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While you guys are on expansion tanks, do you all set the tank air pressure equal to or slightly more than the water PSI? or do you leave it at the factory precharge? Curious what others do.
 

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While you guys are on expansion tanks, do you all set the tank air pressure equal to or slightly more than the water PSI? or do you leave it at the factory precharge? Curious what others do.


You almost never leave it at the factory precharge. The exception being it happens to match the pressure you require. It's also best to install the expansion tank per the manufacturers instructions.

For a well pump system you set the air pressure to be equal to or slightly less than (-1 or 2psi) the cut-in pressure of the pressure switch.

With city water you would set the air pressure to be slightly less than (5psi) the pressure regulator output or the incoming pressure if there is no regulator present. If there is a regulator the expansion tank should go on the house side of the regulator.

The idea being that with no regulator the tank will absorb pressure spikes when a city pump kicks on. When there is a prv present the tank absorbs the pressure increase when the hot water heater goes from high demand (colder) to low demand (hotter, water expands, pressure increases).

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Seems like if the air pressure is lower than the water PSI The expansion tank has little to no room for expansion. Wouldn’t the higher water pressure fill the ballon to capacity? Would Newton’s law not apply here? I was always taught slightly above. 5psi range. I’ve seen others never touch the Schrader and not have problems. Maybe I’m not grasping the tank construction fully. Mainly in regards to thermal expansion.
 

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Seems like if the air pressure is lower than the water PSI The expansion tank has little to no room for expansion. Wouldn’t the higher water pressure fill the ballon to capacity? Would Newton’s law not apply here? I was always taught slightly above. 5psi range. I’ve seen others never touch the Schrader and not have problems. Maybe I’m not grasping the tank construction fully. Mainly in regards to thermal expansion.
1 or 2psi lower is miniscule compared to the 20-30psi swing that the air pressure will see when the system is active. 1 or 2psi has almost no effect on the total room for expansion.

And of course they have problems, eventually all tanks have problems and need to be replaced. What we're trying to do is increase the time the tank lasts. Having a proper air charge is important for that reason, not just because the system may not behave ideally.

There is a greater risk of the bladder getting harmed if the air pressure is higher than the water pressure. This causes the bladder to get forced into the opening in the bottom of the tank. Aside from the fact that the opening is small, possibly sharp, and would abrade the bladder by itself, there is also usually a build up of sediment in the bottom of the tank and you want to avoid the bladder being pressed into this.

On the other hand if the air pressure is slightly lower the bladder is just stretched a little in the direction it is intended to stretch. Ideally the air pressure would exactly match the static or cut-in pressure. But since the town water and the pressure switch on your well are never exact we "air" on the side of caution and go with a slightly lower pressure.





Thermal expansion tanks are pretty easy to get right and I bet you could just leave them at the factory precharge the majority of the time. I believe amtrol brand comes with a 55psi precharge. The air charge should be a little lower than the lowest pressure you expect the system to see. If you're installing an expansion tank on city water you likely have a prv which are often preset for 55-75psi. So a 55psi air charge would be fine. All you're trying to stop is water hammer and the blow-off that would occur from the cold water heating up and expanding which isn't much. That's why they are only the size of basketballs.

With well tanks you need to be a little more precise as improper pressure can cause pressure surges and drop outs.

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In St. Louis county you have to have them in The City of St. Louis you don't the difference is the one has a meter the other doesn't. BUT they have to be set to static pressure and have to use a dielectric union, a brass cuppling or we use the bronze propress union. Also the biggest problem with them isn't that they fail its they are not checked yearly like they are supposed to be just like dielectric unions
 
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