The flow rate of the water at the pressure tank should be measured accurately because many filter media require approximately twice the backwash flow rate as the service flow rate. Timing how long it takes to fill up a measured bucket is an inaccurate method of attaining flow rates.
The proper well water flow rate is determined by counting the gallons drawn down and the time between cut in and cut off cycle of the well pump. To do this, one must allow the well pump to build up to full pressure. Close the main shut off valve to the building to assure that no water is being used. Then, open a spigot below the pressure tank, capture the water, and measure the number of gallons drawn down from the pressure tank until the well pump turns on. When the pump turns on, immediately close the spigot and time the period it takes for the well pump to recover, that is, between cut in and cut out.
The formula for determining the flow rate is gallons drawn down that were measured above, divided by the seconds required for recovery, then multiplied by 60. (Gallons / Seconds) x 60 = Gallons per Minute (gpm) flow For example, if 16 gallons are drawn down and it takes 90 seconds to build pressure back up, then: 16 divided by 90 = .177. Consequently, .177 x 60 = 10.6 gallons per minute flow rate.
Considering that the backwash rate for many filter media should be twice the filter rate, it may be necessary to install a second filter in parallel to accommodate the service flow. In an instance where about ten gallons is determined as the flow, that flow will be adequate to backwash many filter media in an eight to ten inch diameter tank. However, the service flow for many common filter media in eight to ten inch diameter tanks is about four or five gallons per minute. Hence, forcing ten gallons per minute through a single ten-inch tank may pass iron. In order to govern the flow of water evenly through tanks installed in parallel, flow restrictors may be installed on the outlet side of the filter tanks.
(whole house & well units)