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expansion tanks

Paul S_3
Paul S_3 Member Posts: 1,257
how do you size expansion tanks for systems? for example how do you know if a system needs a #15, 30,60,90 extrol tank?
ASM Mechanical Company
Located in Staten Island NY
Servicing all 5 boroughs of NYC.
[email protected]


  • Unknown
    edited March 2010
    The best way,,

    is going by system water content related to the temp you want to raise-it to.

    I use an accurate water meter for this, but you have to drain the system to find-out.

    If it`s an existing tank that gave no-problems,, you can usually cross-reference to another "acceptance volume" diaphragm tank .

    PS- Some "rules of thumb" calculations are out there, but these are strong guesses,, the ONLY WAY to really know is the actual water content.

    Good Luck.  ;-
  • Paul S_3
    Paul S_3 Member Posts: 1,257

    i really dont understand ...is there a calculation?
    ASM Mechanical Company
    Located in Staten Island NY
    Servicing all 5 boroughs of NYC.
    [email protected]
  • Dave Yates (GrandPAH)
    Dave Yates (GrandPAH) Member Posts: 281
    Boyle's Law

    P = pressure

    V = volume

    P1 x V1 must equal P2 x V2 where P = original sys PSI prior to heating the water & V1 = the expansion tank's air volume prior to heating the water. Say, for example you have a 4.5-gallon air volume in a tank. The sys PSI is 12. V1 (4.5) x P1 (12) = 54. 54 now becomes your constant-value. If the pressure rises to 24 after heating the sys, then the volume must equal 2.25. Wessels has a chart showing the multiplication factor for any given temp rise. You can use that to determine total sys volume in gallons if you know V2. Or, if you know total volume in gallons, you can use the multiplier to accurately predict final PSI after heating the water to any given temp.  
  • Well, you wanted to know,,,

    and Dave Yates explained-it,,, have fun! ;-)
  • Big Ed_4
    Big Ed_4 Member Posts: 2,687
    Grunt's Law

    If pressure rises more then a few pounds on temperature rise , add a bigger or twin in a second tank ... Twin tanks work better then single tanks ... .... The # 15 is designed to be equal to a 15 gallon expansion tank , but they lied ... use a #30 instead which would be enough for most residential homes .… Until you Start getting in the three story homes or small buildings with large cast iron radiators , then start off with a # 60 or a #90 ....… When you start getting into big systems the grunt is out of the loop and an engineer will tell you what to use ... Thats when the above method is used , too much time and money involved , now you can get paid to do the math ...
    I have enough experience to know , that I dont know it all
  • JStar
    JStar Member Posts: 2,752
    Another quick chart.

    Extrol has a quick reference chart. I'm guessing it makes a lot of assumptions as to pipe lengths and total system size, but it will get you close if you lose your calculator. 
  • Chris S
    Chris S Member Posts: 177
    expansion tank sizing

    I used the chart in Extrols' literature for my 1100 gallon system.  It was wrong- not really sure why, but on Extrols website if found the formula.  I then calculated the tank size, and added a second tank to add  up to the total expanded volume of water , and now it works great.  The rep could not explain the discrepency.
  • sizing expansion tanks

    I was curious to this question myself.  My research led me to conclude a few years ago that you can run a radiant system with miles and miles of tubing off an extrol 30.  Baseboard jobs even in huge huge houses an extrol 30.  Cast iron radiators newer style tubes extrol 30.  Only if  you have older CI rads that are bigger you will need an extrol 60.  I never put in extrol 15s.  If you cant spend an extra $20.00 on an extrol 30, you wont be in business very long.  Hot water heats and expands but an extrol 30 is able to handle most jobs.  Even if you abandon an old steel tank in the ceiling your new tank has a badder it does not have to be as big.  Hope it helps its a rule of thumb Ive followed for years havent gotten burned yet!
  • Gordan
    Gordan Member Posts: 891
    Don't forget about any buffer tanks, though.

