Expansion tank question

[rh71]

Have a 2gal expansion tank on my indirect water heater that needs replacing. Problem is I'm getting a 90psi water pressure reading at the pipe/valve that leads from it (same pressure the town sends water to us). What am I supposed to set the new expansion tank's PSI at since most max at 80psi? Will that difference matter that much and is it unsafe to do 80psi in the first place? I do have a pressure reducing valve hooked up to the pipes/boiler though it's not in the direction of the water heater but the rest of the house.

This is what I bought and the sticker says max air charge 80psi:

https://www.lowes.com/pd/EASTMAN-Thermal-Expansion-Tank-2-Gallon-Universal-Steel-Water-Heater-Pressure-Regulator/1000522213

Here's another for $20 more, not sure if I should've spent more:

Amtrol Therm-X-Trol ST-5 Expansion Tank ST-5 - The Home Depot

The Therm-X-Trol Expansion Tank can be used in a variety of places such as apartments, office buildings and hospitals. This expansion tank helps eliminate BTU and water waste. The tank helps protect the

www.homedepot.com

 

[Toastedlightly]

The 80 psi is the pre-charge, it has a max working pressure of 150 psi. If your pressure is at 90 psi, it will compress the bladder's volume so it is then at the 90 psi pressure.

Likely the precharge is limited so it doesn't damage the bladder inside it.

 

[rh71]

The 80 psi is the pre-charge, it has a max working pressure of 150 psi. If your pressure is at 90 psi, it will compress the bladder's volume so it is then at the 90 psi pressure.

Likely the precharge is limited so it doesn't damage the bladder inside it.

Sticker says pre-charge is at 40 psi though (and I'll check when it gets here tomorrow). "Maximum Air Charge: 80 PSI"

 

[DooKey]

Your home needs a water pressure regulator. 90 psi is too high. 60 psi is normal.

 

[rh71]

Your home needs a water pressure regulator. 90 psi is too high. 60 psi is normal.

As mentioned, there is one hooked up behind the boiler. Looks like it goes to the rest of the house, but not the ones I tested (washing machine, outside spigot, and under the expansion tank).

What I want to know is if it's safe or even effective to fill the 80 psi air and hook it up.

 

[Greenman]

Don't know about the expansion tank, but 90 psi water pressure is to high. Around here we run at 32.

 

[Toastedlightly]

The tank should work in your application with the 80 psig pre-charge and perhaps with the 40 psig pre-charge.

There are two numbers that are on the tank. The first is the maximum pressure you can pre-charge the air bladder to. The second is the maximum operating pressure (do not exceed this and make sure a pressure relief exists in the system [likely on the water heater] to relieve below this number).

When you are pre-charging the air bladder, it has pressure on the bladder side, but only atmosphere on the other. So that pressure is limited to a maximum pressure (in this case, 80 psig). If you were to exceed this maximum fill pressure, the pressure differential across the bladder may cause the bladder to rupture.

So now the air bladder is pre-charged with air (lets say we put it up to the 80 psig). What happens next is you fill the system with water. That water is at 90 psig. The water coming into the expansion tank will compress the bladder back down until the pressure in the bladder is at 90 psig when compared to the outside of the tank. However the pressure differential of the bladder is now 0 psig. The bladder is no longer under stress and there are no issues with rupture.

If this pre-charge pressure isn't enough, the air in the bladder will become too compressed and not allow the fluid to expand, thus causing an overpressure in the hot water system and likely leading to the water heater's relief valve opening. I am not sure what level of pre-charge would be used, but the two systems I have setup I set the pre-charge to the operating pressure (which should be below the allowable system pressure) to maximize the amount of expansion which can be contained.