Understanding Tank Sizing

[daveward]

Here are a few facts about my residential single pump setup designed and installed in 1979:

1.  300 foot deep 1 1/2 HP pump with a "pump saver" electronic device...as during the summer the water level is scary low...in fact, our lawn/garden irrigation is minimal year-round.
2.  I have a 80-85 gallon tank about 15 years old with a 40-60 pressure switch.  Lately the 38 lb setting is getting hard to maintain without more frequent checks.  I have begun to research tank options and found you.

I’ve been reading extensively about your product on your site and a plumbing forum that mentions you (meaning you and your product have a good reputation).

I totally understand the idea that a larger pressure tank is not needed with CSV, even though I come from the “if bigger is better then biggest must be best” era.   I keep wondering if “only” 4.4 gallons with a 1 gallon drawdown could be correct for me.  Let’s assume the following usage (currently two adults in the home, but it’s a two adult+three kid home and will be again someday):

Activity                                         CSV Cycles with                                   CSV Cycles with proposed
                                                      1 gal drawdown                                    5 gal drawdown
20 "1.6" gallon toilet
flushes throughout
the day…32 gallons                                  20                                                  0-10*

1 shower…35 gallons                                  1                                                     1

1 washer load…35 gallons                           1                                                      1

1 dishwasher load…14 gallons                     1                                                       1

5 meal prep and dish
cleaning…4 gallons                                     5                                                       0
1 steeping bath fill…40 gallons                    1                                                       1
2 toothbrush and
Listerine procedures…2 gallons                    1                                                      0

Totals                                                        30                                                     4-14

*  It’s true that 0 cannot be the true number for toilet flushes, as some of those flushes would trigger a cycle, let’s say half would…that still brings my total to only 14.

OK, OK…that’s enough personal info about my flushing habits…I hope you get the idea.

Your statement is as follows:  “A CSV with a 4.4 gallon size tank (1 gal draw down) will cause the pump to cycle about 40 times per day on the average house. Using the CSV with a 20 gallon tank (5 gallons of draw down) will cause the pump to cycle about 20 times per day on the average house. In most cases, this is not enough difference to justify the use of any larger than a 4.4 gallon tank. The way the CSV controls the pump system, there is really no justification in using a larger tank. Then if you consider the cost and space needed, there is even less justification for using a larger tank.”

Cary…you write a great story…what I suggest you consider is an addition of some language that explains your most important conclusion, which is:

“Using the CSV with a 20 gallon tank (5 gallons of draw down) will cause the pump to cycle about 20 times per day on the average house. In most cases, this is not enough difference to justify the use of any larger than a 4.4 gallon tank.”

To me, it sure looks like it’s enough to justify a larger tank!  Cutting the cycles from 40 to 20 is great, I’d say.  You should explain why it’s not great, if that’s what you believe.  If you suggest on the one hand that cycles are a pump-killer, then getting them down to the very minimum seems a logical goal…you must know something you are not saying…you must.

And speaking of justification, the effort and expense of replacing a pump 300’ underground is quite an eye-popping justification (for me) to spend a bit more on a larger tank (let’s say a 5 gallon drawdown tank, for instance).

The idea that the tank takes up space is true enough except that I already have space being taken up by an 80-85 gallon tank and sure I’d get some of that back by using a smaller tank, but then all I would do is add more collectible goodies (or crap, as my wife refers to it) to my garage.

I would truly appreciate your feedback.  Thanks so much.

Dave
 

[Cary Austin]

You have done a very good job mapping out the demands and how many times the pump will cycle on and off.  The only thing you did not take into consideration was the number of times the pump would already be running for a demand like a shower, while at the same time in another part of the house someone flushes a toilet, washes their toothbrush, or uses water in any other way.  When the pump is already running, supplying water to a shower or washing machine, and someone elsewhere uses water for any reason, the pump does not see an extra cycle.

Since most people are up and getting ready for school or work at the same time, we have found that the pump may run continuously for an hour or so.  The CSV varies the flow to match a shower, then opens more for an additional shower, even more if someone flushes a toilet, then resets itself to a single shower again.  All of this happens with only one cycle.  Only when a toilet is flushed by itself, during a time when no other water is being used in the house, will the single toilet flush cause a cycle on its own.  This actually cuts the number of cycles you have added up in half if not less.

But even considering that every toilet flush will cause a cycle as you describe, the difference between 40 cycles and 20 cycles per day is not going to reduce the life of the pump by very much.  Real damage from cycling occurs when running sprinklers or a heat pump for many hours a day, which can cause hundreds of cycles per day.  Eliminating all these cycles for long term uses of water far outweighs an extra 20 cycles per day for using the small pressure tank method. 

I have even been known to run my irrigation zones during the same time everyone is getting ready for school and work.  This way the pump only cycles once in the morning, because it stays running the entire time everyone in the house is using water.  Not that overlapping the irrigation zone with a high demand time for the house is necessary, but it does eliminate ALL cycling during this period of time.

I also use to believe that a 20 gallon size tank with 5 gallons of usable water was needed.  However, many installers, putting in thousands of systems with a small tank, over more than a dozen years or so, have taught me that a large tank is not necessary.  Although, a CSV will work with any size tank, so use whatever size you are comfortable with.  I just believe that in most cases, any larger than a 4.5 gallon tank is a waste of space and money.

Thanks Dave

[Cary Austin]

The CSV1A would probably be best for your application.  We sell these for $206.00.  But if you need to replace the tank as well, you might consider the Pside-Kick kit that comes with the CSV1A, pressure switch, gauge, fittings, pressure relief valve, 4.5 gallon size tank, wall mount brackets, even a roll of Teflon tape for only $399.00.  This kit puts the pressure tank and pressure switch close together as they need to be.  Then you can add an additional tank if you want anywhere after the CSV.