How do I choose a pump for my house?

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.

I've been doing a little investigating and appreciate your comments on my assumptions below:

1) 1 foot of head of water equates to 0.4335 PSI so the pump at 250 feet requires 108 PSI to the top of the well. Then adding say 25 feet of head to get to the shower on the 3rd floor adds another 10.8 PSI (~ 11PSI) plus the 70 PSI back pressure of the water system itself requires 108 + 11 + 70 = 189 PSI this your value of 190 PSI. So a rule of thumb is 1 foot of water - .5 PSI?

2) I assume you suggest 70 PSI as a CSV pressure setpoint with a 60/80 pressure switch rather than the lower differential systems to provide the 25 feet of head required because it is a 3 story house with the basement and we want a decent pressure at the shower nozzle?

3) #1 above assumes that we will be able to use all the water in the well down to the pump. If we only want to use 100 feet of well capacity, we could get away with a smaller HP pump. Say first thing in the morning, the well will have recovered fully so the pump will only have to over come the (70+11=) 81 PSI (to the 3rd floor shower) since the static suction head with the well being full will provide the 108 psi. As the demand draws down the well water, the required head will increase from 81 PSI to whatever is required by the drawdown of the well water height to a max of 190 as the water level approaches the pump level.

4) I looked at the CSV placement in a number of examples on your web site and am confused with its placement. Were should it go? On the pump side of the pressure tank/check valve or on the house side (downstream side) of the pressure tank after the check valve and T. The main picture with the smiling faces show the CSV right on the T to the pressure tank but other examples show it upstream of the pressure tank. It would seem to me that mounting it downstream after the pressure tank is the right location but one diagram suggests to put it in the well - ie upstream of the pressure tank and its pressure switch. Please clarify.

5) I don't understand the bleeding of 1 gpm through the CSV - where does it go? Is there a bleed line out of the CSV which should go to a sump or does it leak a bit and that is the 1 GPM? I have seem some references to a drain pan or a sump being nearby would be a good idea. In a finished basement , a 1 GPM 'leak' might not probably be desireable. I understand the requirement to provide some cooling flow for the pump do it doesn't deadhead and heat up the water/motor/pump. I assume the CSV is basically a downstream pressure control valve which tries to maintain 70 PSI or a constant discharge pressure (so we have enough pressure at the 3rd floor shower) to the house. So lets assume the system pressure to the house is 60.1 PSI (60 start/80 stop), the well is full and we start the shower which takes say 10 GPM. The pressure tank will provide all flow through the CSV (assuming it is on the downstream side of the pressure system) until the pressure drops below 60 PSI which turns the pump on. With the shower taking 10 GPM, the pump runs up its curve providing lots of pressure (say 190PSI I don't have a curve) and the CSV provides 70 PSI at 10 GPM plus what ever flow goes into the pressure tank which the pump supplies. As the pressure tank approaches 80 psi, the flow reduces. Then at 80.1 PSI the pump shuts off. So really what does the CSV do. I normally see that the air pressure in the bladder supplying the motive force to supply water when the pump is off.

6) When buying a pump, can/should I specify the GPM AND head requirements or are they basically sold in HP increments and you get what you get from a pressure perspective which is dependent on the flow - as flow goes down/head goes up but follows the pump curve. Are there different impellors (thus different curves) available?

Thanks for your help.
Brian