# What's the real reason for 20 psi differential for water well pressure switch setting?

Another forum (https://terrylove.com/forums/index.php?threads/why-is-pressure-switch-differential-always-20psi.95365/) had this advice: "Any more than a 20 PSI differential will stretch the bladder/diaphragm too far. Anything less than 20 PSI and you don't get the 15 gallons of draw down you normally get from a 50 gallon size tank. Even an 80 gallon tank only holds 20 gallons of water."

I don't think it's correct. I think the "stretch" has to do with the max pressure (not the differential).

Compare these two examples - I say the diaphragm has the same "max excursion" (stretch).

1. Differential 30 psi - setting 30/60
2. Differential 20 psi - setting 40/60 (a typical setting)

If I'm wrong, has anyone quantified the decreased life expectancy due to increased differential?

I was researching this topic because I wanted to increase the differential from the typical setting to gain some perceived increased pressure (and increased for a longer time period) at house fixtures with only slight increase in risk of "premature" failure of any system components. I was thinking of going to 42/67. (FWIW, I have a brand new WX-255. I also have an Express Water Heavy Metal triple 4.5" x 20" set up.) I'm too used to (consistent) city water pressure.

• The larger differential will allow the water pressure to drop lower before the pump comes back on. Causing the pump to work longer to bring the pressure back up. Aug 22, 2021 at 11:09
• Running longer is actually better for the pump than starting more frequently (to run less time.) However, it's quite possible to get into a situation where the pump can't pump anymore. I lowered my system from 40/60 to 30/50 because when the well was full, 60 was fine, but when the well gets drawn down near the pump (water 200 feet lower) the pump would max out at 57 PSI to the pressure switch until the well slowly refilled. That sort of thing depends on details of your particular well and pump curve, which is a whole other education for those coming from city water. Aug 22, 2021 at 12:22
• How much water a given pressure tank holds is variable with setpoint pressures. Tank charts from the manufacturer will typically show you 20/40, 30/50 and 40/60 (& rarely 60-80) drawdown (tidal volume) for a properly precharged (2-3 PSI less than the lower setpoint) tank. They are similar but not identical between manufacturers for reasons having to do with how the tanks are built. Aug 22, 2021 at 12:39
• @Ecnerwal - assuming it was your house with fixtures, when you reset you set point to 30/50 from 40/60, did you perceive a drop in pressure at a fixture (e.g., washing machine filling, toilet refilling, kitchen sink, etc.)? Did you ever consider using 30/55? Aug 22, 2021 at 19:05
• For most modern fixtures, it's practically imperceptible since the fixture itself is the major chokepoint with typical water conservation features. Does it bother me in any way - no. Does it bother me when the pump turns on and the pressure goes from 30 to 50 - also no. It barely affects flow out of the faucet, the toilet gets full at either pressure, etc. Then again, I grew up with wells, so I don't have whatever bias you appear to have towards pressure that does not change at all. Aug 23, 2021 at 0:50

Incorrect. If properly set:

A 30/60 set tank would have an air precharge of 27-28 PSI.

A 40/60 set tank would have an air precharge of 37-38 PSI.

So, the 30/60 set tank, starting from 0, will start accepting water into the tank at 27-28 PSI, while the 40/60 set tank won't accept water until 37-38 PSI. Thus, the diaphragm or bladder will stretch more on the wider setting. Whether that's bad for the diaphragm/bladder depends on the details of tank manufacture. Most are happy to be set at 20/40, and that should be the "same stretch" as 30/60, 40/80 or 50/100. 20/100 would be "more stretch" and might well break something. The max pressure has to do with the outer containment vessel, not whether you've broken the internal bladder/diaphragm/membrane.

## All of which is irrelevant to solving your perceived problem with modern technology.

You want city-like constant pressure.

You can purchase variable-speed pump controllers (variable frequency drives) that will give you city-like constant pressure, or a lower-tech and (generally, at present) somewhat less expensive pressure-regulation-valve thing (which I refuse to shill the most widespread version of by name, but I believe they have bought a shill-space on the forum you looked at, so you'll have no trouble finding them) that will get you near-constant pressure after one drop to the lower setpoint to turn the pump on, so long as you are using water at a sufficient rate, followed by one excursion to the higher setpoint (slowly) after you turn the water off.

Both of those will work with no increase in your present pressure tank size, and they will work with a much smaller pressure tank if/when you need to replace your pressure tank or are building a new system.

You could also go older tech and run a bunch of pressure tanks from 60-80 (or perhaps 60/120, but that might be impractical from the pump and pressure on the down-well pipe point of view) into a pressure regulator set at 50-60 output, but that would be much more costly in terms of money and space required.