure tank is controlled by a Snifter Valves mounted on well piping just ahead of the pressure tank, working in concert with a piping drain/vent located actually inside the well itself.
Question: water pressure is surging, bladderless pressure tank, what to do?
my water pressure is surging and i have a bladderless tank. Any tips? no drain at bottom of pressure tank! - Craig
Reply: check for short cycling water pump diagnosis & solutions
I if your water pressure is surging in synch with the well pump turning on and off your water tank is probably water logged and needs its air charge renewed.
Indeed if your water system included an air volume control device (AVC) that has stopped working, the pressure tank may have become waterlogged.
Question: is changing a water tank AVC twice a year normal?
A Friend of mine has to change his AVC about twice a year is this normal? - Tom
Reply: no, look for debris clogging or improper installation
Not in my experience, Tom; if the AVC keeps needing replacement I wonder if there is debris in the water that's clogging it up. Or maybe an improper installation?
Question: Air is coming out of my water pressure tank - is that normal?
I have a lot of air coming out of the pressure tank (a bladderless with an WJ Type ACV). There is an adjustment screw on the ACV. Which way do I turn it to stop the surging of air into the household water? - Dave M.
Reply: hissing at some air volume controls is normal excess air escaping
Dave, there are adjustments on some air volume control valves, but to be confident we know what action to take we need to diagnose the cause of the excess air. For example, if you are constantly getting air blasting out of plumbing fixtures the root cause could be a leak in well piping - not something to fix by just adjusting the AVC.
Indeed some AVCs will vent excess air in the pressure tank. This design is particularly common on water pressure tanks whose air volume control is maintained by a snifter valve (see SNIFTER VALVES) because the snifter valve system can indeed push more air into the pressure tank than needed. So if your water system uses a submersible well pump with a snifter valve, you will probably find a rectangular type AVC control on the side of your water pressure tank at about mid tank height, and it may occasionally vent air - that's actually normal, as we explain at AIR VOLUME CONTROLS, Hissing.
Question: I replaced my old water tank - what was that old 310WJ for - do I need it?
I just replaced my 20yo galvanized bladderless tank with an identical one from the same mfr. (yes, still being made). I was impressed with the longevity of the old tank - I called her "Old Ironsides". I simply copied the identical piping configuration onto the new tank with new fittings.
Two fittings I "scabbed" from the old tank however, were the 310WJ (as pictured above with attached air pressure gauge) and a brass Flowmatic check valve with two inlet ports. The two inlet ports (on the tank side of the check valve) were being used for a air inflator stem (with cap) and the Pumptrol electric well control valve.
Over the years, I have just emptied the old tank completely with a garden hose annually and been happy enough with the performance.
My question is: I did not know that the 310WJ was anything more than a pressure gauge until I removed the old one and saw the rusted off float arm, apparently long since useless.
According to your description, the 310WJ was to allow excess air to drain out when too much was put in by an air inlet. The only place an air inlet could have been would have been the air inflator stem, which I thought was only for adding air by mechanical means. It really does just look like an ordinary tire pressure stem and not like anything automatic. Did I misunderstand what this was all along by keeping the cap tightly in place?
This is more just for my understanding than the thought of me trying to resurrect the old air volume control valve scenario - I can't imagine the old 310WJ would last very long in any event. Also, my old system (now rebuilt) absolutely had no pressure relief valve unless one is hidden along with something else - on the output side is just a boiler drain for draining the system along with a common stop valve before being connected directly into the house plumbing. Should I worry? - Dan 3292
Reply: bladderless water tank + submersible pump + snifter valve: need changes when replacing the water tank
in the article above (see SNIFTER VALVES) we describe a type of air volume control that was used on some wells that use a bladderless type water pressure tank along with a submersible pump. Because during well pump on and off cycles, the snifter valve system (there is a companion device in the well) can admit more air into a pressure tank than needed, that rectangular device on the side of the old pressure tank included a vent that would automatically vent out excess air when needed.
Because you replaced your old bladderless pressure tank with a new bladderless tank, if you intend to continue to rely on the snifter valve system, you'll want that air vent to work properly or your water system may begin to see too much air. (AIR DISCHARGE at FAUCETS, FIXTURES)
Of course if the in-well valve or the snifter valve have quit working, you can go back to manually adding air to the water pressure tank when needed, but honestly it's better if that chore is handled automatically.
