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# Expansion tank quizz

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Member Posts: 22,536

The purpose of adding an expanion vessel to a closed loop system is to "accept" the increase in volume as the fluid is heated and it expands. As the fluid warms, expands, it pushes an amount of that expanded water into the expansion tank/ vessel.

If I had the tank on a closed loop system, started a circulator capable of adding 15 psi to that 15 psi static fill, what would the gauge at the tank show? Would the pressure be the same if the circulator was pumping at or way from the tank?

With this tank precharged to 15 psi, on the air side. What will happen to the pressure, on the air valve, when I put 25 psi water pressure on the tank inlet nipple?

Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream

• Member Posts: 23,728
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You're asking us? You're the guru!

But you sort of cheat on the first question — since you kind of give the answer. The pressure at the tank inlet is responsive only to changes in the volume of fluid in the closed loop system (assuming the pressure doesn't drop so low that the diaphragm or bladder doesn't bottom out). Hence the happy abbreviation "PONPC".

The second question is a little iffy — but assuming that the tank really was precharged to psi, which is 30 psia, when you raise the pressure on the water side — assuming the diaphragm or bladder doesn't do something evil — the prssure on the air side has to rise, too. So it will rise to the 40 psia and the air will compress to three fourths of the original volume (P1*V1 = P2*V2).

Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 22,536
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The tank is charged to 15 psi. That is a T style 3 port ball valve, so the tank is open to the gauge in this pic.

Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 2,512
edited March 2023
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I'll give it a go, but be warned I flunked my pregancy test. Nobody told me there was going to be a test today.

15 psi

Yes

25 psi

• Member Posts: 1,199
edited March 2023
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I will keep my closed system with 54+- gallons of water,Internal Air Separator,Airtrol valve,and 15 gallon steel compression tank as the old method of pressure control continues to work well as it was designed to do thanks to gas law, fluid dynamics, thermal dynamics, the dead men and Dan Holohan.

Thanks anyway.

😘

• Member Posts: 8,184
edited March 2023
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`The purpose of adding an expanion vessel to a closed loop system is to "accept" the increase in volume as the fluid is heated and it expands. As the fluid warms, expands, it pushes an amount of that expanded water into the expansion tank/ vessel`.

`If I had the tank on a closed loop system, started a circulator capable of adding 15 psi to that 15 psi static fill, what would the gauge at the tank show?` 15 PSI

`Would the pressure be the same if the circulator was pumping at or way from the tank?` SAME

`With this tank precharged to 15 psi, on the air side. What will happen to the pressure, on the air valve, when I put 25 psi water pressure on the tank inlet nipple?` It will read the same as the water pressure on the water side (25PSI)because that additional 10 PSI pressure added to the water side wil compress the air as it accepts the increase in water volume.

Remember the PONPC paper by Gil Carlison?

You can send my prise to the address you have on file Bob.

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?