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Expansion Tank Volume

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by ewdudley, Jun 23, 2010.

  1. ewdudley

    ewdudley Minister of Fire

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    [quote author="sparke" date="1277231691"]
    ...
    How big of an expansion tank do I need?

    [/quote]



    Here's some tables for a charged expansion tank.

    There are three tables for three different maximum system design pressures, p_hi=20psig, p_hi=25psig, and p_hi=30psig. If your maximum system design pressure is in between those given then a simple interpolation should get you pretty close.

    A charged expansion tank is either a diaphragm tank or a closed expansion tank that is charged with air pressure to the minimum system static pressure at the elevation of the the expansion tank.

    Expansion tank volume is given as a percent of system volume, so if the table reads 6.9 and the system is 1100 gallons, then minimum tank size would be about 76 gallons.

    These are minimum volumes according to Boyle's Law and the properties of water. The tank is assumed to be perfectly charged (no amount of flooding and no excess air charge).

    [Edit: Near-perfect charging is easy enough with a membrane tank, but maintaining near-perfect charging with a simple closed expansion tank may require more attention. Therefore an over-sized tank would be preferable when considering a simple non-membrane closed expansion tank, e.g., a converted propane tank.

    Have since found a good resource on the topic of expansion tanks (But keep in mind that they are trying to sell membrane tanks.):

    http://www.amtrol.com/enginhandbook.htm ]

    Also changes in the temperature of the air inside the expansion tank are ignored.

    Code:
    Expansion tank size as percent of system water volume.
    
    Low temperature set to 50F, which is close to maximum density.
    
    'p_lo' is minimum static pressure at the expansion tank when
     system temperature is at a nominal 50F.  Elevating expansion tank
     will lower its minimum static pressure.
    
    (Pressures psig, temperatures degF.)
    
    t_lo=50 p_hi=20:
                              t_hi
    p_lo    160     170     180     190     200     210
      2     4.57    5.17    5.90    6.62    7.46    8.18
      3     4.84    5.48    6.24    7.01    7.90    8.66
      4     5.14    5.82    6.63    7.44    8.39    9.20
      5     5.49    6.21    7.07    7.94    8.95    9.82
      6     5.88    6.65    7.58    8.51    9.59   10.52
      7     6.33    7.16    8.16    9.16   10.33   11.33
      8     6.86    7.76    8.84    9.93   11.19   12.27
      9     7.48    8.47    9.65   10.83   12.21   13.39
     10     8.23    9.31   10.61   11.91   13.43   14.73
     11     9.14   10.35   11.79   13.24   14.92   16.36
     12    10.29   11.64   13.27   14.89   16.78   18.41
     13    11.76   13.30   15.16   17.02   19.18   21.04
     14    13.72   15.52   17.69   19.85   22.38   24.55
     15    16.46   18.63   21.22   23.82   26.86   29.45
    
    t_lo=50 p_hi=25:
                              t_hi
    p_lo    160     170     180     190     200     210
      2     4.09    4.63    5.28    5.93    6.68    7.33
      3     4.28    4.84    5.52    6.19    6.98    7.66
      4     4.48    5.07    5.78    6.49    7.32    8.02
      5     4.71    5.33    6.07    6.81    7.68    8.42
      6     4.96    5.61    6.39    7.17    8.09    8.87
      7     5.23    5.92    6.75    7.57    8.53    9.36
      8     5.54    6.27    7.14    8.02    9.04    9.91
      9     5.89    6.66    7.59    8.52    9.60   10.53
     10     6.28    7.10    8.09    9.09   10.24   11.23
     11     6.73    7.61    8.67    9.73   10.97   12.04
     12     7.24    8.20    9.34   10.48   11.82   12.96
     13     7.85    8.88   10.12   11.36   12.80   14.04
     14     8.56    9.69   11.04   12.39   13.97   15.32
     15     9.42   10.65   12.14   13.63   15.36   16.85
     16    10.46   11.84   13.49   15.14   17.07   18.72
     17    11.77   13.32   15.18   17.04   19.20   21.06
     18    13.45   15.22   17.35   19.47   21.95   24.07
     19    15.69   17.76   20.24   22.71   25.60   28.08
     20    18.83   21.31   24.28   27.26   30.73   33.70
    
