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Tank Heat loss, Anyone measure theirs?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by goosegunner, Apr 8, 2013.

  1. goosegunner

    goosegunner Minister of Fire

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    I went on vacation two weeks ago and I thought it would be a good time to see how well my 1000 gallon tank held its heat.

    I charged the tank to 188 top to bottom and turned off the pumps. We we gone for 8 days. When we returned the tank was 144 at the top and 120 on the bottom.

    So over 8 192 hours the tank lost 466,480 btus.

    or

    2429 btus per hour

    Does that seem excessive?

    I do still have some uninsulated copper piping in the boiler room so I might have some thermosiphon flow.

    I do know that once the tank gets to that level it will hold a long time. I have witnessed it in the summer when I stop heating my pool.

    gg

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  2. kopeck

    kopeck Minister of Fire

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    I've noticed that mine will lose 4-5 degrees a day left on it's own. The cooler it gets the slower is looses heat. I've noticed it will stay around 120 degrees for ever.

    K
  3. Nofossil

    Nofossil Moderator Emeritus

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    Mine loses about 10k BTU per day at typical winter temperatures. It's outside, but I have it *really* well insulated.
  4. goosegunner

    goosegunner Minister of Fire

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    My tank is in a attached lean to on my pole building.

    I have over 10 inches of Iso board under it with 12 inches of fiberglass between tank and iso board floor.

    The Top has over 20" of fiberglass and then another 1" of iso board.

    Overall the tank enclosure is R75 or better.

    gg
  5. jebatty

    jebatty Minister of Fire

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    I know I have done this measurement on my 1000 gal tank (approx R40+ bottom, R50+ on two outside walls, R30+ on two inside walls; tank in heated space) but I can't find the data right now. I'll post if/when I find it. I did change my control on the tank though, and what I now do when I plan to be gone for more than two days in winter is 1) let the system continue to draw on the tank until top of tank drops to 80F, 2) then with a aquastat shut down the system draw from the tank, and 3) use a backup 5000w-240v electric space heater set at 45F to keep the shop above freezing.

    With this setup I have been gone for more than two weeks and on return, the tank is still about 70F at the top, bottom is cooler, but with the backup heat, no danger of boiler, plumbing or tank ever freezing, which is the only concern I have. BTW, when the tank is cooled down that much it takes quite a burn to get it back into normal storage temp range.

    Something else I usually do on every burn is to run the boiler-tank circ bypassing the Termovar to bring the boiler up to tank temperature, then shut the bypass off. On commencement of a burn the boiler typically would be about 60F, shop air temperature, and the tank bottom may be 110F-120F. With the bypass the boiler on burn now only has to heat from about 120 to 160F, rather than from 60 to 160F before the boiler circ turns "on." The cost of this is bringing the tank temp down a bit but I feel better the quicker the boiler is feeding the system, plus I think the likelihood of creosote/water condensation and corrosion inside the boiler firetubes due to low temp is reduced.
  6. Nofossil

    Nofossil Moderator Emeritus

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    I also do the boiler preheat. My tank is at a higher temp since I don't have radiant heat yet :-(

    I heat the zones and boiler form storage (usually to around 100 - 110::F) to give the boiler a running start.
  7. mole

    mole Member

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    My wife and I got back from a 10day Florida trip last week. When we left, the tank was 165deg. When we returned, it was 118deg. That's about 4,900btu/day. The tank is in a 55-60deg basement. The insulation is three R-14 polyisocyanurate boards on bottom, two R-14 boards on the sides, and two boards on the top plus a 1" thick polystyrene board. The polyiso board seams are staggerred in the tank and there are no thermal breaks in the insulation other than the pipe protrusions in the top.
  8. goosegunner

    goosegunner Minister of Fire

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    I think you mean 49,000 btus a day right?

    47 degree drop on 1250 gallons is 489,387 btus divided by 10 days.

    gg
  9. maple1

    maple1 Minister of Fire

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    I'm thinking the heat loss would be non-linear?

    More heat lost at first when tanks are hot than later when they are not as hot?
    Taylor Sutherland and heaterman like this.
  10. heaterman

    heaterman Minister of Fire

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    We did a little experiment a couple years ago when a local Garn user went away for 10 days. His boiler sits in an insulated but unheated lean-to on the side of his pole barn. He has a Garn 2000 with the horizontal flue discharge that is insulated with one of the insulation kits I had made up.
    He called wondering if he should drain his Garn while the family was away and since we were working near his place I told him we would keep an eye on it for him. I suggested that he run his Garn up to 180* and then turn off the circulating pump to the house, which he did.
    When they came back the Garn was still at 105*. I think I recall calculating the average hourly heat loss to be around 5,100 btu. Not too bad for a 2,000 gallon tank in an unheated room during February.
  11. BoilerMan

    BoilerMan Minister of Fire

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    My thoughts exactly! The greater deltaT from water to air temp the faster the heat moves to the cooler surroundings. There must be some formula for this.

    I have a 115 gal indirect, and I generally heat it to 140 or so (mixing valve so anything over 120 is ok). It will drop from 140 to 130 in about 24 hours, then from 130 to 120 in 48 hours after that. Not sure after that as thats the longest time I've not had any HW draw and been around to check on it (we were house sitting and I did this expierement). I have 6, 1" copper pipes protruding from the top so there is the majority of the heat loss. I'm going to insulate those for this summer, to burn further into the spring, to avoid useing my 3 year old heating oil.

    TS
  12. flyingcow

    flyingcow Minister of Fire

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    Thats my thoughts....sorry i have no formula. but in general observation in the summer. I seem to have less heat loss if the tank is cooler. Does that make sense?
  13. bmblank

    bmblank Minister of Fire

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    The heat transfer is the same as any heat exchanger formula, just need to adjust the constants for the mediums. I believe the rate changes with the square of the temp differential, hence the asymptote.
    Taylor Sutherland likes this.
  14. arngnick

    arngnick Member

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    I had the opportunity to check mine today...calculated a 100K BTU loss over 10 hours the found out I had left a zone in that therosiphons because I need to add a zone valve...try again in a couple days :(
  15. mole

    mole Member

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    yeah, gg, that would be 49,000!
  16. goosegunner

    goosegunner Minister of Fire

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    That is the same set up I have as far as the tank location. It is in a lean to on my pole building, Main building is unheated for now and also not insulated. The tank Lean to is completely insulated. No auxillary heat in the tank area.

    The high temps during the week were in the lower 30's with teens at night.

    I guess maybe I am doing ok at 2400 average per hour.

    I do agree that the loss is much lower as the tank gets cooler.

    gg
  17. DaveBP

    DaveBP Minister of Fire

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    The heat loss is directly proportional to the difference between the temps and also the area exposed to that difference. One of the others factors (and the hardest to put a number to) is air infiltration, especially pertaining to fibrous insulations. Sealing all the gaps around foam panels or plywood or whatever the outer surface of the tank installation may be will help. Heat does not rise but hot air sure does and the air inside the tank space will have a pretty mean "chimney effect" trying to get out. The spray foam installlations can sit pretty smug at this point of argument. Not much air leakage there.
    Floydian likes this.

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