ASK THE EXPERTS: Shallow-Well Pump

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The Dankoff Solar Force piston pump.
The Dankoff Solar Force piston pump.

The recent drought conditions in Eastern Ontario, Canada, are forcing my well pump to work overtime, which overloads my four 6 V batteries (550 Ah), wired for 24 volts. Normally, my water supply for this off-grid country home is supplied by an artesian well that flows by gravity to the cabin. That well is running too low due to the drought, and I am forced to pump from our lake. This pump is a standard 3/4 hp shallow well pump. The lift is about 22 feet and the run is 80 feet.

If the pump runs five to six times at night, the battery system is deeply discharged. What pumps could you suggest that would alleviate this problem?

Tom Windle • via email

A well pump that runs five or six times per night seems excessive for a single home. Motor-starting surges are wasteful and hard on your batteries. I recommend you use a 40-gallon pressure tank (or larger) to reduce the pump start-ups. If your tank is that big, test its pre-charge air pressure to make sure that it is set properly. Only then will it store all the water it can between pump cycles (one-third of the tank’s volume). It's OK to add a second pressure tank, and they need not match in size. A modern "captive air" tank is always best. You will recognize it by the tire-type air fitting on top.

But before you change anything, check the system performance as-is. Watch the pressure gauge while the pump is on. To observe its full cycle, run some water only until the pump turns on, and keep your eye on the gauge. The pressure should rise steadily without any sudden dip or spike until it reaches the maximum pressure—when the pump stops (the "cut-out" pressure). A dip or spike in the reading indicates improper tank pre-charge. If the pressure gauge just hangs there and takes "forever" before the pump stops, that indicates a worn or inadequate pump. You can find instructions on the Internet for adjusting pressure switches and tank pre-charge—it's easy if you're handy. For professional help, I recommend hiring a pump technician, since most plumbers are not trained to service pump systems.

You also might consider reducing your water pressure. Do you need to open any of your faucets to the max to get the flow you desire? If not, reducing the water pressure may satisfy your needs and will save energy. I have reduced some pumps’ daily run time by as much as half this way! To reduce the pressure setting, loosen the larger of the two adjustment nuts in the pressure switch, let water out until the pump starts, then observe the reduction of cut-out pressure. When you’re satisfied, reduce the tank pre-charge per normal instructions (with water at zero pressure) to minimize cycling.

On to pump alternatives: I assume you have a standard AC shallow-well jet-pump. The efficiency of those pumps is quite low, and the waste is compounded by a typical 10% inverter loss. A more efficient approach would be to use a 24 VDC shallow well pump that is made specifically for off-grid applications. Such pumps use about 40% of the energy used by a standard jet pump and inverter.

Two classic DC pumps are the Flowlight Booster Pump and the Solar Force Piston Pump by Dankoff Solar Pumps (dankoffsolarpumps.com). (Although I am recommending products from my old company, I no longer have any commercial interest in it.) A low-voltage pump requires larger wire than you would normally use. If it is difficult to run new wiring, you can get an AC version (using about 50% of the energy of your jet pump, with a much reduced starting surge). Another possibility is to use a DC/AC high-efficiency submersible pump. The Grundfos SQFlex pumps are efficient and have a soft-start motor that eliminates the starting surge (grundfos.com). A submersible will never freeze and does not need priming.

In conclusion, if your present system is not performing properly, make any adjustments, repair, or improvements that seem economical. If your water use is low, that may solve your problem. The greater your water consumption, the more economical it will be to get a more efficient pump, especially considering your northern climate and the modest size of your battery bank. I suggest you contact an experienced supplier of off-grid solar power and water systems to help you decide.

Windy Dankoff • Founder (retired), Dankoff Solar Pumps

Comments (1)

Donald Giles's picture

I am using a Solar Force 3020 48 VDC panel direct pump to move water from one tank to another tank that is on a ridge. The discharge head is about 65 psi and the suction pressure is always positive about 4 psi.
This unit came with a small accumulator pressure tank as shown in the photo in this article. I have broken two of these pressure tanks off at the fitting where the nipple connects. It happens because this system is a portable trailer mounted unit and is moved a couple of miles over a gravel road.
My question is: "How necessary is the pressure tank when the conditions are as I have described?" I am now running without the pressure tank and the pulsation seems a bit more but not much. I realize that a possible fix would be a flex line from the pump fitting to the tank and remotely mount the tank on the trailer rail.

Thanks
roadrunner@canyontraders.com

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