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Is there any calculation that can be performed to estimate the amount of water a well water pressure tank should output between pump cycles? E.g.- I have a 40 gallon pressure tank (non bladder) with a pump switch set to 40psi on, 60psi off. I suspected the tank was water logged, as when taking a shower, the pump would kick in every 15-20 seconds, which seemed short. I ended up draining it, pressurizing the empty tank to 35 psi, then having the pump refill it, and now I am getting 2-3 minutes between pump cycles, so obviously recharging the tank was helpful.

This worked out great, but was more of a gut feeling. Is there any calculation to see if a pressure tank has too little air? (i.e.- something like my system should output XX gallons between pump cycles).

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up vote 4 down vote accepted

At 2.5 GPM, your 2-3 minutes of shower time between cycles is using between 5 and 7.5 gallons. If your tank starts at 60psi and 7.5 gallons is enough to bring it down to 40psi, that means you're starting out with 15 gallons of air in your 40 gallon tank, and ending the 3-minute cycle with 22.5 gallons of air. I calculated this using Boyle's law, or P1 * V1 = P2 * V2. And of course these numbers would be different if your shower head is higher-flow and your 3 minutes of showering used more than 7.5 gallons.

I believe to get the most output from your tank, you want it to be nearly empty of water when the pump is set to come on. That means draining and charging it to just below the pump setting, as you did. If you wanted to get a little more cycle time you could probably even go as high as 38 psi. Note that with a bladderless tank, you're going to need to recharge periodically as the air in your tank is gradually absorbed by the water.

For what it's worth I think 2-3 minutes is well within your pump's design cycle time. 15-20 seconds was way too short and by noticing it and fixing it you've probably saved yourself an expensive pump replacement.

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Great information! The engineer in me likes to see hard numbers, rather than gut feelings. Wow.. Boyle's law! Looks like that physics class I took long long ago will have some direct application! ;) –  MarkD Sep 13 '10 at 14:10
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Bladder-less tanks are notorious for this. You need to keep filling them with air. 15-20 seconds will shorten the life of your pump for sure. Another option to consider if you ever have to replace your pump is a constant pressure pump (eg SQ series from Grundfos) - this type of pump changes speed depending on demand. I put in quite a few back when I was doing that type of work, and have one at my cottage - they are quite nice. –  gregmac Sep 13 '10 at 16:42
    
Thanks gregmac. My neighbor has a constant pressure/variable speed pump, that he loves. When the time comes to replace our current pump, I'll be carefully considering that option. –  MarkD Sep 13 '10 at 18:09
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