Bob999 - I see your points. The 80 seconds I measured (before really addressing the need to adjust the pressure tank per Gary Slusser's recommendation) was from pump off to pump on. When I get back over there I'm going to properly adjust the pressure tank, redo my calculations, and see how it operates. The other thing I'm going to do is to disconnect the backwash line (there's a 1" PVC union in the line) and test the actual flowrate during a backwash cycle - then I'll know without any question what sort of rate I'm getting through there.
Gary Slusser - this house is on the waterfront of the Chesapeake Bay and is on well, but with public sewer. The owner directed the plumber to install a 6" drain line to the sewer tie-in. In this case the backwash is helping to keep the drain clean and not much else. The well is down 340 ft. and the water is sitting at 58' according to the well report and the pump installers closeout sheet. I'm pretty sure I'm getting enough flowrate... I sure hope so because there's not much more I can do - I put the correct size button in the DLFC port, ensured I had a short run to the filter, installed a 1" drain line, etc. If I were doing this again I would evaluate the wisdom of using two smaller tanks. If I can confirm the backwash rate... and I will... I might have a good solution though; one that will last a long time and solve a perplexing problem for the owner.
Dittohead's comments about frequency of backwash are consistent with what I heard pretty much across the board. The folks at Fleck (this is a Fleck 7000SXT valve) and at Matt-son insisted on 12 GPM and waiting no longer than every other day to backwash. So I have it set now for 15min backwash/5 min rinse every other day. I bumped up the button to 14GPM based on Dittohead's recommendation. I'm not too worried about lifting the bed and dumping material out during backwash because from everything I've read this media just doesn't move very much during backwash - the little bit of extra flowrate can only help.
Your note about being worried about using this much water is something very much on my mind. This past summer was absolutely brutal and though we didn't get a drought ban on water usage, if this kind of heat keeps up I'd be worried about a lot of wells drying up. The homeowners are frugal users of water, BUT they did just install an irrigation system having a drip side and a pressure side with large garden areas. The house is equipped with rainwater capture and recirculation systems, but in the summertime the wellpump really cranks when the irrigation system come on - it's all plumbed with 1" lines. You're right about water temps - this water never gets above 45 degrees. Temperature does determine the backwash flowrate, but at these temperatures 12GPM is what's recommended.
I've attached a before and after picture of the water color, shown with a few inches in a white jacuzzi - the iron filter seems to be doing its job.
You guys have been terrific help on this project and I'm very grateful!