    t_lo=50 p_hi=30:
                              t_hi
    p_lo    160     170     180     190     200     210
      2     3.79    4.28    4.88    5.48    6.18    6.78
      3     3.93    4.44    5.06    5.68    6.41    7.03
      4     4.08    4.61    5.26    5.90    6.65    7.30
      5     4.24    4.80    5.47    6.14    6.92    7.59
      6     4.42    5.00    5.70    6.39    7.21    7.91
      7     4.61    5.22    5.94    6.67    7.52    8.25
      8     4.82    5.45    6.21    6.98    7.86    8.62
      9     5.05    5.71    6.51    7.31    8.24    9.03
     10     5.30    6.00    6.84    7.67    8.65    9.49
     11     5.58    6.31    7.20    8.08    9.10    9.99
     12     5.89    6.67    7.60    8.53    9.61   10.54
     13     6.24    7.06    8.04    9.03   10.18   11.16
     14     6.63    7.50    8.54    9.59   10.81   11.86
     15     7.07    8.00    9.11   10.23   11.53   12.65
     16     7.57    8.57    9.77   10.96   12.36   13.55
     17     8.16    9.23   10.52   11.80   13.31   14.59
     18     8.84   10.00   11.39   12.79   14.42   15.81
     19     9.64   10.91   12.43   13.95   15.73   17.25
     20    10.60   12.00   13.67   15.35   17.30   18.97
     21    11.78   13.33   15.19   17.05   19.22   21.08
     22    13.25   15.00   17.09   19.18   21.62   23.72
     23    15.15   17.14   19.53   21.92   24.71   27.10
     24    17.67   20.00   22.79   25.58   28.83   31.62
     25    21.20   23.99   27.34   30.69   34.60   37.94
    

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  2. sparke

    sparke Minister of Fire

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    Thanks for all your input. This helps alot!
  3. sparke

    sparke Minister of Fire

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    So if my system reads 15 psi at ambient basement temp. and 200* F when wood boiler is cranking. Does this mean I use the 26.86 value from chart? 26.86% x1100 ? ( 1100 is a guess as to my total gallons in the system)... That Cant be right I would need a 295 gallon exp tank... What am I doing wrong? I am using 15psi as my base because that is roughly what the water recharge valve charges the system to...
  4. ewdudley

    ewdudley Minister of Fire

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    First note that there are three tables, one each for p_hi=20, p_hi=25, and p_hi=30. P_hi is the maximum design pressure of the system at the expansion tank.

    Suppose you have a two-story house with a basement and radiators on both floors. The basement is 9ft below the first floor and the second floor is 9ft above the first floor. The highest point in the hydronic system is the top of a 3ft high radiator on the second floor. The boiler sets on the basement floor.

    Now we fill the system until water reaches the top of the system. A pressure gauge at the bottom of the boiler would read (3ft + 9ft + 9ft) * 0.434 psi / ft = 9.1 psig. This is your static water column pressure.

    Minimum system static pressure at the lowest point should be static water column pressure plus 3-5psi, depending on who you ask, call it 14psig even in this example.

    Maximum design pressure is some pressure less than the what your system can comfortably handle, typically 30psig at the boiler blow-off valve.

    So now we place your expansion tank hanging from the basement ceiling, 6ft from the floor. When you have minimum system static pressure of 14 psig at the basement floor, the pressure at the expansion tank will read 14 psig - (6ft * 0.434 psi / ft) = 11.4psig. When you fill the system you have to add/drain water and add/blow-off air until the expansion tank pressure reads 11.4 psig when there are just a couple inches of water in the bottom of the expansion tank, and the average water temperature in the system is cool, nominally 50F.

    So now to the tables. The boiler is going to see a couple psi more than the expansion tank, and if the boiler blows off at 30psi then that means we can't take the expansion tank any higher than 30psi minus a couple psi, so lets use the p_hi=25 psi table to get an idea.

    So p_lo at the expansion tank is 11.4, call it 12. P_hi at the expansion tank is 28 or so, call it 25. Go to the p_lo=12 line of the p_hi=25 table and we have

    Code:
     160     170     180     190     200     210
    7.24    8.20    9.34   10.48   11.82   12.96
    
    So for a system of 1100 gallons with every drop of water at 200F you're looking at 11.82% * 1100gal = 130 gal minimum.

