How important is expansion tank for DHW

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Old 03-09-10, 06:24 AM
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How important is expansion tank for DHW

A plumber was giving me an estimate for replacing my boiler and when I mentioned that I just recently replaced my waterlogged dhw expansion tank (6 yrs old), he said that happens quite often with dhw tanks in his experience. I looked at a bunch of his photos of installations and only one or two of more than 20 had that little tank (the heating lines always had the bigger one). My question is, how important is that tank? I'm sure he would put one in if I insisted.
 
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Old 03-09-10, 08:42 AM
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In todays systems it is normally required due to check valves in water meters. As you heat water the pressure goes up and water is a non-compressible so it drive up pressure. The pressure may go high enough to make the relief valve open. The tank having an air pocket in it will compress as the water pushes into the tank while heating. Therefore controlling the water pressure.
The only time I do not use a tank is when the house is on a well. The well tank is the expansion tank in that situation providing there are no check valves between the hot water tank and the well tank.
 
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Old 03-09-10, 09:56 AM
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I like check valves on the cold supply even with a private well. I have found that since the pressure swings with a well system, that the water heater will 'breathe' in and out on the cold line every time the pump cycles.

Keep in mind this fact:

Bladder tanks will lose 1-2 PSI annually, due to the air 'weeping' through the bladder internally. For the same reason that childrens baloons deflate in a few days, so will the 'baloon' inside those tanks.

On a heating system the tank pressure should be adjusted to the nominal cold system pressure, i.e. 12-15 PSI.

On a water system, the tank should be set 1-2 PSI BELOW the nominal system pressure. If I were installing a tank on a water heater with a well system, I would set the tank 1-2 PSI below the nominal cut off pressure of the pump (the high side).

Bottom line is, don't install a bladder tank and forget it's there... check the pressure at least bi-annually!
 
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Old 03-09-10, 12:06 PM
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Thanks for the info and insight. I appreciate you guys sharing your knowledge and time.

Mike
 
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