Increase PSI and GPM - Booster Pump & Pressure Tank

mjp24coho
mjp24coho Solar Expert Posts: 104 ✭✭✭
in Solar Water Pumping #1
I'm looking for some insight on an upgrade I'm thinking of doing for my offgrid water system at my cabin. First - a bit of background on my current system. I have a Grundfos submersible deep well pump that runs off my solar panels. It pumps into a 500 gallon holding tank buried below ground. That tank gravity feeds (2-3' drop) into a shurflo 12V (5 GPM) pump that is used to send water into my cabin. The shuflo pump feeds into a 5 gallon pressure tank, which then pressurizes water into my cabin. I'm expanding my cabin, so I need to be able to have ~10GPM in the cabin (so all the showers/faucets can run at the same time). I'd also like to have considerable more pressure than I currently have. I'm thinking of the following two upgrades:
  • Replace the shurflo pump with a 10 GPM 1/2 HP AC shallow well jet pump
  • Change the pressure tank to a 60-80 gallon tank

I have plenty of battery and inverter capacity to run the 1/2 HP AC pump. My question is whether that will make the difference I'm looking for. I don't fully understand how the pressure tanks work, and how many GPM they output. If the pump only puts into the tank at 5 GPM, can the tank output at 10 GPM? If I keep the existing shurflo 5 GPM pump, but upgrade to a 80 gallon pressure tank, will that give me any increase water pressure in the cabin, sufficient to run 2-3 showers? The current setup gives weak pressure, especially if 2-3 faucets are running at the same time. I'm trying to get it as close to possible as the water pressure and GPM capacity that I have at home. Any advice/direction would be appreciated.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Increase PSI and GPM - Booster Pump & Pressure Tank

    Most water outlets like showerheads for example only flow about 3 GPM. The Shurflo should be able to keep up with this. Especially if you increase the size of the pressure tank. Frankly a 5 gallon tank is useless; the pump is doing all the work anyway.

    I'd up the size of the pressure tank first. All pumps prefer to cycle long and infrequently rather than short and often, so whatever pump you end up with will appreciate a larger tank. And maybe you won't have to change the pump if you like the result.

    BTW you could also run two Shurflo pumps in parallel, connected electrically as well so they both run at the same time (more volume, equal pressure). They'd both need to be controlled from the same pressure switch (not the built-in switch some have).

    Another option is to use separate pump & tank with appropriate check valves per shower/bath; each becomes its own little unit operating as needed. Still should be bigger than 5 gallon PT though.
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  • Texas Wellman
    Texas Wellman Solar Expert Posts: 153 ✭✭
    #3
    Re: Increase PSI and GPM - Booster Pump & Pressure Tank

    Change your 5 gallon tank into a 40ish gallon tank. Something like a Well X Trol WX-202 (20 gallon) would be my starting point. The bigger the tank the longer it will take the pressure to peter out. A WX 202 only holds about 6-7 gallons of water. A WX 250 is a 40 gallon tank with a 13 gallon draw down.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #4
    Re: Increase PSI and GPM - Booster Pump & Pressure Tank
    Cariboocoot wrote: »
    Most water outlets like showerheads for example only flow about 3 GPM. The Shurflo should be able to keep up with this. Especially if you increase the size of the pressure tank. Frankly a 5 gallon tank is useless; the pump is doing all the work anyway.

    I'd up the size of the pressure tank first. All pumps prefer to cycle long and infrequently rather than short and often, so whatever pump you end up with will appreciate a larger tank. And maybe you won't have to change the pump if you like the result.

    For a given pressure tank, and a given air charge (the air bubble above the water inside the tank that acts like a spring to maintain the pressure, there is a number called the drawdown.

    This is the amount of water than has to be removed from the tank to go from the pressure where the pump shut off to the pressure where the pump turns on again.

    For an 80 gallon tank and low and high pressures of 50 and 30 PSI, the drawdown is somewhere around 20-30 gallons. Once the pump is started, the low pressure set point, if water is being pulled out faster than the pump can push it in the pressure will continue to go down. Eventually the pressure gets low enough that the decreasing amount of water drawn out meets the increasing volume that the pump is pushing in. That may be a satisfactory pressure and flow, or it may be disappointingly low.

