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Old 03-10-2011, 09:20 PM   #1
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water heater question.

Is a captive air water tank the same as a thermal expansion tank?


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Old 03-11-2011, 01:55 AM   #2
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I'm not a pro plumber, but I think that the answer to your questions is no.
A thermal expansion tank goes near a water heater to absorb hot water expansion into supply lines.
A "captive air water tank" I believe would be a pressure tank such as at a pump that controls the cut-on/cut-off of the pump (air pressure in a bladder).


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Old 03-11-2011, 08:00 AM   #3
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I am installing a new water heater today and want to add a thermal expansion tank to the system. I got this captive air water tank at a sears -Hdw that is closing, thinking it is a thermal expansion tank. When I got home and looked at it I realized it is is not. However they seem to be the exact same thing, a pressurized tank with a bladder that absorb excess pressure. Am I wrong ? can they be used interchangably ?

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Old 03-11-2011, 09:14 AM   #4
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Subject to their capacities the two kinds of tanks will work interchangeably. An expansion tank is suggested (sometimes code-required) right at the water heater on the cold pipe after the cold inlet shutoff valve so it is not accidentally isolated from the water heater if the latter valve is shut off. The disadvantages of a large one is that additional bracing or guy wires are needed to support it and also the tank is more expensive. It should be above the water heater, not put on the floor.

The captive air/water tank is fairly large so pressure changes will be more gradual and the pump (well pump) does not go on and off very frequently for short periods when you are using the water.
The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit.

Last edited by AllanJ; 03-11-2011 at 09:22 AM.
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