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I'll have a 3000 gallon water tank (around 8 feet tall) located at the same elevation as my house. The distance between my house and the tank will be about 90 feet, with the water traveling through a 2" line. I use about 10000 gallons per month, or 323 gallons a day. My house (1700 sq. ft.) has a total of 14 'water outlets' (spigots, faucets, toilets, showers etc). The water tank is filled through the top, and water will travel to the pump through an aperture located at the bottom of the tank. The placement of the pump in relation to the tank is show below:

Pump placement

How should I choose the correct size pump and airbag? I won't have access to the inside of the tank, and I would rather have the pump located outside of the tank.

I'm looking for a formula, or some general guidelines.

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By "airbag", I presume you mean "pressure tank"? (FYI: I've never heard it referred to as "airbag", in North America at least it's called "pressure tank" or occasionally "bladder tank".) Is the pump gravity-fed from the bottom of the tank, or are you looking for a jet pump to pull through pipes that go over the top? How high is the tank? Any objection to just putting a submersible pump in the tank? (this is what we did where I used to work - more reliable, no priming, only thing it requires is access to get it in). –  gregmac Dec 13 '12 at 18:42
I've just betrayed my ignorance in these matters: by 'airbag', I meant 'pressure tank'. I'll update my question to provide more specifics. –  cg433n Dec 14 '12 at 0:26
I should also add, the sizing depends on a bit on the type of system you get. A constant pressure pump is a variable-speed pump that has a pressure sensor and automatically adjusts the speed so it always delivers within a couple PSI of your target, and they use much smaller pressure tanks than typical constant-speed pumps, but of course they also cost more. Aside from a smaller tank, the benefit is the pressure doesn't surge up and down between the typical 40/60 psi range that constant-speed pump systems do. –  gregmac Dec 14 '12 at 0:36
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1 Answer

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Two factors matter: flow rate and pressure. Pressure is determined by friction loss and elevation adjustment. Since the tank and house are at the same elevation, we don't need to account for that. We do need to account for your friction loss, though. To do that, we need to know what your pipe diameter, length, and flow rate.

We start with flow rate because the amount of water flowing through a pipe determines its friction loss. That's what we size the pipe from. Your flow rate should be based on maximum flow: everything that could possibly ever run at one time. Your shower, dishwasher, washing machine, somebody washing their hands, etc. There's no uniform minimum standard for that, so check the fixtures around your building to find out how muh they would draw.

According to a plumbing design guide from Michigan, this can vary from 7 gpm for one bathroom to 17 gpm for a 3-4 bathroom residence. After you've determined the flow rate, you must select a desired pressure for the house inlet pressure.

If your pump is able to fully supply your house without the aid of a tank (sufficient flow at operating pressure), then your only consideration for tank size is the cycle time of the pump. For smaller pumps, you want your tank to provide at least 2 minutes of water. For larger pumps, you may desire a higher cycle time. There is no harm in a larger tank size.

If your pump isn't able to provide that level service, then you'll need to size your tank to provide supply for a period of time. Remember that pump supply in gpm is also related to pressure. The more pressure the pump needs to provide, the less flow it will have. Calculate your pump flow based on the pressure needed to charge the tanks.

The ultimate calculation is (maximum flow * maximum minutes) > (tank capacity / minutes + pump capacity * minutes). You can use any combination of tank and pump capacity as long as the tank + pump capacity meets your demand needs. A 1 gpm pump and a pressure tank that can supply 300 gallons could probably hold you over just as well as a 14 gpm pump and 28 gallons of supply tank. Remember again that supply is not equal to full capacity: you only have supply if it is higher than your necessary pressure. Your goal is to find a combination that is most cost effective.

For another good resource related to tanks, see Flotec's pre-charged pressure tank FAQs.

By the way, since you gain 1 psi for every 2.31 feet of elevation gain (referred to for some reason as "feet of head"), you can get a lot of "free" tank supply pressure by putting your tank at a higher elevation, though that will increase your pump charging pressure.

For absolute luxury, keep your entire supply system able to supply above your desired household pressure and slap a pressure reducing valve on the supply line just before it reaches the first appliances. 60 psi is the ideal dream coming out of your pressure valve (fantastic showers!), but pressure does cost in terms of electric usage for lower pump capacity because, again, higher pressure means lower flow. Most systems are designed to start charging at 40, 30, or 20 psi, though. Note how the pressure corresponds to the amount of water that can be supplied in this example tank.

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