This has been a long journey, so I apologize up front for the walls of text. Short story is, when our whole-house (carbon) filter goes through its regen process, it will occasionally result in an extremely loud water hammer that lasts for about 5 seconds, then we have massive amounts of air in our lines.

Our house was built in rural Missouri in 2004. It has 650' well with a 2 HP submerged pump at 500'. We purchased the house in 2020 and had a new water softener and whole-house carbon filter installed professionally by a local reputable company.

Soon after that install the problem I described started. The regen runs at 3a - imagine the hilarity of how scary the water hammer was the first time it happened... The water treatment guys and the well guys know one another, so, after a year of the water treatment guys troubleshooting the carbon filter, they talked to the well guys and suggested a service call.

The well guys pulled the pump and found a broken shaft. They also determined that the wrong flow rate pump was installed for the depth of the well and replaced the motor/pump unit with a new one at a lower flow rate. The original pump was a 15 GPM and they replaced it with a 12 GPM. The water treatment guys also changed the inlet regulator from a 6 GPM to a 3 GPM. The pressure switch and gauge were replaced and the pressure tank in the house adjusted as well, though it is the original tank from when the house was built.

Unfortunately, the problem remains. I'm looking for advice on what I might be able to do to help provide better data or even fix myself. The water hammer is violent and I'm concerned about the damage it may be doing to our water system.

What I really don't understand is where the air is coming from. The pump is submerged and the pressure tank has a bladder. The water treatment guys are telling me that the pump isn't kicking on quickly enough and is sucking the pressure tank dry, which is introducing air into the system. I just can't wrap my head around that.

I appreciate any thoughts / experiences you may have.



2 Answers 2


This is a long shot, but it's worth considering.

You have a backflushing carbon filter. It's possible to use a water softener's control valve (the mechanical gizmo on top of the cylinder) for that purpose by reprogramming the valve for just two steps: backwash, then fast rinse to re-pack the carbon bed.

But what if the installer simply left the default regeneration program for water softening (as mine did!) on your carbon filter? That would be backwash, brine draw, slow rinse, fast rinse, brine refill.

Of course there is no brine tank to draw from on a carbon filter, but if your control valve is going through a brine draw step, there's a venturi in there trying to draw brine into the cylinder. And if the brine line is not properly plugged, what does the venturi draw? AIR!

So you get a slug of air in your carbon tank and a tremendous water hammer when the program moves to the next step, slow or fast rinse.

Does your carbon filter control valve have a digital display that shows what it's doing when it regenerates? Can you manually trigger a regeneration cycle? If yes to both, I suggest you trigger a regeneration cycle and watch what the unit does. If it simply does a backwash and fast rinse and it's all done in 10-15 minutes, this theory is all wet. (Sorry.) But if the regeneration takes over an hour and the display shows "brine draw", your tech has some 'splainin' to do. In that case, the brine line fitting should be plugged and the unit should be reprogrammed for only the regeneration steps that a carbon filter needs.

  • A very reasonable speculation, IMHO. I can't see it sucking air from the well, when the pressure tank empties water just stops flowing, it doesn't magically suck air from nowhere. The only way you get air out of a 500 foot deep pump is when the water in the well is at 501 feet.
    – Ecnerwal
    Commented Dec 3, 2022 at 15:51
  • If a brine draw step is occurring but the brine line fitting is well and truly plugged, the culprit could be a worn O-ring on the brine draw valve. That could allow air to sneak in via the venturi but water under pressure in the opposite direction -- brine fill -- could be stopped. If the control body is the 2-part type, undertightened assembly screws could be allowing air into the venturi as well.
    – MTA
    Commented Dec 3, 2022 at 16:16
  • I do believe that my softener and filter are the same heads with different tanks. My filter does have a "brine" tank that it sucks from. It's a much smaller "bucket" that has a water/bleach mix in it. We dump a gallon of bleach in it each month. We definitely see differences in the fluid level in that bucket, and it has a "straw" with holes. In the past we have noticed that those holes are above the fluid level after then problem has occurred. The fitting on that line also had a crack in it at one point, which the water treatment guys thought was the problem (they've been out a lot)...
    – Jaso
    Commented Dec 3, 2022 at 16:56
  • 1
    @Jaso Well that's it then, your problem is solved. The "brine" line and all the holes in its "straw" must be submerged at all times, or it will draw air into the carbon tank during regeneration. There's your water hammer! If budget allows, get a real brine tank with a float and cutoff valve so if the bleach level falls below minimum, the valve will close and air won't get sucked in. Otherwise replace the holey straw with a frit or filter basket and secure it at the bottom of the tank. Then you need to keep after the bleach level and make sure it doesn't get too low.
    – MTA
    Commented Dec 3, 2022 at 19:25

The water treatment guys are telling me that the pump isn't kicking on quickly enough and is sucking the pressure tank dry,

IF that's true, it's trivial to fix. Just need to lower the pressure on the bladder 2-3 PSI so there's water in the tank as the pump starts. But if that were true, it would be an issue any time where was high water use and the pump started, so it might not be true.

i.e. stock advice is either 2 or 3 PSI below the pressure switch cut-in. But if the pump takes a long time to spool up, 4 or 5 psi below cut in might be more appropriate.

If you watch the pump cycle you'll either see the gauge drop to cut in and start rising to cut-out, [this is normal expected behavior.]

Or you'll see it drop to cut-in (or even slightly above cut-in), then drop further very fast, then rise very fast, then rise slower until cut-out. [this is a sign that there is more air in the pressure tank than is good for your particular system.]

I would expect that your well guys have set it properly, but that's how you can check that part. Which will put the ball back in the water treatment guys' court.

  • Thanks a lot for this info. I'll check the tank pressure and monitor behavior of the fill cycle.
    – Jaso
    Commented Dec 3, 2022 at 5:44

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