My setup is a bit different

[reddog]

I've got a 3hp deep well (pump set at 785 ft) pumping about 10gpm, and after burning a start winding after 7 years I decided that it wasn't a good idea to be cycling the pump to pressurize an 86 gallon tank, especially when pulling and replacing the pump is a $4000 operation. I installed a 1500 gallon reservoir with float control, and now my pump runs a fill cycle only once every 3-4 days during low use periods and once a day, if that, when I'm irrigating. My well discharge is 1 1/4 inch and is fed into a fitting near the bottom of my partly buried (freeze protection) reservoir tank. There is a Tee tapping that line and a 1 inch suction line to an MQ3-35 in my wellhouse, with a 1 inch discharge to my domestic system and sprinklers. This has worked just fine for 7 years, but I am well aware that, even if nothing goes wrong with the pump itself, that the electronics on the circuit board are likely to fail at anytime. I'm considering replacing the MQ with a small pressure tank, Grundfos JP07 jet pump and CSV, but I am concerned about the CSV back pressure to that Tee on the well pump discharge, and what effect that will have (if any) on the well pump if it is filling the tank while the jet is running. The only check valves in the system are at the well pump. Thanks in advance.

[Cary Austin]

Wow!  You have gotten more out of an MQ type pumps than most people.  But you are on borrowed time now.   I think the MQ has a built in check valve?  If not it must have an external one. 

Back pressure from a CSV would only be seen in the pipe between the pump and the CSV, not on the suction line.  Plus a JP07 can only build 76 PSI max, which would be the back pressure on the 6" long nipple between the pump and the CSV. 

[reddog]

Thanks for the fast answer! It is good to know that 'nothing untoward happens'. Just a FYI, the MQ comes with a check valve factory installed in the discharge port and an optional check valve to install in the suction port if the pump is drawing from depth. I have only the discharge port check installed.

Now for my next question. Looking at the pump curve of the JP07, the output at 50psi is about 7 gpm, am I reading that correctly? If I need more than 7 gallons at full system load (all my sprinklers on at once, which I would prefer to do, approaches 9gpm), does that preclude setting the pressure switch at 40-60 and setting the CSV at 50psi? The curve chart I have shows the JP07 producing about 13gpm at 40 psi, so I would guess that a CSV setting at about 40 would be better and the pressure switch at 37-57 would be what I want? I guess I'm unclear as to what the CSV does, if anything, when the demand exceeds the amount the pump can supply at its pressure setting. There's a reason I'm not a plumber, and you'll probably tell me if I want 9gpm at 50psi to just get a JP10

[Cary Austin]

That is a  very good question.  You can still leave the CSV set for 50 PSI and use a 40/60 switch setting.  The CSV will hold 50 PSI constant as long as the pump can do that as well.  But when demand increases and the pressure drops below 50 PSI, the CSV just turns into a piece of pipe and does nothing. You can still get your 9 GPM, but the pressure will only be 40 PSI as that is all the pump can do.

And yes a larger pump would keep the pressure up.  You can install a pump as large as you think you need, and the CSV will make it act like a smaller pump when smaller amounts of water are being used.  But you must have a pump large enough for peak demands as the CSV cannot make the pump act larger than it is.