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Open Expansion Tank- How high up does it need to be?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by DaveBP, Oct 23, 2009.

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  1. DaveBP

    DaveBP Minister of Fire

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    When open expansion tanks (Euro style, vented out on the roof) are discussed on this forum they are often described as "in the attic".

    I understand that the system pressure has to be greater than the static head of the system. In other words the 'weight' of the water column from the circulator at the boiler to the very highest point in the system.
    If your highest system water was upstairs in the radiant floor tubes and say those tubes are 20ft. above the low point at the boiler circulator in the basement, you would have a static head of a little less that 9psi. As long as system pressure is more than that the circulator can 'circulate' and not have to 'pump' that water upstairs.

    How much more does that pressure have to be in the real world?

    If only a couple psi, does the open expansion tank need to be all the way up into the attic?. Is it enough to mount it high in a closet on the second floor only a couple or three feet above the same floor that contains those tubes that are the system high point? That would be a little less than 2psi extra. Is that enough?

    I'm combing through Siegenthaler's book but I haven't found the answer yet.

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  2. julien

    julien Member

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  3. emesine

    emesine Member

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    Estimate somewhere around 30 feet to equal 15 psi. The higher you put it, the higher the pressure in your system. If your tank is 10 feet above your pump, then that pump will be under about 5 psi. Pumps can "suction boil," meaning they create a little low pressure behind them. If your water is very hot, near boiling anyhow, that is enough to cause that water to boil, which can be a problem. If you keep your entire system under pressure this can't happen.
  4. Gooserider

    Gooserider Mod Emeritus

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    Exactly... The laddomat manual says 2 meters above the highest radiator, or about 6-7 feet. Optimal seems to be in the 30-35' range as that will give you about 15 psi at the boiler. You get about 1/2psi for each foot of height, which also suggests that if you are in a really tall house, you might want to be careful if you get much over 40-50' as that is going to be crowding your 30psi pressure relief valve...

    I have seen other sources that suggest you could get away with .5 meters over the highest radiator, or about 2 feet, but IMHO that would be pushing it a little bit.

    My take on it would be to use an electric water heater tank in the attic, box around it really well with insulation, but leave the area under the tank un-insulated so that the heat loss from the room underneath can keep the tank from freezing...

    Gooserider
  5. Hansson

    Hansson Feeling the Heat

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  6. Floydian

    Floydian Feeling the Heat

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    Hi Hansson, thanks for the information you have provided on open pressurized systems. How much pressure does your system have?

    Noah
  7. heaterman

    heaterman Minister of Fire

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    1PSI = 2.31ft of head

    You want the tank to be the high point of the system. 2-4 feet above your highest radiation will be sufficient for pressure and aid in purging. The reason most of the old open expansion tanks were in the attic was simply a matter of convenience and appearance.
  8. DaveBP

    DaveBP Minister of Fire

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    Gentlemen, thank you for your responses.

    I actually have a copy on my computer of the Laddomat pdf that Julien posted. I have studied it some but never noticed the 2meter note for the height from expansion tank to high point of the system water. My take on many recommendations from manufacturers is that they tend to be a little conservative, probably for good reasons.

    I like Hansson's and heaterman's empirical rules of a couple feet or so (60 or more cm). Hansson's system is even a little less. If that is enough to work for them, then it works for me.

    I will have about 3 ft.(1 meter) of height available to me in my upstairs closet before I come up against the ceiling. Now I feel confident that I'm not doing something that I will regret later.

    Thanks again.
  9. heaterman

    heaterman Minister of Fire

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    The thing to watch for if you get down to 1 or 2 ' above the highest radiation is that you have enough size to "absorb" the expansion of the system fluid. Otheraise you may find a new meaning of the word "leak". :(
  10. DaveBP

    DaveBP Minister of Fire

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    The tank I'm planning to use is an 80 gallon stainless tank 5ft. tall. I was thinking about mounting it with its bottom 3ft from the same floor that holds the highest water in the system. It can expand up from there. I'll have less than 1200 gallons overall so it shouldn't overflow, but if it does ever steam over it will go out onto the roof.