    Or reverse indirects.
  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 16,488
    You can use this from our friends at Amtrol.

    Retired and loving it.
  • Al Roethlisberger
    Al Roethlisberger Member Posts: 194
    Conflicting results....

    My system specs:

    Total volume:  380G

    Max system temp:  190F

    Boiler net:  172MBH

    Type radiation:  cast iron radiators

    If I use the online calculator at:  http://www.amtrol.com/sizing.htm

    Plumbing & Heating -> Extrol

    The recommended tank is the SX-30V (total volume of 14G, accept volume 11.3)

    However, as noted earlier in this thread, if one uses the formula Extrol provides on page 8 of their Expansion Tank Brochure using the following values:


    1.  Total system volume:  380G

    2. Min system temperature:  40F (chose worst case)

    3. Max system temperature:  190F

    4. Max system pressure:  27psi (minimum they provide data for)

    5. Min system pressure:  20psi

    6. Net expansion factor from their table:  .0313 (40Fmin/190Fmax)

    7. Multiply line 1 x line 6:  11.894 calculated acceptance volume

    8. Acceptance factor from their table:  .168 (20psimin/27psimax)

    9.  Divide line 7 by line 8:  70.80G calculated total tank volume

    One is then instructed to match these values on pages 6 & 7 to the correct tank using line 9 for "total volume" and line 7 for "acceptance gallons".

    If I then use that data, the "acceptance gallons" value would likely allow any tank from an SX-30V through SX-60V (or larger).   But the "total volume" could only be satisfied by their largest SX-160V, which has an acceptance of 46G.

    So, do I need an SX-30V(or maybe a 40), or the the giant 160V?  There's a significant price difference *laugh*

    Just a DIY'er trying to learn, and improve and maintain his converted ca 1929 overhead gravity hot water system since there is no one local that can.
  • JStar
    JStar Member Posts: 2,752
    Does it make more sense...

    ...to size by water volume and not btuh rating. I mean...water is going to be hot when it's hot. The tank shouldn't care how fast it gets hot.

    Here a some helpful links.



    Maybe we'll eventually get a good formula to work with. 
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Sizing expansion tank.

    John Siegenthaler's book


    has a CD-ROM in the back that will run in Windows. It includes a neat "Design Studio" that will do heat losses, compute circulator requirements, and things, including sizing expansion tanks. This is the educational version of the program; it seems to me a professional might wish the commercial version. But it will sure give you a good idea how the program works, and do calculations like this for you.

    I am not qualified to say how good his program is, but my guess is that it is very good.

    The book sure is.
  • Al Roethlisberger
    Al Roethlisberger Member Posts: 194
    Me too....

    I hope so as well, because the difference in price for a SX-30V or 40V is about $400 less than the 160V, and I'm about to embark upon that project.

    Any thoughts based on the data I provided which model is more "correct"?

    Just a DIY'er trying to learn, and improve and maintain his converted ca 1929 overhead gravity hot water system since there is no one local that can.
  • MIke_Jonas
    MIke_Jonas Member Posts: 209
    Not to hijack the op..

    Al, your figures do not look correct.

    Why 40d for minimum temp? Are you really going to run 40d water through your house when heating is required?

    20 psi minimum..Why so high? Tall building?

    190d max...is this your system temp for a design day?

    380 gallon system volume. This is not unusual, I see residential systems this size often. I do measure them on occasion, when draining them. Is this how you arrived at 380 gallon?
  • Al Roethlisberger
    Al Roethlisberger Member Posts: 194
    Yes, accurate specs.... for the most part

    I chose 40F from their chart to simply test  for "worst case" for the acceptance volume.  The factor generated from that low temperature is only relevant in the acceptance volume calculation as they use in their worksheet formula.  This generated the ~11.8G acceptance volume figure.  If I had used something more realistic, say 70F as the min temp, it simply would have decreased the acceptance volume.  In this case, my problem with the differing figures from the online calculator and Extrol worksheet are for total tank capacity, not acceptance volume.

