Water pressure tank pressure with Grundfos pump

hidrologohidrologo Registered Users Posts: 8 ✭✭
I recently purchased a Grundfos MQ3-35-1 (115V) 3/4hp pump to replace a previous similar Grundfos pump that was cycling at my off-grid home. Although, the Grundfos comes with an internal bladder and check.valve, I ordered a 20 gallon water pressure tank in order to extend cycling intervals. I had a local plumbing company with Grundfos experience install the pump. They did so, but before they left, I tested the system by flushing a toilet and found that the pump started on the second flush. The installer from Petroglyph plumbing had no idea on the correct water pressure tank pressure to minimize pump cycling, snd had no adjusted the water pressure tank psi. As such I had him lower the pressure until the pump started at about 4-5 1.6 gallon flushes. I do not believe this is optimal.  I believe the water pressure tank came set at about 38 psi. Under low flow the pressure gauge at the tank was reading around 35 psi when the pump started after we reduced the water tank pressure. 

My question to the forum is  - What static psi (pump shut off and a faucet turned on)  should the water pressure tank be set at to minimize pump cycling for this pump?

Comments

  • mvasmvas Registered Users Posts: 383 ✭✭✭
    edited December 2019 #2
    It is the difference between the Turn-On pressure and Turn-Off pressure that determines how long the pump will run.
    I believe mine is set at a 20 psi differential =  20 psi turn-on and 40 psi turn-off

    What is the Air Pressure inside of the tank?
  • EstragonEstragon Registered Users Posts: 4,494 ✭✭✭✭✭
    It seems to me a "20 gallon" pressure tank might be a total size.  The effective capacity of the internal bladder would vary a bit with pressure settings, but likely ~7 gal.  If so, 7 ÷ 1.6 gives your 4-5 flushes.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • AguarancherAguarancher Solar Expert Posts: 313 ✭✭✭

    The internal bladder air pressure should be set at the cut-in pressure of the pump switch. The bladder pressure is measured with the tank empty of any water pressure.



  • 706jim706jim Solar Expert Posts: 423 ✭✭✭✭
    edited December 2019 #5

    The internal bladder air pressure should be set at the cut-in pressure of the pump switch. The bladder pressure is measured with the tank empty of any water pressure.



    So in your case set the bladder pressure (with pump turned off) to either 20 or 30 psi.
    Island cottage solar system with 2400 watts of panels, 1kw facing southeast 1kw facing southwest 400watt ancient Arco's facing south.Trace DR1524 MSW inverter, Outback Flexmax 80 MPPT charge controller 8 Trojan L16's. Insignia 11.5 cubic foot electric fridge. My 28th year.
  • AguarancherAguarancher Solar Expert Posts: 313 ✭✭✭
    edited December 2019 #6
    "set the bladder pressure (with pump turned off) to either 20 or 30 psi"
    I disagree, the correct internal bladder air pressure is determined specifically by the pumps cut-in switch pressure setting. Nothing else is close enough. And all bladder air pressure adjustments are made with the tanks empty of any water pressure.
  • 706jim706jim Solar Expert Posts: 423 ✭✭✭✭
    Well unless you have an adjustable pressure switch (who ever changes the default setting?) it would either be 20 or 30 psi.
    Island cottage solar system with 2400 watts of panels, 1kw facing southeast 1kw facing southwest 400watt ancient Arco's facing south.Trace DR1524 MSW inverter, Outback Flexmax 80 MPPT charge controller 8 Trojan L16's. Insignia 11.5 cubic foot electric fridge. My 28th year.
  • m151m151 Registered Users Posts: 27 ✭✭
    The bladder pressure should be set 2 psi below the cut-in pressure to reduce "pressure collapse" as the pump cycles. My 30 gallon tank has a draw down of about 10 gallons. This will very with the cut-out pressure. An accurate air pressure gauge is a must. An upgraded quality water pressure gauge doesn't hurt either.
  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    edited December 2019 #9
    Set the pre-charge pressure of the tank to 26 psi.  The Grundfos recommendation of 22 to 25 psi is also fine.

    I am available for custom hardware/firmware development

  • Dave AngeliniDave Angelini Solar Expert Posts: 5,910 ✭✭✭✭✭
    To the OP, who never came back, you are using a constant pressure pump. Since you can't really adjust the pumps switch internally, this idea is not going to do very much at all as far as reducing cycle time.

    I have done this many times for folks and it really does not add much capacity to the system and won't save you many pump cycles or electricity.

    If you want to do this and are determined, you need to add an external pressure switch to power the pump. This will work in what you want to do.

    A normal size system, with decent battery capacity really is not worth this complexity. if you have the time, go for it.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
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  • LoidLiuLoidLiu Registered Users Posts: 2
    Hi, I've got a little lost. I want to change some bathroom stuff and consider changing the original toilet to a low-flow one. Should there be done any changes in all this pump-pressure system to get the benefit of a low-flow toilet? 
  • 706jim706jim Solar Expert Posts: 423 ✭✭✭✭
    No there is no need to alter your existing water system for the low flush toilet.
    Island cottage solar system with 2400 watts of panels, 1kw facing southeast 1kw facing southwest 400watt ancient Arco's facing south.Trace DR1524 MSW inverter, Outback Flexmax 80 MPPT charge controller 8 Trojan L16's. Insignia 11.5 cubic foot electric fridge. My 28th year.
  • BB.BB. Super Moderators, Administrators Posts: 31,684 admin
    The new toilets (at least in California) are in the 1.2 (down to 0.8 gallon for liquid waste flush) to 1.8 gallons per flush... The old toilets were ~3.0-3.5-5-7 gallons per flush.

    If you are buying a new toilet, look at the reviews. A fair number do not do a very good job of flushing "solids" and paper (stoppage in toilet itself).

    Depending on your sewer piping, angle of drop, and septic system... The lower flow of water could hurt (clearing of waste in pipe, possibly a problem with 0.8 gpf) and probably help with septic (less water to process).

    No fat/oil/grease in septic systems. No garbage disposal recommended.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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