Pump sizing and numerous questions

[Sphyrna]

Hello,

I think our deep well jet pump is failing due to age and a faulty venturi, it's not able to produce adequate pressure to switch off the pressure switch at its normal setting. Last water equip. redo was about 12 years ago - installed 1 hp jet pump, 75 gal. pressure tank, and added a Waterboss acid "filter" and water softener. We have mixed piping - some elderly copper, pex and pvc, as well as acid water at 5.5 pH. We have two bathrooms now and probably will have a third once the upstairs is finished (highly unlikely that we will ever use more than one or two at a time, (only two people))

I've been advised that if we are going to have to open the well anyway, we might as well replace the pump with a submersible. I've been looking at the stainless Goulds 10 gpm 1/2 hp, 3/4 hp and the 7 gpm 1/2 hp.

I've been told, (I haven't been in there to check yet,) the well is 75' deep, say we would put the pump at 65', the well is at the basement ground level and the house is two stories, so say 30' head in addition to the well = 95' which would make the 1/2 HP 10 gpm adequate according to the specs. (static level is not known, but we are between two creeks)

My confusion is that when you add up the total dynamic head it seems the 1/2hp 10gpm is undersized or just borderline. (Borderline for the current one level of the house, lowered psi to 50 and would give us no option if we wanted higher pressure)... (We think we have some narrowing copper that might warrant higher pressure settings, as there are areas where we have never had nice water pressure, (kitchen) of course I guess it could blow out the old copper and really put us in a pickle.)

60psi x 2.31 = 138.6'

depth of pump = 65'

stories, 3 x 10 = 30'

pressure tank distance, 25' (100' 1 1/4" pipe = 5.26 feet of head pressure) =  1.32'

138.6' + 65' + 30' + 1.32' = 234.92' total dynamic head pressure (or 201.82' with 50 psi and 2 stories)

Goulds 10 gpm, 1/2 hp tdh max is 210'

I guess my primary question is - will the smaller pump be adequate and I am over complicating this, or do we need one of the larger pumps? Will the CSV prevent a larger pump from gargling at the low head and damaging itself or will the higher dynamic head mean this won't be an issue? How is a CSV different from a slightly closed ball valve used to create pressure for an "over sized" pump and will it be helpful with the 75 gal. pressure tank?

The pump specs can be seen at my other forum question here - http://www.terrylove.com/forums/showthread.php?47227-Submersible-pump-choice-help

Thanks for your time, sorry this is so long.

Elizabeth

[Cary Austin]

I would go with the 3/4 HP, 10 GPM.  You will be better off with a 50/70 pressure switch setting.  With the large tan you have, that would mean a CSV setting of 65 PSI.  You will need the extra pressure for the elevation of the top floor, and the CSV will make the pump work like a smaller pump when you need it.

[Sphyrna]

Thanks, that's kind of what I was thinking.

Thanks again.

[Rory]

I would go with the 3/4 HP, 10 GPM.  You will be better off with a 50/70 pressure switch setting.  With the large tan you have, that would mean a CSV setting of 65 PSI.  You will need the extra pressure for the elevation of the top floor, and the CSV will make the pump work like a smaller pump when you need it.

I thought the CSV needs to be set at 10 psi below the cut-off pressure. Is this not the case? Under what circumstances would you ignore this requirement?

[Cary Austin]

The CSV must be set below the cut-off of the pressure switch.  How much lower depends on the size of pressure tank.  In this case the 75 gallon tank will hold about 20 gallons of water with a 50/70 switch.  So setting the CSV at 65 PSI means the tank will quickly fill to 65 PSI, then this size CSV will let the last 5 gallons in the tank fill at 1 GPM.  This will give at least 5 minutes of run time.  Setting the CSV at 68 PSI, will give a minimum of 2 minutes of run time.  You just don't want to take too long to fill the tank after all the faucets are turned off.  So the larger the tank, the closer the CSV setting is to the pressure switch off setting.