Adjusting Water Pressure to Reduce Energy Waste
Water well pumping can be the largest load on a remote home power system. There are several ways to minimize its energy consumption. Here is another technique that applies to any pressurizing system, especially if it uses an AC pump.
If you look at the performance curve of any centrifugal-type pump (including all AC submersibles and jet pumps) you will see that as the pressure increases past a certain point, the flow drops drastically. Here is an easy way to detect this situation at the job site. Release some water, just until the pump switches on. Watch the system's pressure gauge and observe its rate of rise (this indicates flow rate). Does it rise to a certain point and then slow way down? If so, then that is the pressure at which the pump "slips" and loses efficiency.
We had a customer in Colorado whose cut-out pressure had been set to the typical 50 PSI (pounds per square inch). As the pressure got past 40, the flow rate slowed way down. There seemed to be more than enough pressure at the faucets, so we reduced the cut-out to 36 PSI. In doing so, we cut the energy use of the pump nearly in HALF. The owner couldn't detect a change in the water delivery but, as it was gardening season, she saw an immediate increase in the amount of energy available from her PV power system!
Why do most Americans want more than 35 PSI at their home? It's because of undersized plumbing! Most houses in the U.S.A. are plumbed to the legal minimum requirements of the plumbing codes (1/2" and 3/4" pipe). At the end of a long pipe run, the dynamic pressure may be diminished by 30%. Where a house has not yet been plumbed, we recommend using one size larger than minimum, for all cold water lines.
Similarly, when using garden hose, 3/4" hose will cause far less pressure drop than 1/2" or 5/8" hose. When these measures are taken, a pressure setting of 25-35 PSI will please anybody. Where a house is already plumbed, observe water delivery at the faucets. If water flow is satisfying without opening faucets all of the way, then a reduction in pressure may be acceptable.
How to reduce water pressure
Pressure adjustments are made at the pressure switch. On a standard switch there are two adjustment nuts, with a spring under each one. Turning counterclockwise will lower the settings. You will see the result by watching the pressure gauge as the pump cycles on and off. First, loosen the nut on the longer screw. This will reduce both cut-in and cut-out pressure. Set it for the CUT-IN that you desire. Second, adjust the nut on the shorter screw. It adjusts the CUT-OUT only. Cut-out pressure should be around 2/3 of the cut-in pressure.
Once the pressure is set and everyone is satisfied, reset the precharge air in the pressure tank. This will maximize its storage and minimize on/off cycling. To reset the precharge, first make note of the cut-in pressure. Now shut off the power to the pump. Release water until the pressure gauge drops to zero. Measure the pressure of the tank's air bladder using a tire pressure gauge at the fitting on top of the tank. Set the air pressure to 2 or 3 PSI less than the cut-in pressure. Restart the pump. Finally, write down the running time per cycle. Write it on the wall, so the performance can be checked later to detect pump wear or other problems.