Water Heater Expansion Tank - What Pressure?

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Old 01-20-03, 02:20 PM
agent428
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Water Heater Expansion Tank - What Pressure?

I have a Rheem 50 gal gas Power Vent water heater with an expansion tank teed off of the cold water inlet. What pressure should the tank be at? I checked it and it is at about 68 psi. It indicates that the maximum working pressure for the tank is 150 psi and the pre-charge pressure is 40 psi.

Thanks in advance for any advice.
 
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Old 01-20-03, 02:28 PM
fjrachel
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More of a plumbing question, but I do believe the expansion tank is there to help stop water hammer.
 
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Old 01-20-03, 02:38 PM
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The tank should have about 100psi in it. It is there to accept water that expands as it is heated and is only necessary in a closed system. That is one with a check valve of some kind on the supply. Most systems today will require this as plumbing code is more strict on water leaving a house and going back to the street. The tank will not really help with water hammer. Or I should say it isn't designed to stop water hammer. Location of the device is more critical to stop water hammer and your tank can be installed anywhere in the house.
 
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Old 01-20-03, 05:20 PM
brickeyee
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The expansion tank should be placed to make sure it traps and retains its air charge. Sooner or later it will need to be drained also.
 
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Old 01-20-03, 05:32 PM
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I assumed it was a pre-pressurized diaphragm type tank. Like a Therm-X-trol. If so, it can be mounted anywhere in any position.
 
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Old 01-20-03, 11:59 PM
brickeyee
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Is therm-x-trol rated for potable water?
 
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Old 01-21-03, 03:42 AM
agent428
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Thanks for the responses and sorry about posting in Heating forum (newbie!).

The tank is an AMTROL Therm-X-trol expansion tank. I raised pressure to 100 psi (per KField's suggestion) but it didn't seem to impact hot water pressure or volume.

I'm curious to understand how you decide what pressure the tank should be at.
 
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Old 01-21-03, 05:28 AM
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Amtrols Therm-X-Trol is meant for potable water systems, unlike the regular X-Trol tank which is for closed loop systems only.

The tank is only there to accept the change in water volume due to heating. The pressure in the tank should always be more than the normal pressure in the house. That way water will not sit in the tank under normal circumstances but will push its way in after pressure increases. You could put 80 psi in if you wanted but it won't make a difference. Since the relief valve opens at 150psi, you just need to buffer the expanding water. Probably not even a quart. Thats why the tank can be so small.

Maybe I missed something but why were you suspecting the tank for a pressure or volume problem?
 
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Old 01-22-03, 03:47 AM
agent428
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Thanks. The reason I suspected the expansion tank is that when I cut the hot and cold water lines to move the water heater, I lost hot water pressure in my tub/shower. I thought maybe the expansion tank pressure had something to do with it. I think now it is some stray material in the tub/shower mixing valve. Thanks.
 
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Old 01-22-03, 07:16 PM
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Technically, when you close the shut off valve for the hot water heater, you have created a closed system, which requires an expansion tank. The current UPC requires expansion tanks on ALL new hot water heaters, whether you have a check system on the main or not. I know most inspectors don't care but I always try to put one in anyway.
 
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