Constant Cycling Variable Speed Freshwater Pump

[Excalibur5]

I have a fairy simple freshwater setup on my boat. Cold water only, two sinks and a watermaker flush tied into it, along with the ability to directly connect dock water. I completely re-plumbed the system a year ago with brand new piping (reinforced semi-clear hose).

The question is, I have a Jabsco 18750 series variable speed freshwater pump powering it all. Before re-plumbing it, I constantly fought a incessant battle with the pump constantly short-cycling every 30 to 60 seconds. I have rebuilt the pump with a new factory kit, replaced it with another new-old-stock unit I was able to source, and now rebuilt the pump and completely re-plumbed the water system to be sure my piping wasn't shrinking and swelling causing the pump to cycle.

It worked good for a couple months after the re-plumbing, but now it's short cycling again... Anyone had a similar issue with these pumps? Any recommendations? I'm about ready to tear it out and go to the old-school single speed pump with accumulator tank.

Oh yeah, no leaks anywhere in the system, and all air is purged out of the piping.

[Richard Harper]

I had the same problem with a Jabsco single speed pump. It would occur every few months and clear up in a day or two. I assume it was some crud stuck on a valve that eventually cleared itself. When the pump failed I replaced it with a variable speed unit, no problems so far and I really like the constant flow. I fill and flush (syphon) out my tank periodically to keep crud down.

[Dsanduril]

What Richard said. If you have no leaks and no air then there is most likely a "leak" backward through the pump from the pressurized side back to the suction side. The check valves/devices keeping that from happening are not fully closing/seating.

[Tellie]

Also, if this is tied into your watermakers fresh water flush you may be leaking small amounts of water through the fresh water flush coil. These are notoriously weak in many brands and if the pump used has enough pressure it will leak by the coil creating sporadic house pump engagement.. A quick test (Do this is calm water like a marina as your brine discharge thru hull will be open to the outside water) is to disconnect your brine discharge overboard hose at the brine discharge overboard thru hull. Direct the hose into a dry bucket and let it sit like this for a few hours. If the bucket has water in it there you go. If dry, back to the drawing board. Good Luck.

[Shrew]

If that is a Parmax variable speed pump, then do what I did and toss it out. My buddy currently has the same issue I had.

The pump would never completely come to pressure, but would slow way down to an amazingly slow and quiet 'tock, tock, tock" like a clock until it over heated and shutdown. Then you had to turn off the breaker, wait a few minutes then turn the breaker on again.

I gave up.

[BillKny]

The problem is that there IS a leak. It might be back through the pump, but something is leaking—somewhere.

Hopefully you have installed a pressure gauge somewhere in the system. That will show you. if you turn off the pump with the system pressurized the pressure will slowly fall.

Do you have a fine debris screen on the inlet of the pump to catch crud that can jam the check valves open?

[rwidman]

Originally Posted by BillKny

The problem is that there IS a leak. It might be back through the pump, but something is leaking—somewhere.

Yep, there is a leak. The check valve built into the pump can leak.

You can get around this by installing an external check valve on the inlet side of the pump. If it still cycles, you have a leak on the pressure side. If your boat has a transom shower, make sure it is turned all the way off.

[thinwater]

Add an accumulator. Water is non-compressible, and even the tiniest change in system volume will cause a large pressure change and cycle the pump. Earlier posts list many of the reasons. Including an accumulator in the system is better design. Vital if there is a hot water heater, but it will also smooth the operation of cold-only systems.