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ideal size of thermal storage

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Jersey Bill, Feb 2, 2008.

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  1. Jersey Bill

    Jersey Bill Member

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    So, my biggest mistake so far on building my own house is the lack of high temperature thermal storage. By high temp, I mean less than 200f.
    taking up interior existing floor space is not an option. I was reluctant to go outside with an underground tank, but a valid option. I think that I found a way where I can dig horizontally from my boiler room into an existing crawl space under my family room. Extending between the footings could give me a tank 7 foot wide and as long as 18 ft, either 4 or 5 foot deep.
    A 4x7x7 foot tank is 1466 gallons. an 18 ft tank would be about 3770 gallons.

    The peak heat load of my house is about 100 MBH, but its the low load days, like 30 mbh that I have a problem with now because the boiler cant go that low (efficiently).

    BTW, this is no small project excivating by hand and carrying out in buckets so a minimum size would be preferable.... but, i usually dont do anything minimum.

    How Big?

    thanks for your help

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  2. Bob Rohr

    Bob Rohr Minister of Fire

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    Fill in some blanks and I can calculate a buffer volume. Working temperature of the tank ie 160- 180. Air temperature. Size of all the loads. them smallest load, in BTU/hr.

    Here is a 100K boiler with a 30K load and 120 gallons of storage.

    hr

    Attached Files:

  3. Jersey Bill

    Jersey Bill Member

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    Thanks.
    I have that program. I never considered using it because in my mind, a buffer tank is different. This looks like it could work. I will have to play around with some numbers.
  4. Nofossil

    Nofossil Moderator Emeritus

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    You have a heat load of 100,000 BTU/hr in Jersey? That seems really high - might want to double check.

    I have 3500 square feet with a lot of glass in Vermont, and my peak load is less than a third of that. Using a number that's too high might discourage you or drive you to spend a lot more than you need to.
  5. Jersey Bill

    Jersey Bill Member

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    without getting into the heat load calcs, I can tell you that I just put in a new 80 mbh boiler and @ 17 deg outside it doesnt shut off.
    also, I have a master addition that is just framed and currently storage, that will someday need heat.
    This is how I came up with the 100 mbh number.
  6. hkobus

    hkobus Member

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    I have the same question about how to decide what is the best size is. As a rule of thumb Orlan commends 50 liter per Kw capacity. What is the right way to determine the size, or do we just go bigger than the last guy..?

    If I understand the above program right, it determines over capacity to allow for enough storage to prevent idle right? What other criteria should we consider to decide on storage size, in relation to cost of the storage?

    Henk.
  7. Eric Johnson

    Eric Johnson Mod Emeritus

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    I think you're right, Henk, that preventing idle when the boiler is cranking away at the optimum is the primary consideration. Beyond that, it gets more into lifestyle considerations. If your goal is to only have to fire the boiler up once a day in the coldest weather, calculate your heat load and then make your tank big enough to satisfy it for the desired time period. Or, look at it from the other end. If you want wood-fired DHW in the summer, calculate your needs and size the tank to get you, say, one firing session per week. Beyond all that, bigger is usually better.
  8. Jersey Bill

    Jersey Bill Member

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    I played around with the numbers some. It seems that bigger IS better.
    The only downside that I can think of is the cold start up.
  9. Eric Johnson

    Eric Johnson Mod Emeritus

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    Like any wood heating situation, it's a matter of momentum. If you get a big tank charged up, it's likely to stay that way longer, which could mean less cold starting time over the course of a year than you'd have with a smaller, more responsive tank. Plus, if you pipe a bypass around the tank and directly into your heating zones, you can charge the tank up at your system's convenience. Other than the expense and space involved in building one, I can't think of a downside to a bigger tank.
  10. Bob Rohr

    Bob Rohr Minister of Fire

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    Bigger tank= more heat loss. No amount of insulation stops heat transfer. The more you store the more you lose to the surrounding room. Even with 4" of Dow Blueboard around my 500 gallon tank it loses enough to warm my shop without running the floor radiant.

