I have a ½ HP 115V/230V 1PH 60Hz water pump (inside my house) that starts and stops hard. It is connected to a simple on/off pressure switch. The noise at startup is jarring. The quarter-turn ball valve shutoff between the supply and the pump has now sprung a leak twice (first on the output side, now on the input side) which leads me to believe there is at least some water hammer effect in play when the pump cuts out (there is now a tiny jet of water for 1-2 seconds from the input-side leak when the pump cuts out, otherwise a steady drip). The water supply coming in to the whole system is probably at around 5 PSI from a shallow well via several hundred feet of 1½" ABS pipe.

Here's an attempt at a visual representation of the configuration:

  shallow dug well
  200-300' 1½" ABS
  ball-valve shut-off
    check valve
  ½ HP water pump
  ↓                  ↓              ↓ 
20 G pressure tank   ↓              ↓ 
                     ↓              ↓
         1" cold water manifold     ↓
                       1½" outdoor supply line
                           ball-valve shutoff
                           frost-free spigots

Are these problems that would be alleviated by a soft start water pump? If so, can the current pump be adapted to a soft start system or is a pump that is natively soft start required?

  • I am by no means an expert, but have you considered installing a water hammer arrester to alleviate what you suspect to be a water hammer situation? It would likely be far cheaper than replacing the entire pump, and certainly wouldn't hurt, even if you do end up having to replace the pump in the long run.
    – FreeMan
    Commented Oct 7, 2020 at 13:28
  • A diagram showing the relationship of the pump, pressure tank, valves and particularly any check valves might be very useful in getting to the actual core of your problem. There is a tendency to "throw check valves at" water pumping systems which can create water hammer problems. In a properly set up system the pressure tank acts as a very large capacity "arrestor" but if it's valved off by improperly placed check valves that won't work. The other possibility is that the pressure tank has failed and is waterlogged.
    – Ecnerwal
    Commented Oct 7, 2020 at 13:45
  • @FreeMan Yea, I have considered that, but I'm reluctant to believe that any additional plumbing complexity can't hurt (at least when I'm doing the plumbing). ;) Commented Oct 7, 2020 at 14:54

1 Answer 1


I would say a soft start controller would not change the problem I have many booster pumps and the only thing a soft start would be good for would be throttling the pump. Centrifugal pumps create a large hammer action in how they work.

A expansion tank may be your best option. The quality of the valve may also be the issue I can warn you I had a irrigation pump that when throttled to 45hz with a VFD (can be used as a soft start) that frequency would tear up 4” irrigation lines I could run fine from 50 up to 70 hz and had no problems (below 40hz the motor would over heat)

There is cavitation at start up on the pump this is normal but a soft start won’t stop that. Some water filters can not be used without a pressure tank or expansion tank as the vibrations tear them apart.

I did see the comment on the water hammer arrestor that may work but I would think a 2-3 gallon expansion/pressure tank would solve the issue better than a electronic method.

I thought I should add most 120/240v pumps are capacitor start with a centrifugal clutch pulling them in. A soft start in this case would probably overheat the motor they are designed to spin up quickly and change windings or the cap depending on the type.

  • With text diagram to look at: Add a small pressure / expansion tank to the input line before the check valve. You've got 200-300 feet of water (roughly a couple of hundred pounds) moving down that pipe which is in need of a "shock absorber" when the pump shuts off.
    – Ecnerwal
    Commented Oct 7, 2020 at 16:30
  • 1
    You ran a pump with a VFD... I love it so much... the problem with a VFD on a pump is that many structures, including hydraulic structures, are like a "tuning fork" in that they have one or more "resonance frequencies"... if one is inside the operating range of your VFD, you're gonna find it... Commented Oct 7, 2020 at 17:13
  • @harper that was true on my water pump but I have dozens of both hydraulic and water pumps running on vfd’s
    – Ed Beal
    Commented Oct 7, 2020 at 19:02

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