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Author:
asummers (MN)
Hi -
My extended family - 3 summer-only (older) houses - have always shared a well/water system. We had a unique gravity-feed set up using an 800-gallon milk tank with a 10gpm pump from a deep well (180ft). Well was re-drilled 2 years ago. This year we are switching to a pressure tank with rubber bladder. We have consulted a plumber who thinks that an 80 gallon tank should be enough using our existing Grundfos 10gpm submersible. We may bump up to an 119-gal. I assume pressure setting would be 40/60.
Bottom line, is can 3 houses use a 10gpm pump with only 25-40 gal drawdown? Our old system had so much storage that the pump size wasn't an issue with 800-gal tank filling overnight.
From knowing nothing, I've tried to learn alot in a short time - I understand drawdown rule of thumb, that only 1/3 tank size is water capacity, wanting at least 1 min on/off for pump preferably 2, etc. I also read about the 7-min peak demand formula for determining pump capacity/supplemental storage.
So here's where I am confused - For 3 houses isn't the GPM usage at peak going to be way more than 10GPM pump? 4 showers at once or 3 showers plus lawn watering and laundry is already 25-35 GPM right? The formula states that a 3-4 bath house (we will exceed this with 3 kitchens, 3 laundry, etc) will need 122 (gal) peak use in 7 minutes - our pump will produce 70-gal and then we have the storage in the tank but is that enough to make up the difference?
What am I missing? Does the pump crank out more depending on pressure setting? How much water reserve is there below the low pressure level when the pump kicks on?
Any help is greatly appreciated,
Andrew
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Author:
m & m (MD)
You will be okay with the 80 gallon, better with the 119 gallon (recommended, but pricey). Don't worry about the peak demand factors in your case. The well/pump gpm is all you have to work with and you're not going to change that. If/when you do hit peak demand, at that point you will be
"riding on the pump" at whatever it can deliver which can vary depending on the pumping water level in the well- all variables that you can't control.
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Author:
asummers (MN)
Thanks for replying - I was asking the question assuming a new pump could be changed out but maybe that's a big job - I see now better option is to run the system and then address pump size if need be - thanks!
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Author:
hj (AZ)
You use the water in the pressure tank FIRST. Then the pump kicks in to try to keep up with the demand. If it cannot do so, the tank will keep outputing water until its pressure is ZERO, or whatever the pump is producing.
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Author:
packy (MA)
if you have a question or a concern about capacity, contact the manufacturer of the tank. ask for technical help and give them all the statistics. they will give you the advice/information you need.
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