Residential Well System Pressure Tank Sizing

[kbuhagiar]

Hello GJ Nation,

I need some information regarding the proper sizing of a well pressure tank on a residential water well system.

My existing pressure tanks (2 each Challenger model #266, 85 gallon size but 34 gallon water capacity each) are almost 20 years old. Everything on my well water system is working fine, but I need to relocate one of the pressure tanks, so I figure this my be a good time to consider replacement.

Here are the specifics: Three toilets, and three bathroom basins, three showers, two kitchen sinks, a dishwasher and a washing machine on two water heaters (long ranch house, one for each side of dwelling). Just me and my wife, with the occasional weekend guests. Other than the 12-zone landscape irrigation system (which runs 3 days a week for two hours), no other out-of-the-ordinary water demands. Well pump is 13GPM, pressure switch operates on at 35PSI and off at 60PSI. Water pressure is good to excellent.

How big a pressure tank do I need? My first impulse was to size a new tank/tanks according to my existing capacity (68 gallons total), but every resource I have referenced seems to indicate something more like 28 to 36 gallons capacity.

Does my existing capacity seem excessive? I wouldn't mind downsizing (as I could use the extra room in the garage), but I don't want to reduce the efficiency of my existing system.

Any and all input is greatly appreciated.

 

[ard]

I seem to recall that the size is based on the GPM delivery of the pump as installed (i.e. Not it's rated GPM, but how many GPM it delivers in the speciif installe (i.e. Depth, pressures, etc)

The idea is that you want to make sure that when the pump kicks on it runs for a long enough period- ie doesn't 'short cycle'. I think it should run a minute at least

So with your 13GPM, it will run ~5minutes to refill 68 gallons of storage.

I had two 60G tanks, added a thrid 120G tank when I upgraded my pump to a 5HP. I estimated 27GPM delivery. It was my impression that going too large was not an issue.

Anyway, there are also some devices to eliminate tanks ... Cycle stop valves. I did a bunch of looking at these recently, looks interesting: http://cyclestopvalves.com/csvapplications_1.html

WOuld definitely save space. I have a 6-7 ft crawl under part of the house, so miles of space. But if the tanks do fail, I'd need to look hard at this.

GL

 

[pstnbly]

Tanks are based on pump delivery, well static level, well recovery, total head and operating pressure. Basically the tank should be sized to allow the pump to run for at least 1 minute on recovery. Any cycle times less than 1 minute are considered a "short cycle" and affect pump life.

 

[sberry]

1 large tank will be fine. The 1 minute formula is correct. When operating sprinklers add as many as it takes to keep the well running just below kick off pressure, if it is 50 then add till it operates at 45.
Water is similar to electric, you can have 10 fixtures or showers but if you are using only 1 at a time the rest don't matter. Occupancy ratings in residential are normally based on amount of bedrooms not on amount of bathrooms.

 

[zak77]

Before you go guessing at the size you need you may want to check on the capacity and recovery rate of the well. There's probably a good reason you have such large tanks so you may want to find out if you need to keep a system like that or you can change it. I just had the pressure tank in my house replaced but my situation is a lot different from yours where i know what me well's depth is and recovery rate plus the usage so i calculated what i needed and so far it's working good. Without knowing all the facts all i'd be doing is guessing.

 

[sberry]

I would relocate and not bother replacing till it was needed. If one failed simply remove it. A flow test with a barrel isn't a bad idea.

 

[ard]

OP-

How long does the pump run NOW, from turn on to turn off?

 

[kbuhagiar]

OP-

How long does the pump run NOW, from turn on to turn off?

Just checked - from turn on (35 psi) to turn off (60 psi - not 65 as I earlier stated) - almost 4 minutes.

Spoke to my well/pump guy this morning, and he echoed the notion that, based on my water usage needs and the in-service life of my equipment (pump is 18 years old and the tank is 20) I should just remove the extra tank, as opposed to replacing everything now; he is pretty sure that my existing equipment should be good for another 3 or 4 years.

Of course, he mentioned that if I was bound and determined to replace everything now, he would love to sell it to me.
And he spoke very highly of the new cycle stop valve systems you mentioned earlier.

 

[sberry]

Sounds like you have a good well guy.

 

[holdover]

I use a big tank for this reason, when the power goes out, which it often does here in the winter, I have a bigger draw down before I have to start the generator to recharge the system.

 

[why worry]

By your avatar I see you are in northern California, though not as prone to drought as the southern portion of the state, a draw down test is important to have as you don't want to over pump the well. I have had troubles with over pumping on my well due to a lowering of the water table especially during the summer months.
Dave

 

[padroo]

I have had bad luck with those tanks , I am on my fourth one. I put a smaller one in and didn't get too scientific about it. I don't have any regular large water usage very often to worry about it. My original submersible pump lasted 23 years.