For many years I have heard form owners of open loop geo systems that pumping cost quickly takes a back seat to the cost of continually replacing the pump system. With variable demands form a house/geo combination, regular pumps using pressure tanks, would cycle themselves to death on a regular basis. It doesn’t take long to figure out that spending an extra 20 dollars a month on electricity and having a pump system last 20 years, would be far less expensive than saving 20 bucks a month on electricity using pump systems that must be replaced every 3 to 5 years or less.
In the last 10 years or so I have been hearing these same complaints from people who are using variable speed pumps. I have been working with variable speed pumps for more than 20 years, so I understand them very well. Whether you want to believe it or not, variable speed pumps were designed as a cash cow for the manufacturer, not to save you money. Manufacturers claim they save energy, which is the “hook” that snags most people. The reality is VFD’s always increase the energy used per gallon. Even when running at full RPM, the parasitic losses of a VFD cause you to get fewer gallons per kilowatt used.
There is nothing more efficient than a properly sized standard pump, running at it’s best efficiency point, and drawing pure sinusoidal power directly from the grid. The VFD control itself uses extra energy, and the harmonics and stray voltage they produce cause the motor to be about 5% less efficient. Then when you have a unit drawing 1.5 HP while producing 30 GPM, it is using more energy per gallon when slowed to produce 5 GPM and still drawing a 3/4 HP load. 1.5 HP producing 30 GPM is drawing .05 HP per gallon. The same pump slowed to 5 GPM and still drawing a 3/4 HP load is using .15 HP per gallon produced. This is 3 times more energy used than a properly sized pump.
However, the biggest expense is because of “planned obsolescence”, which is the main reason manufacturers have for designing variable speed pumps. They can more easily predict and plan the length of time for failure of a pump system. With standard pumps using pressure tanks, the number of cycles would determine the life of the pump system. Manufacturers had built in enough quality for the pump to cycle an average of 7 years before it failed. Then things like Cycle Stop Valves came along that reduce the number of cycles considerably, and would triple or quadruple the life of pump systems. This was completely unacceptable to pump manufacturers, so they quickly devised a plan to use variable speed technology that had been used in industrial applications since 1968, to compete with the constant pressure performance of the CSV in residential applications.
They designed little Driemel tool size pumps that only weigh 11 pounds and spin 10,600 RPM, with computerized electronics built into the motor. These will last many times less than standard heavy-duty pumps that were built like bench grinders that weigh 40 pounds, only spin 3450 RPM, and do not have any electronic components. Then they lie about variable speed pumps saving energy to make you think it is the “green” thing to do. Once you buy into the variable speed hype, you are locked into a perpetual cycle of regular and expensive replacements, which is how they keep the cash flow flowing through these big corporations. Usually after people have been through 3 or 4 of these, they realize that saving energy is more about longevity of the equipment, than supposedly saving a few bucks a month on the electric bill. Then after switching back to a standard type pump, they discover that they were never actually saving even a few bucks a month with the previous variable speed systems.
The best way to save energy and make the pump system last with a geo open loop is to use a 2 pump set up. You need as small a well pump as you can get that will produce 30 GPM at low pressure. Then you can use a jet pump as a booster to the house, to increase pressure for the showers and sinks.
