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Thermal shock

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John B_2
John B_2 Member Posts: 46
My existing boiler has no controls or additional piping to prevent thermal shock. It has three zones and I fear that a replacement cast iron boiler may not hold up to what this beast did without shock prevention. Is it simply cutting into each return and connecting it to the supply with let's say 3/4" pipe? Is it really that simple? I don't mind spending the money on more hardware, just want to know what the best method is.

I can see a taco air scoop first out of supply connected to a huge expansion tank above it. Then there is taco check valve. Supply then splits to three lines each having own control valve. The returns each have a circ pump each connected to it's own relay.

Regards

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  • Brad White
    Brad White Member Posts: 2,398
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    Thermal Shock

    is not so common and is less of an issue than condensation. Yes, it does happen but more rarely than you think. Still, you are wise to be ahead of this.



    The basic cause, obviously, is a disparate temperature, where cold water meets an already hot boiler. Keep the temperature differences fairly narrow and you minimize the possibility.



    Your bypass line size is a function of total boiler capacity. A 3/4" line may work in a small boiler, say up to 80-100 MBH, but a 200 MBH boiler may use a larger size.



    Properly done, a bypass is simple, but is a fixed device. It is always open and spending some boiler energy, denying the radiation, to protect the boiler all of the time.



    A better approach is an automatic-to-need type such as a temperature controlled mixing valve (ESBE or Danfoss), or a temperature controlled circulator (Taco 00 series VS for example).   This is more geared to condensation prevention than shock, but it also helps the shock issue.



    The use of constant circulation is an additional, or may I say, other core strategy. The boiler loop and radiation loop are separated and the boiler circulation is constant. This keeps a fairly even temperature across the boiler.



    The issue with thermal shock is a sudden slug of the coldest water hitting your hottest boiler. By this I mean more of the extremes, not necessarily an absolute temperature.



    Other integrated approaches within the above:



    1) Use of variable-speed circulators on your radiation side, to lessen the volume of returning colder water. (Control valves take control of the zones and the circulator responds to their opening and closing.)



    2) The use of outdoor reset (ODR) on both boiler and radiation. The boiler ODR control may have a narrower band (Say 140 min. to 180 max.), but your radiation side can have a range of nearly room temperature up to nearly the maximum.



    The benefit here, lessening of thermal shock potential aside, is energy savings. Let thermal shock be your reason but reap the energy savings and greater comfort.



    Such a deal.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"



    -Ernie White, my Dad
  • John B_2
    John B_2 Member Posts: 46
    edited October 2010
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    thermal shock

    thanks so much!!

    So if I do the odr, should it do the job alone? Can you please give me a part number on the odr that I could use with this. Is it one odr or two? Leaving for son's football game now. Be back later!!



    Regards
  • kcopp
    kcopp Member Posts: 4,443
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    Give us some....

    more specifics about your system. Size of boiler and model/make, how big are the zones, what type of emitters( cast iron rads, fin tube, radiant...) You may not need a bypass at all.
  • John B_2
    John B_2 Member Posts: 46
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    ok

    size going in next week (hopefully)  300,000 btu slant fin gg series

    3 zones: 2 have about 160 feet each of 3/4 fin

    the 3rd zone has about 100 feet of 3/4 fin

    Regards,

    John B
  • kcopp
    kcopp Member Posts: 4,443
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    you have ...

    2 zones w/ 160' of copper fin  each fed w/ 3/4" copper?  typically the max you should be running on 3/4" is 70' the last bit of baseboard is pretty ineffective.

    Are you sure it is all fin and not dummy baseboard?
  • John B_2
    John B_2 Member Posts: 46
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    just got back and verified length of fin pipe

    yes it is correct.
  • CMadatMe
    CMadatMe Member Posts: 3,086
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    Boiler is oversized

    Forget the by pass spend your efforts on a heat loss.

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  • John B_2
    John B_2 Member Posts: 46
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    I did heat loss before which showed loss of 240,000

    but I don't remember anythingt about the length of fin pipe.

    My building is a simple "old style motel" you know the "off road type from the old days. It is approx 180' long and 28' deep.

    It is divided by 10 units which have a living room, bedroom and small (very small bathroom). Two good size sliding windows each unit.

    Flat roof.

    Boiler room is almost in the center with 4 units on one side and 6 on the other.

    2 zones handle 4 apt's each. The other handles 2 apts plus a very small utility room and office.

    Everything seems to work fine heatwise but the boiler is so big and old that I believe an 80% will save on energy now. The flue on the existing boiler is 12".

    If someone believes that I can replace with anything smaller than 300,000 btu please speak up now because I still have time to downsize the new boiler.



    Regards



    .
  • CMadatMe
    CMadatMe Member Posts: 3,086
    edited October 2010
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    New Heat Loss

    Just by the number provided my gut is telling me oversized...To keep is simple lets use round numbers. The building is roughly 6,000 sqft and we know all of that is not heating space. Based on your information you calculated 40 btus a sqft and I can't see that being an accurate loss.



    I don't know what part of the country you are in so I'm basing my thoughts on a zero outdoor temp. The outside units heat loss will be more than the inside units loss as they would have 3 outside walls all the inside units would have 2 outside walls. If I take your 180 length and divded that by ten then I am to assume each room is 28x18 which is roughly 600 sqft. If I were a betting man I would say each room has a heat loss of between 12k and 15k and these numbers may even be high. I will take the 15k high x 10 and my load is 150,000 btu's. I also based this off R11 walls and R-30 in the attic.



    That is why you have been getting away with the long baseboard runs. The amount of board is oversized to the loss so it's decreased output can satisfy the room. To me that gives away the fact that the boiler is over sized!  All that cold water returning is also the reason why you want to pipe a by-pass in the new boiler.



    This cries for a condensing gas boiler. You would save tons of money. If you budgeted a boiler that size then a proper heat loss may show you that you could reduce the size of the boiler which may put a mod/con in your budget. What do you do for hot water?

    There was an error rendering this rich post.

  • John B_2
    John B_2 Member Posts: 46
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    Thank you

    I want to say that my ceilings have barely any insulation.

    So if I downsize to a (lets say) 150,000, will I still be alright with the oversized radiation pipe?

    I still have questions later about the bypass.

    Regards,

    John B
  • John B_2
    John B_2 Member Posts: 46
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    Was so impressed with response

    that I didn't see your last paragraph about the mod/con stuff.

    Yes if I was told that I could get away with a smaller mod/con, then I would do it. But I've been basing everything on 300 which is above my budget for a mod/con.



    Regards,

    John B
  • John B_2
    John B_2 Member Posts: 46
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    Oh domestic water!

    100 gallon 199,000 btu direct
  • CMadatMe
    CMadatMe Member Posts: 3,086
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    Have Your Contractor

    Do a new heat loss. Start over.. Burnham makes a nice 84% atmospheric boiler in the Series 3. Even if you can down size and stay with a 80% or 84% boiler you can then take the few extra dollars you had budgeted and insulate the place. What are your monthly gas bills?

    There was an error rendering this rich post.

This discussion has been closed.