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One expansion tank per system

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Wayco Wayne_2
Wayco Wayne_2 Member Posts: 2,479
I'm helping a friend of a friend. I know. Big mistake. I'm just a glutton for punishment. :) His friend is installing a Wood Boiler in his garage piping it to a large buffer tank, and from the buffer tank he is piping into the main house basement to bring heat into the pre-existing hydronic system., with Oil fired boiler. He wants to keep his expansion tank on the house system and then add one to the wood boiler system. I said no, you are coupled with the existing system so they will affect each other. What could happen if he kept the both? I need a simple answer for a homeowner who raises pigs in Montana. He's a smart guy but doesn't have too much experience and is flying by the seat of his pants. Thanks for your help. He may be looking in as I told him to come to this site to educate himself.

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  • Ironman
    Ironman Member Posts: 7,378
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    Is the wood boiler pressurized?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
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    There was an error rendering this rich post.

  • HVAC_Service_Mentor
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    One way or another, you will need extra expansion tank volume to handle the additional volume of heated water in the new boiler, buffer tank, and boiler.
  • ChrisJ
    ChrisJ Member Posts: 15,767
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    Is there a such thing as "too much expansion tank" ?
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    A piping diagram would be helpful. If the entire system is pressurized and all shares the same water, the you (he) is best served by one large expansion tank. Arranged to be pumped away from. There is no such thing as too much expansion. A buffer and boiler will likely add 100+ gallons to the system fluid. His existing tank is likely an EX30 or EX60, not worth keeping. Pump away from wherever you locate the expansion tank connection! You will save a world of headache, also I'd recommend a good microbubble air separator at the wood boiler supply. I head reoccurring air issues which were completely eliminated buy a spirovent.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • kcopp
    kcopp Member Posts: 4,438
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    WW you are correct 1 per system.
    Pumping away from the circ in the primary loop.
    Make it a big one.


  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 16,544
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    You can have more than one, but they all have to connect to the piping system at a single point. If you don't do this, the point of no pressure change will appear somewhere in the system between the multiple tanks, which will cause water to move in and out of the tanks. Not a good idea.
    Retired and loving it.
    ChrisJ
  • Noel
    Noel Member Posts: 177
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    ChrisJ said:

    Is there a such thing as "too much expansion tank" ?

    The way that I learned it, yes. An oversized expansion tank will absorb water volume until the water temperature it takes to hit 30 psi (or whatever relief valve setpoint is on the system) can be approaching 300° or more. If a relief valve opens at very high water temperatures, it can be dangerous for property and people that are in the area, as the water will flash to steam pretty violently.
  • ChrisJ
    ChrisJ Member Posts: 15,767
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    Noel said:

    ChrisJ said:

    Is there a such thing as "too much expansion tank" ?

    The way that I learned it, yes. An oversized expansion tank will absorb water volume until the water temperature it takes to hit 30 psi (or whatever relief valve setpoint is on the system) can be approaching 300° or more. If a relief valve opens at very high water temperatures, it can be dangerous for property and people that are in the area, as the water will flash to steam pretty violently.
    I'm not understanding how this makes sense?

    If the tank is precharged to the proper pressure, why wouldn't it work like any other in the same system, oversized or not?
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Noel
    Noel Member Posts: 177
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    As far as expansion, it would work fine. A properly sized tank wouldn't allow the relief valve to open unless the temperature limit failed, then the rising temperature would build pressure until the relief valve opens. The tank size determines how far it rises before opening the relief valve.
  • ChrisJ
    ChrisJ Member Posts: 15,767
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    Noel said:

    As far as expansion, it would work fine. A properly sized tank wouldn't allow the relief valve to open unless the temperature limit failed, then the rising temperature would build pressure until the relief valve opens. The tank size determines how far it rises before opening the relief valve.

    30 PSIG is 30 PSIG, it doesn't matter how much water is behind it.

    That's the part I'm not getting. Would the oversized tank hold more water? Yes.But I can't see it mattering, especially considering the huge size of the tanks from the days before bladder tanks.

    That's the part I'm confused by.


    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Noel
    Noel Member Posts: 177
    edited September 2017
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    30 psi at 240° is safer than 30 psi at 340°. The question isn't what the pressure is when it opens, it is what the water temp is when it hits 30 psi and opens.

    Its a tough concept to explain, and most guys training people don't even bother trying.
  • ratio
    ratio Member Posts: 3,649
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    The bigger tank will accept more water than the smaller one. Therefore, the water will have to expand more to fill the larger tank than the smaller tank before the relief pressure is reached —this means more heat will be accepted by the water before the relief valve will be forced open.
  • ChrisJ
    ChrisJ Member Posts: 15,767
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    ratio said:

    The bigger tank will accept more water than the smaller one. Therefore, the water will have to expand more to fill the larger tank than the smaller tank before the relief pressure is reached —this means more heat will be accepted by the water before the relief valve will be forced open.

    Yes, but the temperature will remain the same regardless of btu's taken in. Because as you said, there's more water.

    How much more water will a larger tank accept, a gallon? Do guys limit piping runs for the same reason?
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • bob_46
    bob_46 Member Posts: 813
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    Noel's chart assumes the limit is busted .

    There was an error rendering this rich post.

  • ChrisJ
    ChrisJ Member Posts: 15,767
    edited September 2017
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    Noel said:


    30 psi at 240° is safer than 30 psi at 340°. The question isn't what the pressure is when it opens, it is what the water temp is when it hits 30 psi and opens.

    Its a tough concept to explain, and most guys training people don't even bother trying.

    In order for 340F degree water to exist, the system would need 106 PSIG in it.

    The highest you can go with 30 PSIG is around 273F before boiling.

    30 PSIG water at 340F does not and can not exist regardless of expansion tank size.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Noel
    Noel Member Posts: 177
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    Um, no. Maybe ask Amtrol about that. That's where I got it from.
  • ChrisJ
    ChrisJ Member Posts: 15,767
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    Noel said:

    Um, no. Maybe ask Amtrol about that. That's where I got it from.

    With all due respect, I don't need to as Amtrol about that, that's how it works.

    You cannot exceed the boiling point of any liquid at any given pressure. Even if Amtrol say's you can. :D
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,607
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    @bob iscorrect, only if the limit's busted. that's why 2 limits (1 regular and 1 man reset is best)

    Other than that a larger expansion tank has no dettrimental effect cept for cost
    HVACNUT