    If your high flow use (shower, watering, dishwasher or washing machine, etc.) lasts long enough, you will end up at the pressure and flow that can be handled directly by the pump anyway.
    If your water uses are within the drawdown range, you will have relatively stable pressure and flow.
    For a 2GPM shower head (the right design can actually be comfortable), it would take a 10-15 minute shower to take a "full" 80 gallon tank down to where the pump will start.
    For a 5 gallon tank, that will be less than a minute. :-(
    SMA SB 3000, old BP panels.
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  • Ralph Day
    Ralph Day Solar Expert Posts: 1,022 ✭✭✭✭
    #5
    Re: Increase PSI and GPM - Booster Pump & Pressure Tank

    A larger pressure tank for sure, and some common sense. If person A is doing dishes (manually) suggest that person B wait for the shower so there's no temperature or flow surprises. Similarly if person C is showering person A should hold off on doing the dishes (or flushing) unless person C is taking too long and using too much hot water:D.

    A little common sense and preparation will save a lot of money on upsizing a water delivery system.

    \ralph
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  • mjp24coho
    mjp24coho Solar Expert Posts: 104 ✭✭✭
    #6
    Re: Increase PSI and GPM - Booster Pump & Pressure Tank

    Thanks all for the advice. I think I'll start by increasing to a larger tank now. I also think I may experiment with increasing to two pumps. Can anyone provide direction on how to wire them both electronically and plumbing-wise to accomplish this?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: Increase PSI and GPM - Booster Pump & Pressure Tank
    mjp24coho wrote: »
    Thanks all for the advice. I think I'll start by increasing to a larger tank now. I also think I may experiment with increasing to two pumps. Can anyone provide direction on how to wire them both electronically and plumbing-wise to accomplish this?

    The plumbing is very straightforward: 'T' input and output to match pipe sizing.
    The wiring is also pretty straightforward: both pumps controlled off a pressure switch on the pressure tank (the built-in switches, if so equipped, are bypassed).
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #8
    Re: Increase PSI and GPM - Booster Pump & Pressure Tank
    Cariboocoot wrote: »
    BTW you could also run two Shurflo pumps in parallel, connected electrically as well so they both run at the same time (more volume, equal pressure). They'd both need to be controlled from the same pressure switch (not the built-in switch some have).

    Why can't two shurflo pumps run in parallel, using their own pressure switches, and WITHOUT being controlled by an external pressure switch?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: Increase PSI and GPM - Booster Pump & Pressure Tank
    vtmaps wrote: »
    Why can't two shurflo pumps run in parallel, using their own pressure switches, and WITHOUT being controlled by an external pressure switch?

    --vtMaps

    Because the pumps and switches will not be identical; one can run higher PSI than the other and may keep running after the first one shuts down. You want them to behave as one pump for best results.
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #10
    Re: Increase PSI and GPM - Booster Pump & Pressure Tank
    Cariboocoot wrote: »
    Because the pumps and switches will not be identical; one can run higher PSI than the other and may keep running after the first one shuts down. You want them to behave as one pump for best results.

    One could also think of it as two charge controllers feeding one battery bank. As the batteries come up on full, one CC will switch to float while the other continues to bulk for a bit longer. No harm done.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
    Re: Increase PSI and GPM - Booster Pump & Pressure Tank
    waynefromnscanada wrote: »
    One could also think of it as two charge controllers feeding one battery bank. As the batteries come up on full, one CC will switch to float while the other continues to bulk for a bit longer. No harm done.

    By the same token, have you heard of MidNite's "Follow Me" function? :D
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  • mjp24coho
    mjp24coho Solar Expert Posts: 104 ✭✭✭
    #12
    Re: Increase PSI and GPM - Booster Pump & Pressure Tank
    Cariboocoot wrote: »
    The plumbing is very straightforward: 'T' input and output to match pipe sizing.
    The wiring is also pretty straightforward: both pumps controlled off a pressure switch on the pressure tank (the built-in switches, if so equipped, are bypassed).

    Do I need a check valve on the output line for each pump to prevent backflow into the other pump if its not working, or do the pumps have their own internal check valves?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #13
    Re: Increase PSI and GPM - Booster Pump & Pressure Tank
    mjp24coho wrote: »
    Do I need a check valve on the output line for each pump to prevent backflow into the other pump if its not working, or do the pumps have their own internal check valves?

    They're positive-displacement pumps; they have valves built in so no additional check valve should be needed.
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