    If you take it to the hairy edge and take p_hi to maybe 28 psi you could shave off maybe 15gal. And if you can claim that maximum average water temperature will be more like 185F you could shave off a couple dozen more.

    So work through the example while plugging in the elevations for your system and see what you come up with.

    --ewd
  5. sparke

    sparke Minister of Fire

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    Got it thanks!
  6. rkusek

    rkusek Minister of Fire

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    Great info Eliot,

    I'm adding 1000 gal storage this summer. I've been eyeing the 86 gal bladder tank by Pex Supply. Looks like two of the smaller ones would be a better choice. Too bad they are not priced proportionally. Not sure I feel brave enough to make my own tank. I was going to leave the small expansion tank connected on top of boiler and place the big one(s) to the primary loop like most diagrams show or possibly to the 1000 gal tank (which seems to me where it should be). Anyone have thoughts on this? Will do more research before I do any plumbing.
  7. sparke

    sparke Minister of Fire

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    I would love to hear more discussion on this topic of installation/design of expansion tanks as well. Also, the 2 other choices which would be a smaller tank in the attic or a tank on the ceiling in the basement (old school solutions to expansion with large water volumes).
  8. in hot water

    in hot water New Member

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    You can still buy those blank tank, compression tanks. I bought a 20 gallon one to build a solar drainback tank. They are much cheaper that the bladder style expansion tank until you add the Airtrol fitting and sight glass, etc. A good bladder tank, properly sized and connected to a system that doesn't allow O2 ingress should last the life of the system.

    Bladder tank failures are often related to non barrier tubing installations, or poor water quality.

    hr
  9. RobC

    RobC Minister of Fire

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    Here's a tank design that I used thanks to "leaddog's suggestions. This is a 100 GALLON propane tank and works well for 1000G pressurized storage. I have a Tarm 60 and many feet of 1.25 and 1.5 pipe and from practical experience it works well. There is a 100 POUND tank which is much smaller don't get them confused.
    Rob

    Attached Files:

  10. sparke

    sparke Minister of Fire

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    RobC, What prevents the 100 gallon tank from filling up with water? Do you need to add air periodically? If so what isolates that air from the system? Where is the location of your 100 Gallon tank, floor, ceiling, attic, etc... Thanks.
  11. ewdudley

    ewdudley Minister of Fire

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    The air stays in the tank because it has nowhere else to go, same as a diving bell or the caissons they used to do the foundations of the Brooklyn Bridge.

    My neighbor has a conventional expansion tank in the floor joists above his boiler that hasn't required any attention since the early fifties when it was installed. However his system has a Bell & Gosset air separator that discharges up through a stem up inside the expansion tank, which may be a critical advantage.

    The tank can go at any elevation, sitting on the floor in the far corner of the basement or anywhere else is fine. In a conventional system it was important to "pump away from the expansion tank", but speaking only for myself I have come to the opinion that this is not a problem for a gasification boiler connected by fat pipes to a storage system.

    Nonetheless, the rest of the system should be reviewed to be sure there is no opportunity for a high-head pump to pull some portion of the system below atmospheric pressure such that air is pulled in past valve stems and whatnot.

    If you have a multi-story installation the size of the tank needed can be significantly smaller if the tank is placed as high as possible, but for a normal residential system you wouldn't want to fool with putting one in the attic unless it made the difference between buying one tank or two.

    The best, easiest, and most foolproof is to get a diaphragm tank and be done with it, otherwise get whatever suitable tanks you can get your hands on an tuck them away wherever it's convenient. You need a way to fill/bleed air; and the water needs to enter and leave from the bottom of course.

    To perform the same as a diaphragm tank of the same volume, a simple closed tank has to be charged perfectly. By perfectly charged I mean that when the system is at minimum temperature/maximum density, there needs to be enough air pressure in the tank such that the highest point in the system sees a pressure of at least 4 or 5 psig. And also when the system is at minimum temperature/maximum density there must be some water remaining in the bottom of the expansion tank.

