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Thread: Boiler expansion tank replacement: steel expansion vs bladder?

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  1. #1
    DIY Junior Member grider's Avatar
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    Default Boiler expansion tank replacement: steel expansion vs bladder?

    My 48" x 12" horizontally mounted steel expansion tank has developed a pinhole leak (it's 40 years old but has otherwise served me well) and needs to be replaced as the PRV is opening due to excess pressure build-up each heat cycle. I realize that the standard tank nowadays is the bladder type, but they will never last as long as the old type, so, my question is: Should I replace the old tank with a new bladder type or one like the old one? If so, what would be the equivalent size bladder tank to the 4' x 1' steel expansion tank I now have, and is it crucial where it should be located in relation to the boiler? Thanks for any helpful comments! Greg

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    My personal opinion...no 'closed' system heating system likes air; the only way to keep air out is if it is truely closed. A bladder tank does that. Each time you open yours up, drain the tank, you introduce new air into the system. This slowly gets purged out, but in the process provides oxygen to rust any ferrous parts. A bladder tank can last 5-years or more, and are fairly inexpensive.

    The way to size the thing is to determine how much water is in the system when it is full, then specify how big of a swing you have (i.e., from cold to max hot). That determines the volume you need to provide for expansion. Watts makes a decent tank and has a sizing calculator. Useful, even if you choose a different brand. http://www.watts.com/pages/support/sizing_ET.asp

    The best place to install it is on the feed side near the circulator, but it will work anywhere in the system. this way, when the pump turns on, it has a little extra volume available.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Member tk03's Avatar
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    I prefer the old steel tanks due to the fact the bladder is oxygen permiable and the air from the tank get into the water. The old steel tank is a maintenance free tank , no draining, if properly installed. The biggest downfall of the old steel tank is the systems have auto air vents installed. No auto air vents on the old steel tank systems. Pipe uphill all the way, no valves in the horizontal piping.
    For more info check out this link.
    http://www.comfort-calc.net/expansion_tank_info.html

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    In the Trades Tom Sawyer's Avatar
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    What? In 36 years of servicing boilers I have NEVER ever run across a steel tank that was maintenance free. The air cushion gets absorbed into the water over time (usually within a couple years) and requires the tank to be drained which can be time consuming and difficult to do especially for a homeowner. Nobody installs steel expansion tanks anymore. Even on very large commercial jobs we install bladder type tanks.

    A direct replacement for what you have would be a extrol #60

    For the record, whoever wrote that web page needs to do some serious editing for typo's and some more research for content because many parts of it are just plain wrong, (cool down period? there is no cool down period unless the boiler is cold starting) though he did do a nice job of cutting and pasting information from manufacturers.
    Last edited by Tom Sawyer; 02-25-2011 at 05:44 AM.

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    DIY Member tk03's Avatar
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    The air cushion gets absorbed into the water over time (usually within a couple years) and requires the tank to be drained which can be time consuming and difficult to do especially for a homeowner
    Where does the air go when it leaves the tank?

    Many people leave the old steel tanks in due to never need draining.
    The reason for draining is due to auto air vents venting the air that leaves the tank.
    Sure the water goes in and out of solution and leaves the tank. It than goes thought the system and back into the tank instead of venting out due to some moron installing an auto vent in the system.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    Oxygen will combine with any ferrous materials in the boiler. The nitrogen anc carbon dioxide will form acids. Air has NO place in a closed system.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #7
    In the Trades Tom Sawyer's Avatar
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    The air does'nt leave the tank it stays at the top of the tank. the loop does not circulate through the tank. There are no auto air vents on the tank. If there was an auto air vent there would be no air in the tank and no cusion for water expansion.

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