Expansion tank size?


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Old 10-06-08, 04:38 PM
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Expansion tank size?

Anyone know what determines the size of the expansion tank you need in your boiler system? Does it depend on total amount of water in system, highest height of radiation piping, size of pumps, size of boiler, or other? Should an old tank (I have a huge one on my ancient system at basement ceiling level) be routinely replaced when a new system is put in - if so by what size?
 
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Old 10-06-08, 06:30 PM
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www.amtrol.com

they have a sizing guide.

The simplest way is to fill the system with water (minus the old expansion tank), then drain it into a 5 gallon bucket until it's empty. A converted gravity system might have 100 gallons in it.

I would definitely go with a bladder tank(s) and ditch the old one.
 
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Old 10-06-08, 07:27 PM
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Water volume, and the min-max water temperatures determine the size of the tank.

If you have 100 gallons, and heat the water from 70 to 180, you will have roughly 104 gallons. Your tank needs to be big enough to admit that 4 gallons of expanded water.

Keep in mind that if you do ditch the old tank, you will also need to add an air separator and an automatic air vent in addition to the new tank. You can't simply connect the new tank to the old pipe... well, you _could_ but then the air that's in the system would have no way out, where it used to (hopefully) find it's way back into the expansion tank. But you don't want it there with a bladder tank.
 
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Old 10-07-08, 06:28 AM
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Thanks for this excellent explanation and link. Given my 4" c.i. main heating run and radiator sizes, it sounds like I may need more than one tank - more measurements needed!
 
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Old 10-07-08, 06:42 AM
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You can find the water volume in radiation and piping here. Look under the water column at the last two choices. You will have to add the boiler water volume which you should get from the manufacturer.
If you have copper tube baseboard use the manufacturers chart. They do vary the size from one to another.
http://www.comfort-calc.net/tech_area_index.htm
 
 

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