Watch out: for readers replacing water pressure tanks: if you are changing from a bladderless water pressure tank on a submersible pump well system, check to see if your old system included a snifter valve system that needs to be properly removed - details are at SNIFTER VALVES
I have owned the house and well pump tank for 17 years now - only 3 years after the sticker on the tank said that it was installed by our local well driller. I don't think I ever heard the 310WJ AVC ever hiss to release air in all my years here, and the snifter valve (that's a new one on me) has had it's cap tightly screwed down for years - even with teflon tape around it as it used to leak water on the floor. This tank has been relocated around the basement maybe four times (all by me).
That said, I am really doubting how much the air volume control ever worked. I have never experienced excess air in the tank to the degree of getting air coming out of the faucets - more the reverse that I got short cycling of the pump from not enough air, so I started an annual program of draining the tank in the fall. I see that the AVC 1-1/4" opening on the tank is approximately half way up, implying that tank is meant to basically ever be half full of water, half full of air - another revelation. So my tank which is marked 42 gals. pneumatic actually only holds about 20 gals. of pressurized water?
Question: Water tank leaks when the pressure drops off
Hello, I was wondering if someone could tell me what this is on our 82 gallon pressure tank. When the pressure drops it leaks water at around 40 # . It also sounds like it is sucking in air. This only happens when i run the sprinklers. The well will put out about 12 gallons a minute. But i have the pressure set at 45 and 65. It only takes 8 gallons of water to drop the pressure from 65 to 45 and that is when the pump comes on. Is this some kind of automatic valve? is there a way i can stop the spitting of water or is that necessary.
I do not believe this tank has a bladder. It looks like it has a schroeder valve in that thing on the side of the tank. But that is out side the well house on the well side of the check valve along with the pressure relief valve. I thought the pressure relief valve should be on the tank side of the check valve. I do not think much of the man who installed this 3 years ago as he wants nothing to do with it now, So i need to figure this out. I will be changing the heads on the sprinklers to use less water. But the largest zone only uses 10 gallons a minute.
Your photo shows a rectangular type air volume control on a water pressure tank - details and more photos of that device are found in the article above, at WATER TANK AIR VOLUME CONTROLS
Your valve does not include a tube connecting it to the water pump as other air volume controls sometimes used because your model uses an internal float that opens and shuts a valve to allow air to enter the water tank. I often find these valves just stop working, because of debris clogging or because the float is stuck. You can leave the valve in place and add air manually or you can replace the device.
Question: air surging out of faucets after tightening up the air volume control
I recently had to repair a small pinhole leak in the cold water line to my kitchen sink. To do so I had to shut off the well pump/water supply. After the repair I turned everything back on and then discovered a dripping leak from the air release valve on the tank's WJ type AVC and water spraying out from the snifter valve. I tightened up both valves to stop the leaks and thought all was fine but a day or two later we began to get air surging at the toilet and other fixtures.
This seems to occur for a minute or two after the pump starts up. I have not noticed short-cycling and the pressure gauge cut on and cutoff points seem ok(32 and 52psi respectively)and the tank seems to hold pressure ok as well. Any ideas/suggestions would be greatly appreciated. Thanks. Tim B. 5/3/12
Reply: replace the leaky air volume control on the water tank
Sometimes when we tighten fittings on the air volume control to stop a water leak we are also stopping the valve from being able to purge excess air - sending that excess air out at the faucets. If the water system uses a submersible pump and an in-well snifter valve that admits air into the system, that design can admit more air into the pressure tank than is needed. If the excess air can't be released at the aboveground AVC it heads for the faucets. Try replacing the leaky AVC that you found on your water tank.
I have a 60 gal vertical tank. after a lot of water use , it contains only air, which then comes out of facets . draining tank cures it for a while. what can i do? - George Cain 10/15/2012
Question: short cycling waver pump, plastic bowl type AVC
My water pump cycles on for about 8 seconds then off for about 35 seconds. I have replaced the (plastic bowl type) Air Volume Control. I thought it was ok but it is back to cycling too often. I have checked for tank leaks with soapy water but found none. What do you think would cause this?