    Which brings up another question. Would 3/4" pipe be adequate for this overflow pipe out onto the roof? Is that overkill for what will usually be a gentle flow of air? Larger pipe less likely to block up from water vapor frost? Any opinions?
  11. Hansson

    Hansson Feeling the Heat

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  12. DaveBP

    DaveBP Minister of Fire

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    ....that's about 9 psi...


    With the boiler circulator on the colder temperature return line, I'm liking the open expansion tank idea more and more.
  13. Hansson

    Hansson Feeling the Heat

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    If the Expansion tank is in a closet and there is no risk of freezing you can do like this.
    Then the cold water is going up to the expansion tank.

    If you have the Expansion tank to the top of the tank the expansion get hot.
    When the water cools in the expansion the water going down to the top of the tank and that is not good for the stratification

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  14. Floydian

    Floydian Feeling the Heat

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    Thanks Hansson-thats almost exactly the same pressure I should be able to get. Glad to know that it works well.

    Does your expansion tank get any warmer than room temp? Do you have it insulated?

    Noah
  15. heaterman

    heaterman Minister of Fire

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    As an aside here as far as piping goes, the same rule applies to this type of expansion tank as well as a sealed system. You want your curculator placed to pump away from the point where the tank ties into the piping. (PONPC)
  16. Gooserider

    Gooserider Mod Emeritus

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    Thanks for confirming that HM. I had been thinking it would have to be the case just from lack of alternatives, but I wasn't quite sure.

    Just out of curiosity, I did have one other question about the PONPC in a system that has occurred to me - can you have more than one? I'm thinking mostly of the case where you have a sealed system that the owner has chosen to use several smaller expansion tanks rather than one big one.... The usual way I see that done is to tie the two tanks together and then hook them both into the system at the same place, but is there a physical reason to have to do it that way? What happens if a person puts two tanks in at different points in the system and then pumps between the two points?

    Gooserider
  17. Hansson

    Hansson Feeling the Heat

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    Yes it gets much warmer than room temp. You got to insulate it.
    The room whit the closet got really hot before I got it insulated.

    If you do it like my drawing it should get colder.I wish I have done it like that
  18. Paso

    Paso Member

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    I have a 250 gallon OWB and a 300 gallon storage tank along side of the OWB. ( heating boiler room )
    I piped the expansion from the storage tank ( old HHO tank ) using 3 of the 4 threaded fittings on the top of the tank to go up in varying heights and pipe diameters. ( 1.125 + 2.00 + 1.00 pipe.
    They all dump into the expansion tank on top of the OWB. Not pretty but functional.
    Once I got my baseline of water/ storage it is intresting to find out I can control which pipe the water exits the storage tank going to the expansion tank on top of OWB.
  19. heaterman

    heaterman Minister of Fire

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    A pair or any number of tanks should be tee'd in at the same point which would preferably be the suction side of the main circ in the system. If the system was sealed this would also be the place that you would want to bring in your feed water. I've never seen multiple tanks at different points on the system but I would think that the one doing the actual work would again be the one on the suction side of the circ. I have however done multiple tanks with air vents in large primary secondary systems. (Think 3" pipe and larger with 120gpm going through the secondary and 400 in the primary) In that particular case additional tank capacity was needed in the system due to another large zone being added. There was no room for another tank in the existing mech room so I added it to the new secondary. Worked like a charm.
  20. Gooserider

    Gooserider Mod Emeritus

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    I was sort of thinking of the situation where someone was adding a closed gassifier and storage to an existing system. It wouldn't seem unreasonable to me that they might want to leave the existing (paid for) ET in place, and just add another ET sized to fit the added boiler and storage - which given how big the tanks are might not be convenient to plug in next to the existing tank... I wondered what would happen if the second tank was added some place other than where the first one was... It hasn't come up yet, but it seemed like an interesting problem.

    Gooserider
  21. heaterman

    heaterman Minister of Fire

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