    Similarly with the 190F max temperature, I can't recall if my max mix temp is 180 or 190, but 190 simply gave a little cushion to the acceptance volume calculation.  This doesn't impact the total volume figure, using their worksheet.  The ~11.8G acceptance volume seems relatively consistent in both the online and worksheet examples.

    Regarding the 380G figure, yes I drained the system using a water meter and obtained 380G.  This is an old overhead gravity system.

    What bothers me is that the online calculator delivers a spec for the SX-30V(so I'd probably opt for a 40V as some cushion), and the worksheet delivers the same results for acceptance volume, but the total volume specified by the worksheet would require their largest 160V.   Something isn't right.


    Just a DIY'er trying to learn, and improve and maintain his converted ca 1929 overhead gravity hot water system since there is no one local that can.
  • Al Roethlisberger
    Al Roethlisberger Member Posts: 194
    Any thoughts?

    Coincidentally with the start of this thread/question by the original poster, I'm just about to start ordering various parts, including my expansion tank.

    So any clue on which size is right based on the discussion above?

    Will that SX-30V really be right as generated by their online calculator?


    Just a DIY'er trying to learn, and improve and maintain his converted ca 1929 overhead gravity hot water system since there is no one local that can.
  • The Grundfos Handbook

    explains a method of determining system water volume without draining, based on temp-rise,, but it`s not available online.
  • Ken Wickre_6
    Ken Wickre_6 Member Posts: 12
    Expansion tank formulas - not as tough as they look

    Assuming water, but no glycol, whether you use 40 deg or 70 deg, the amount of expanded water drops from your calculated 11.9 gallons to 11.3 gallons.  So using your 40 deg start doesn't make much difference, but it will require - within the Amtrol family anyway - going to a minimum 22" diameter diaphragm expansion tank - SX-90V or larger for example.  Also, assuming the height from the tank location to the highest point in the system to be less than 20', you can operate the system at 12 psig (with the expansion tank factory charged to 12 psig - but double check it) and accept the 27 psig that the charts recommend - which is the 30 psi relief valve setting less 10% (again, assuming you have a 30 psi relief valve). 

    Now look at your acceptance factor from the charts based on the above - .36;  which divided into your expanded water of 11.9 equals 33.1 gallons required tank volume.  Based on your data and the above calculations (assuming a 30 psi relief valve and 20' or less system height) I would select  the SX-90V. 

    The reason you are seeing different tank volume requirements is based on Boyle's law as GrandPAH stated above, which for all practical purposes - the closer we operate our system to the maximum pressure allowed - the more air (tank)  volume you will need to prevent the relief valve from opening. 

     So to re-cap - and not to dive into an extensive dissertation on fluid properties, thermodynamics and other laws of physics better left to persons like John Siegenthaler or Mark Eatherton - determine expanded fluid (remember water and glycol are different) and select a tank with an acceptance factor meeting or exceeding that amount.  And for residential applications where we have a 30 psi relief valve and less than 20' high system, divide the expansion volume by .36 and select a tank volume equal to exceeding that volume. 

    You have done the most exacting work by determining the system volume as accurately as possible; the rest of the selection process is based on your systems operating temperature and pressure parameters which you may want to review before you select your diaphragm tank (not compression tanks as they are sized differently).  And as far as the on-line calculator, inserting your parameters yielded "Water capacity exceeds residential sizing specifications" and defaulted to a recommended tank size based on boiler net output and radiation type which, of course, we can see from above would not be right.  Hope this helps.


    Gosh, now that I have written this all out and re-read it, I think I am even beginning to understand it... 
  • Big Ed_4
    Big Ed_4 Member Posts: 2,687
    Gravity Feed

    Big cast iron radiators in the home ? You normally set the limit to 160* Max ...
    I have enough experience to know , that I dont know it all
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