Where does the AVC get the air it puts into the tank? Could the line from the pump to the AVC be restricted? - Wayne 12/24/12
The air source depends on the air volume control type - byt typically from a vent on the control.
Question: should I scrap the old unused water tank
We have one of these in our crawl space - it hasn't been used in over 40 years - I suspect it is much older. It looks very much like the picture above (Horiz_Tank_PumpBefore_022_DJFss) Should I just scrap it out for the metal or is there any value in it? - Karen 2/16/2013
Karen, if the water pressure tank is no longer connected and thus really no longer in use there is no reason to keep it around - it's scrap metal. Just be sure someone takes a careful look to make sure that the tank is not connected to working plumbing before it is scrapped.
Question: I don't see how to adjust the US Gauge Type air volume control
I have read through all of your pressure tank articles searching for some details. I am not the first to ask this question as I see where others have asked the same question but never answered. I don't think the correct answer is switch to a bladder tank as bladderless with the volume control can be found new for sale in most plumping stores in my area at high altitude mountains. It is very simple.
Thanks Mike, I reviewed the article above and added details on adjustment as well as repair of these air volume controls.
Basically, if you're getting air hissing out of valves and fixtures in the home, and provided that your water system indeed is using an air volume control like the US Gauge float type controls above, the device is intended to vent excess air from the system. Air is entering elsewhere at a snifter valve or similar device, usually on the vertical water pipe riser inside the well. You may be able to correct the problem by simply following the adjustment procedures in our article above.
Question: what are normal pressure gauge readings on a water tank?
(Apr 10, 2014) Anonymous said:
what is the gauge reading on a deep well sopose to be
Anon, the water pressure gauge reads water pressure, not a function of well depth. The pressure will range between the pressure switch cut-in and cut-out pressures, typically 20/40 psi or 30/50 psi.
Question: what does the float do in a water tank?
5/26/14 robert said:
What does the float do in a water pressure tank
The "float" in a water pressure tank is probably referring to the small float that actuates an air volume control on a water tank that does not use an internal bladder.
For details see
in that article where we discuss the US Gauge & Similar Type 310WJ, Type 300SL, or Type 6 Rectangular Air Volume Controls = Excess Air Vents
Question: air coming out of faucets after fixing a pump
2/1/2014 Author: Rchard (no email)
Comment:i have a bladder tank; and i recently changed the well pump after 25 years of use. Since then air has infiltrated my system with spurts of air.Any advice would be truely appreciated. Thanks!!
about air discharge in the system after changing a pump, I would look for an air leak into the system or a bad air volume control, or a snifter valve that should not be in use with a bladder tank.
To see diagnostic details
Question: changed bad AVC, now no water pressure, pump won't come on
(Mar 27, 2014) email@example.com said:
I have changed bad AVCs before but this time is a mystery. My AVC was discharging water and needed replacement so I cut power to the well, drained some water off the 200 gal galvanized tank, disconnected the tubing at the AVC, unscrewed the AVC from the tank and installed the new AVC. I did not completely drain the tank becacuse there was water coming out of the fitting when I took out the old AVC.
There is almost zero water pressure in the house, looks like the tank has water in it because I removed the AVC again just to see if there was water in the tank. Reinstalled AVC but no water to the house.
What did I do wrong. When I checked the new AVC installation, I took off the copper tubing and water was shooting out of the AVC so I screwed it back on. Pump won't come on because pressure is already at 65 lbs and no water to the house or out of the hose at the bottom of the tank. HELP !
Try turning off power and draining the tank completely. Then close tank drain and turn on the pump - assuming its a submersible pump, right?
You did make sure that water was turned on between the tank and the building, right?
Question: hissing at the air volume control?
(Apr 8, 2014) AVC - WJ said:
I just install a new one. If my cut-in pressure is 30 and my AVC unit is set to 25, I should hear hissing out this until the float closes the valve, correct? I don't hear a thing ever. The only thing it could be is the water level never goes down far enough?
You didn't tell us the brand and model air volume control, WJ; in some cases you will never hear air hissing unless there is excess air in the pressure tank.
t's a AMETEK-WJ