My existing well pump (at 550 ft deep) is a nearly brand new (installed last July) 3 wire pump and I do not want to replace it. However, I now have a 1500 gallon underground cistern in which to store water. I can use the existing pump just fine, but filling the cistern rapidly depletes the well, which doesn't produce water as fast as the pump can drain it.
Is there a way to take my existing pump and reduce its operating speed so that it pumps slower than the recovery rate of the well?
Simplest solution from scratch: - put the deep well pump on a timer that only lets it run for a short length of time when filling the cistern. Say 15 minutes every hour, or whatever ratio actually works with the pumping rate and refill rate. So you'd have a (float switch?) calling for water into the cistern, and you'd have the deep well pump set to pump for 15 minutes (or whatever time works) and be off for 45 minutes (or whatever time works) when the cistern is calling for water.
You can try restricting the flow from the pump (which will, counterintuitively to most, reduce its power draw) but this will result in high pressures in the piping from the pump to the restriction - possibly higher than is advisable, depending on the pipe and fittings.
Simplest solution if the parts are already in place: If the deep well pump was set up with a normal pressure switch and bladder/pressure tank, you could also simply use that arrangement as the source of a restricted fill to the cistern, and the pressure switch would act as the "timer" and the restrictor valve would act as the restriction, without any excessively high pressures. The cistern float switch (or the float itself, mechanically) would control a valve in line with the restrictor valve. Set the restrictor valve for a flow slightly lower than the well refill rate.
Well--Pump--Pressure switch--bladder tank--restrictor valve--float-controlled valve
If your pressure switch has a low-pressure cut-out it will shut itself off (until you reset it) if the water level gets too low in the well. In that case, you could close the restrictor valve some to reduce the flow before restarting the pump, as you'd know the pumping rate was exceeding the refill rate.
You do not mention but presumably must have either an additional pump on the cistern, or you have located the cistern sufficiently far above your dwelling to provide gravity flow from the cistern; but that does not materially impact this question.
With a three wire pump, you could also investigate whether VFS (variable frequency drive) pump controller/inverter could be used. It would also soft start/stop the pump, and depending upon the inverter one can set it for a lower flow. The downside is that the inverter is about $1k, but the upside is that it may extend the life of your pump, and has greater protection of the pump motor than conventional controllers.
A second tune parameter would be to put a restrictor valve in to limit the filling rate, however that may not be quite as energy efficient.
For a ground source heat pump, I matched the flow orifice and pressure to be at a point which did not cycle the well pump and kept the pump in a reasonably efficient aprt of the curve during heat pump operation.
