Why is pressure switch differential always 20psi?

[Anthony W]

I discovered our 30 year old pressure tank has failed. Pump cycles often and water comes out of the air valve at the top. I wanted something in the 80 gallon range like the one we have now, but there's apparently a supply issue (COVID related?) and the only tank within a 100 mile radius I could find is a 52 gallon. So I'm planning to install that this afternoon.

Anyway, I thought I would install a new pressure switch and pressure gauge while I'm working on things. I chose a 30/50 switch like we have now, but noticed there are also 20/40 and 40/60 switches available. So I'm curious, WHY do the switches always have a 20psi differential? Why aren't there 30/60 or 20/60 switches available?

 

[Reach4]

20 psi is a good combination of having fairly consistent water pressure and limiting the stretch of the diaphragm. That said, most pressure switches can be adjusted to give 30 psi differential, and probably more. Tighten the nut (differential nut) on the smaller spring to do that. Usually you leave that nut untouched.

Adjust the differential nut if you want to raise or lower the
cut-out setting while keeping the cut-in pressure constant. The
differential nut is the 3/8-inch nut that adjusts the smaller of
the two springs in Models FSG, FYG, FRG, and Type G switches. Turn
the differential nut clockwise to increase the cut-out pressure and
counter-clockwise to lower the cut-out pressure. Adjusting the
differential nut will change only the cut-out setting while the
cut-in setting remains unchanged.​

Some premium pressure tanks have an internal dome (with a hole to pass air) to limit the stretch on the diaphragm, which primarily comes into play if the air precharge is too low.

There exist pressure switches that can do 10 psi differential. The ones I know are electronic and cost more and/or are less reliable than the cheap mechanical units.

Pressure tanks for reverse osmosis units support a higher differential.

If you want to keep the 20 psi differential, turn the range nut (on the bigger spring). 3.5 turns clockwise is about 10 psi. If you are not sure if you want 30/50 or 40/60, no problem. Go to 35/55, or whatever. Just remember to adjust the air precharge.

 

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[Valveman]

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. Your water doesn't come from the tank, it comes from the pump. The pressure tanks only purpose is to limit the number of on/off cycles, as that is what destroys everything in a pump system.

 

[Anthony W]

Any more than a 20 PSI differential will stretch the bladder/diaphragm too far.

Okay, that makes sense. As long as it works, I don't care. I was just curious why the differential was the same regardless of the switch options.

I got the new tank in yesterday. Had an issue draining the old tank, could only get about 1/4 of the water out. At first nothing came out, but letting air in the top valve released a bit. Then it stopped. I don't know if it was clogged with 30 years of sediment, or if the bladder somehow blocked the drain. Either way I had to wrestle the mostly full tank out of the pump house. Then I drilled holes in the bottom to get the rest of the water out.

The new 52 gallon tank looks like a toy compared to the old 80 gallon tank, but seems to be doing it's job. Let's hope for another 30 years!

 

[Reach4]

Anthony, nice that you got that swapped. What was the brand, model, and date code of the old tank? Just curious.

What is the brand of the new tank? Menards in our area has State brand in stock State Vertical Presurized Well Tank at Menards®, and I suspect those are the same/similar as A O Smith tanks from Lowes. A.O. Smith 52-Gallon Vertical Pressure Tank in the Pressure Tanks department at Lowes.com On paper they look good-- butyl diaphragm and stainless steel connection, tho only 100 psi max pressure rated. I understand that you will never approach 100 psi for your tank.

The next time that you have the water pressure zero, note that the tank sounds empty when you knock on it. That could be useful to know for the future.

 

[Valveman]

Once a bladder or diaphragm in a tank is broken, it is very common to need to drill holes in the side of the tank to get the water out.

 

[Anthony W]

What was the brand, model, and date code of the old tank? Just curious.

What is the brand of the new tank?

The old tank was an 80 gallon Waterworker tank I bought in 1991. I'm not sure of the model number, I think it said XW350.

The new tank is a Reliance PMD-52. The guy at the store said it was the same as the AO-Smith brand (Reliance bought them?).

Home Depot and Lowes both list 52 and 80 gallon tanks, but every store within 100 miles was out of stock (estimated delivery was Dec 31st!). I did see the 80 gallon tank at a Lowes two hours away, but when I got up the next morning to go get it, they had already sold it.

I ended up getting the Reliance tank at a local plumbing supply. They only had two in stock and he said they weren't expecting new deliveries for at least a month or two.

 

[Reach4]

The old tank was an 80 gallon Waterworker tank I bought in 1991. I'm not sure of the model number, I think it said XW350.

Water Worker is in the Amtrol family.

The new tank is a Reliance PMD-52. The guy at the store said it was the same as the AO-Smith brand (Reliance bought them?).

Yes! https://www.aosmith.com/About/Brands/ has a graphic with green left and right scroll arrows. Both State and Reliance are in the A O Smith family. https://www.reliancewaterheaters.com/media/33532/RPTBR0613.pdf

Reliance Diaphragm Pump Tanks

• For dependable protection of your jet or submersible well pump
• Steel shell with powder-coated exterior for maximum corrosion resistance
• Metal air charge valve is conveniently located and resistant to mechanical damage
• Strong butyl rubber parabolic diaphragm delivers dependable service
• Powder-coated inner shell protects the water reservoir