    A sight-tube is desirable, even if only a piece of translucent plastic tubing with a plastic marble floating in it. A spritz bottle and a hair dryer will work if you're as cheap as my dad. At any rate if you use a simpled closed tank you need to keep an eye on it and keep a log of water levels vs system pressure so you can tell if something's not right.

    Cheers --ewd
  12. sparke

    sparke Minister of Fire

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    Thanks again Eliot and all other posters, you have explained many of the factors of my install with this thread and the 1000 gallon tank thread.. We have covered much ground when it comes to expansion tanks. I am sure this thread will help many people in the future.

    Cheers!
  13. stee6043

    stee6043 Minister of Fire

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    I'm a little shocked at how complicated this post makes expansion tanks out to be. Sheesh. When I did my system I was told by a very knowledgeable boiler sales person to take "4%-5% of your total system volume and use that as your required acceptance volume for expansion" and "add a little for future system add-ons if you'd like to". Sure seemed a lot easier than fancy charts and graphs....
  14. RobC

    RobC Minister of Fire

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    For judging the level of water in a home made propane tank you can use the existing propane level gauge. When propane is in the tank it's in the liquid form, so the gauge works just as well for water.
    WoodWacker likes this.
  15. woodsmaster

    woodsmaster Minister of Fire

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    The numbers I get when I do the math dont come close to the 4% or5% your boiler sales person told you to go with. I'm heating a two story with the bottom of the boiler about 12' below the highest baseboard.
    even if I raise the tank 7' above the bottom of the boiler I come up with about 89 gal expanshion for 1100 gal. does this sound correct. that would be 8%
  16. RobC

    RobC Minister of Fire

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    I think some people build in a 2X safety margin. If your using pressurized storage extra acceptance isn't going to hurt as your storage water temp is going to have large fluctuations in temp therefore expansion. This will be more true if you build your own expansion tank, if your buying tanks they are pricey and you would want to only purchase what's necessary.
    Some where out on the storage tanks you might want to add a pressure relief valve, Not necessarily a Temp and Pressure relief, known as a T&P like on a boiler, just plain Pressure Relief for extra margin of safety too.
    Rob
  17. DaveBP

    DaveBP Minister of Fire

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    Liquid water is densest at about 39F. It expands below that and above that. It expands a lot as it gets hotter. From densest at 39F to maximum expansion up near boiling the total expansion is about 4%. I think it's safe to say that for most people on this forum 39F is colder than their system water will ever get in normal use and boiling is hotter that they ever 'intend' it to get. So 4% is a safe round number to use for total expansion. The amount of expansion you 'design' for will want to add some kind of safety factor in addition to that.
    Large commercial and especially multi-story systems are another level of design complexity, but like I said, not what most of us here are working on.

    The greatest single source of the perennial confusion that i see about expansion tank size is the difference between expansion "tank size" and "acceptance volume". The terms get used interchangeably sometimes and really messes up minds that are new to this hydronic stuff.

    If you take your total system volume and use 4% of that as your MINIMUM acceptance volume and get an expansion tank with at least that much acceptance volume you'll be minimally safe. Realize that with that minimum tank size your system pressure will probably about double its starting pressure as it heats up. So that if your system starts at 12PSI when it is cold it might get to 24 PSI at its hottest part of the cycle. A larger tank will help reduce that pressure cycle maximum but larger tanks are more expensive and so it goes. Bigger is better but bigger is more money.

    It doesn't have to be complicated but it is easy to get confused.
  18. stee6043

    stee6043 Minister of Fire

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    Roger that. There is a big difference between adjusting for elevation in the system and adjusting for thermal expansion. I suppose I didn't realize this thread could have been covering both??? Thermal expansion is a known, your max expansion should be 4.2% between 4degC and 100degC. Elevation changes are certainly a different animal.

    For what it's worth I have 112 gallons of acceptance volume in my system. I adjusted for the future of in-floor radiant and potentially garage heating. Both of which are above my boiler (which is in the basement)...

    [​IMG]
  19. jason elmer

    jason elmer Member

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    stee with roughly 112 gallons of acceptance volume you are at about 10% of total system volume. Just curious what is the pressure running at your systems coldest temps and what pressure is the system at it's highest temps?

