Dynamic or pumping level is needed to size the pump. This is the maximum depth of water in the well when pumping max flow. The static level is the highest water level of the well before the pump is started. The static level is needed to size the CSV. In say a 300' deep well the pumping level could be 250' deep. So, the pump needs to be able to produce water from 250' deep plus 50-60 PSI (138') to supply the pressure a house needs. However, if the static water level is 20' from the surface, even though the pump is set at 300', it is only lifting from 20'. Water under water doesn't weigh anything. Drop a bucket on a rope down 300' to get water, and it won't weigh anything until you lift it to 19', which is above the 20' static water level. This means a pump that was designed to pump from 250' plus 138' for the 60 PSI must deliver 388' of head or 168 PSI. When the pump is first turned on and the water level is still at 20', that pump will produce more pressure than needed. Only after the water level has pulled down to 250' will the full capacity of the pump be needed.
When using a Cycle Stop Valve and only taking a 3 GPM shower, a 20 GPM pump is only producing 3 GPM. At 3 GPM rate the water level in the well may not drop below the 20' static level. When the CSV closes to make the pump produce only 3 GPM and the water level is only at 20', the pump might produce more back pressure than the CSV can handle. We only recommend a 125 PSI difference between what the pump can do and the setting of the CSV. Setting the CSV to deliver 50 PSI constant to the house means the inlet or back pressure to the CSV cannot be more than 175 PSI. 175 PSI is the same as 404' of head. So, if the pump is capable of building more than 404' of head, it will make more pressure than the CSV can handle. We figure the maximum pressure the pump can build from the static level instead of the dynamic level, because when using very low flow rates with a CSV the water level in the well may not pull down at all.
