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sizing expansion tanks

Glenn Sossin_2
Glenn Sossin_2 Member Posts: 592
Check out page 4. That should help you.


  • Bob_36
    Bob_36 Member Posts: 83
    sizing expansion tanks

    I want to size an expansion tank for my system. I have six ci rads. I read Dan's book "How Come", to find the EDR. Is there a conversion to gal. or do I have to fill a rad with measured water to get an idea of total gal? I don't know if I can use a thirty gal size or need to upgrade to the sixty gal size. Any help to save me from filling rads would be appreciated.
  • Brad White
    Brad White Member Posts: 2,398
    Don't Sweat It Too Much

    The cost of a #30 tank is less than half of a #60 that I have seen. Worst case is, you buy two and they work together connected to the same point, for less equipment cost. If one fails you have a back-up too!

    What is your time worth compared to the cost of a second tank?

    There are tables on water volume of radiators and piping but it is not all that critical. Normal expansion from 50 degrees to 200 degrees is about 4 percent.

    Houses I have owned with CI radiators and in the 1500 to 1800 SF floor area range had water volumes of between 50 and 80 gallons given old iron pipe too, just a ballpark point of reference. Yes, your volume may vary but just for comparison there is a number for you.

    If you are off by 20 percent or say 20 gallons, that is 0.80 gallons to be absorbed. Not a huge amount. Just keeping it in perspective.

    Here is an alternate method I was taught years ago:

    The advice I was given by a favorite mentor, Ray Stevens, was to take the cubic volume of an imaginary box around the radiator and use 25-30 percent of it as the internal volume. It would still be likely high.

    Thus a 38 inch high column type radiator, 8 inches wide and 30 inches long (5.28 cubic feet at 7.49 gallons per) would have a gross volume of 39.5 gallons. 25 to 30 percent of this would be 10 to 12 gallons. Ray even suggested to throw in the run-outs as included in that number, it would be that conservative.

    Forget sizing based on the BTU output; those are averages based on small diameter copper pipe as you probably well know.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Dave_4
    Dave_4 Member Posts: 1,405
    another site

    goto www.watts.com and type "expansion" in the search box - they have built in calculators for potable and non-potable and lots of other good stuff - i have used them to size my commercial jobs with 2 Turbomax’s, each with 119 gallons of "boiler" water - i also use dan's rule of thumb, for pipe volume:- take twice, the width, length, and height of the building, to calc the total piping len and plug that number into the pipe vol calcs - re-did a school with 5" steel pipe, used dan's method and it worked just fine and this building was odd shaped - just treated it as if it was a larger rectangle on the outside - i doubt though, that in your case, that you would need nore than a #30
  • Bob_36
    Bob_36 Member Posts: 83

    Thanks for the quick replys. As usual, your info was right on. Brad showed me how much I forgot or wasn't paying attention in math class. I couldn't figure how you arrived at the result. So I looked up the Watts' website and the chart listed a #30 as all of the posts suggested. Thanks again.
  • Ted_9
    Ted_9 Member Posts: 1,718

    Gsossin, that pdf is a 2 page brochure and I didn't see where it says how to size expansion tanks. Is that the wrong pdf?

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