    I'm putting in a system with about 1000 gallons total. I could buy one tank with 52 gallons of expansion which would put me at 5% total. Or I was thinking of buying two tanks with a total of 84 gallons of expansion at about 8%. Difference in price would be about $240 and a bigger foot print.

    Are there any problems associated with the higher pressure's at full load other than the obvious bursting fittings and leaks. For instance do pumps cavitate more easily, ghost flows more problematic? I guess what I am saying, is it advantageous to maintain pressure as steady as economically possible? For example is there rule of thumb that system pressure should vary no more than 8 psi for optimum performance?

    For the record I don't see my system dropping much below 70 degrees ever (summer use pool). Fill water will likely be around 50 degrees not 39. If it ever reaches 39 the system is likely no longer in use and should have been drained.
  20. stee6043

    stee6043 Minister of Fire

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    Pine, I set my system at 14psi cold (60 degrees in both tanks). When I max my temps (180 top to bottom in both tanks) I'm usually running 22psi, maybe a touch higher.

    I went with extra expansion for future capacity, not necessarily to keep my pressures down. As long as you stay below 30psi I can't imagine any downsides to running consistently in the 20's.
  21. Der Fuirmeister

    Der Fuirmeister Member

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    Thank you very much Eliot. Your explanations and suggestions have been very helpful. Your dad sounds like my kind of guy.
  22. in hot water

    in hot water New Member

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    When you chose a pressure for your system you want to have 5 psi gauge pressure at the highest point in the system. So fill pressure varies depending on how tall your building is. Generally 12psi is the min. fill pressure and most boiler fill valves come pre-set at 12 psi.

    There is no reason that you could not increase the fill pressure. Increasing fill pressure will also increase the boiling point. Some installers would run very high temperatures 220F and above so they could install less fin tube baseboard and shave some cost. Steam tables will show you the boiling point at various pressures.

    If you increase the fill pressure you need to increase the pressure in the expansion vessel also.

    In Europe they use a much higher fill pressure on closed loop, glycol solar installations. 4 bar, around 58psi is common. They do this to raise the boiling point of the glycol above 300F so when the collectors stagnate they don't flash to steam.

    Higher fill pressure makes air removal, especially on troublesome systems, much easier. Higher pressure = smaller air bubble size and it is easier to push the air back to the air removal devise.

    The small wet rotor pumps like to have some pressure to help prevent cavitation. I believe Grundfos recommends 5 psi min for the 15 and 26 series wet rotor pumps. This is one of the reasons un-pressurized OWF tend to go through a lot of pumps. They often recommend high head pumps and little to no pressure on them, so they cavitate to an early death.

    The problem with non bladder expansion tanks is the water can absorb the air (O2) over time seeing as water is H2O. that is the one reason you see those B&G airtrol fittings on non bladder tanks. Air removed from the purger is pushed back to the top of the expansion tank by that device.

    Bladder expansion tanks are charged at the factory with nitrogen as compressed air (O2) will migrate through the EPDM bladder after time. I notice tire shops now offer nitrogen as an option to compressed air for tires, for the same reason.

    If you use a non bladder tank install a piece of clear pex, like Uponor from top to bottom to act as a sight gauge. The fluid in the system will be dirty enough to see the level just fine, no need for a float. The glass sight tubes are great, but expensive when you add in the fittings needed.

    hr
  23. virvis

    virvis Member

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    Are the blue expansion tanks available at the big box stores suitable for expansion? I may have to pick up a couple quickly and want to make sure these will work my radiant system.

    thanks,
    Jim
  24. RobC

    RobC Minister of Fire

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    You may be looking at well water expansion tanks and they may not handle the heat.
  25. ewdudley

    ewdudley Minister of Fire

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    Can't promise that I've thought it through enough, but considering the tiny gpm flowing to the tank you might be able to prove that a section of finned copper could guarantee that the tank would never see water above lukewarm. Of course you'd need to guard against thermo-siphoning as well.

    [Edit: If you're buying new it would be pretty hard to justify not using the correct tank. But if somebody was going to let me have a used 100 gallon membrane water system tank for next to nothing I'd sure figure out a way to make it work correctly.]

    --ewd

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