Fresh Water Pump/ Will not pressurize

Edwin G Littke

New Member
Sep 30, 2018
6
Boat Info
1996 Sundancer
Engines
Cat 3126
My 1996 450 Sundancer has two fresh water pumps. The number 1 pump seemed to have very little pressure, just tiny amount of water would come out, so I replaced it. The new pump still does not pressurize and pump water. The number 2 pump works fine all by itself. Looked for leaks and cannot see any. I did notice there is an accumulator tank in the system. Not sure what that is for and maybe contributing to the problem. Any thoughts are much appreciated. Thanks!
 
How does the pump cycle (run time) in comparison to the 2nd pump? Higher cycle time would indicate a restriction or cavitation in the low pressure side of pump. Shorter cycle time would indicate there is blockage or possibly a kink in the high pressure circuit.
A bad electrical connection could also cause a voltage drop at the motor which would reduce it's efficiency. You would have to check power at the motor with it loaded (running) to identify that. A bad connection can look good with a volt meter but open up under load. If it is tough to check that then find a 12v 70-100W bulb and plug it in instead of the pump. It should be bright.
 
Are both pumps connected to accumulator? How many outlets does #1supply? I have seen accumulator tanks rust out from the inside. Shut off pumps, do you still have pressure at faucet? Presuming #1 gets most of the usage perhaps remove faucet aerator to look for deposits.
 
How does the pump cycle (run time) in comparison to the 2nd pump? Higher cycle time would indicate a restriction or cavitation in the low pressure side of pump. Shorter cycle time would indicate there is blockage or possibly a kink in the high pressure circuit.
A bad electrical connection could also cause a voltage drop at the motor which would reduce it's efficiency. You would have to check power at the motor with it loaded (running) to identify that. A bad connection can look good with a volt meter but open up under load. If it is tough to check that then find a 12v 70-100W bulb and plug it in instead of the pump. It should be bright.
The #1 pump does not pressurize so it does not turn off.
 
Are both pumps connected to accumulator? How many outlets does #1supply? I have seen accumulator tanks rust out from the inside. Shut off pumps, do you still have pressure at faucet? Presuming #1 gets most of the usage perhaps remove faucet aerator to look for deposits.
The #1 does not get used cause it does not pressurize
 
Sounds to me like you just haven't/need to prime the pump since you changed it out?
 
I just started experiencing the exact same issue. I just turn off the second pump at the panel and the first pump is sufficient. Mine started when I ran the water tank empty. The second pump will not pressurize and runs continuously. I will have to troubleshoot at a later time. Interested in seeing how this thread unfolds.
 
I don't have this set up, but I"m curious...are the pumps in parallel or series? Are the pumps identical in terms of their rated capacities and pressures? I would think that they are.

Here is a generic pump curve.
Pump Curve.png

In a single pump operation, the flowrate is inversely proportional to the differential head supplied by the pump. Higher flow; lower pressure. My guess is that on the bigger boats with higher fresh water demands and further distances to pump, it is cheaper to install two smaller pumps rather than one large one. I would also guess that they are in parallel and are of the same rated capacities. So here are some things we know from the curves. In a single pump operation, if the flow is reduced from Q1 to Q2, then the pressure has to go up from P1 to P2.

Now think of a two pump in parallel operation. Pump #1 can operate by itself until the flowrate is approaching its maximum and thus its pressure is approaching its minimum. Somewhere, above that minimum pressure (represented by P1 above) the second pump should kick on. Now, here is where it gets interesting. Because the pumps are in parallel, the pumps will be putting out the exact amount of pressure AND (assuming they are the same size pumps), each pump will be outputting one half of the total required flowrate. In other words, Q1= 2 x Q2 in the above illustration.

With regard to the Pressure Switches, it helps me to think of them as a High Pressure Limit switch. I want the pump(s) to shut off when the pressure is above X.Y psig. Then, the converse is true in that I want the pump to run when the pressure is below X.Y psig. Pump #1 should have the highest pressure setting and Pump #2 should have a pressure setting below Pump #1 but it has to be above the pressure that Pump #1 will achieve at maximum flowrate...which is P1 on the graph. I show these graphically by the red lines on the graph below.
Pump Curve 2.png

Jaybeaux
 
Is the filter plugged? That was my problem
 

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