    Every energy transfer or change of state costs you a % every time. Burning the wood to heat water costs at least 30% loss. Pumping loss cost you some, and the heat loss from the tank. then the energy to pump it back out and the piping loss. Of course if you consider it "free" heat i guess it doesn't matter. Nothing is truly free, is it.

    Use the formula I showed in another post to get the storage right where you need it for your conditions. No need to overkill.

    hr
  11. Jersey Bill

    Jersey Bill Member

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    ok,
    the formula from hogstroker was 50 litres/kw
    assuming 150 MBH= 44 kw
    44*50=2200 litres
    580 gallons

    I ran the HDS program for 550 gallons (R21 insul, 55 deg, 50 mbh load, 150mbh boiler) and it came up with 108 minutes ON and 216 min OFF time. That would give me 3.6 hours of sleep time before reloading. I think its a little light, at 80 mbh load the cycle is 2.25 hrs on and off.

    Ideally for me, I would not have to get up to fire the boiler in the middle of the night. The question is whether I should be allowed to sleep through on a peak night or not.

    If I figure on 70 mbh load, and 5 hours OFF time, the tank size comes out to 1200 gal
    and a 50 mbh load, and 5 hours OFF time, the tank size comes out to 600 gal

    the tank standing loss shows as 551 btu/hr.

    the total storage capacity of the tank is somewhere around 400,000 btu's (1200 gal, 40 deg delta) if I did the math right.

    551/400k= 0.14%/hr loss --- could that be right? it sounds low to me. One of the reasons I wanted the tank in the crawl space is because a heat loss to the tank, is a heat gain to the house.

    master of sparks, I hear you about the losses though. Firewood is not free to me by any stretch of the imagination. But is a cheaper, tax-free alternative energy that I prefer.
  12. Bob Rohr

    Bob Rohr Minister of Fire

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    Ah yes, the catch 22 of storing energy in hot water :) You can see how powerful the math is before pulling the trigger. Any way to widen the delta t on the tank for more capacity. 190- 200 draw down 60 degrees?

    In 1989 a Swiss solar pioneer set out to heat his home 100% with thermal solar. It became a media spetacle and he invited the world to come swimming in his solar heated pool. To this end he heats a 130sq. meter home with 84 sq meter of panel and a whopping 118 cubic meters of storage! 31,000 gallons if I have my numbers right. Folks describe his home as a storage tank with a residential annex.

    Now how do you feel about storage?

    So most anything is possible if you are bent that way.

    hr
  13. Nofossil

    Nofossil Moderator Emeritus

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    A few more things about heat loss from storage tanks:

    1) It's a squared - cubed thing. Heat loss increases with the surface area of the tank, which increases as the square of its size. Storage capacity increases as the cube of the size. A tank that's twice as big in each dimension will have four times the heat loss and eight times the volume. The total losses will be higher, but the storage efficiency will be better.

    2) If you invested in insulation for the house, invest proportionally (at least) in the tank. The house walls see an average temp difference of maybe 50 degrees. The tank is something like three times as much, If the house is R-35, make the tank R-105 or better.

    3) There are four heat loss mechanisms: Conduction, Convection, Radiation, and Evaporation. Consider all of them in designing your storage insulation. Radiation is a factor at higher temperatures, and evaporation is potentially a killer with an open tank. R-value ignores radiation and evaporation losses.
  14. jebatty

    jebatty Minister of Fire

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    Excellent argument for locating the storage tank and even the gassifier in an area to be heated. Tank/boiler heat loss then becomes non-existent, as all lost heat is used. Someone who can do the math could calculate the efficiency gain, which I estimate to be quite significant.

    I too heat my shop from the steel storage tank, partially insulated, so the tank acts as a radiator. Another benefit of this is no electricity to run a pump, valves, etc. to get the heat. No moving parts. I do have to roll up or down insulation batts sometimes to get more or less heat depending on outside temp.
  15. Como

    Como Minister of Fire

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    We were sem seriously looking at a house in Colorado with very similar charachteristics, I think I still have the plans somewhere. In foreclosure and heading our way from a price perspective, but then somebody else came in.

    But I am sure this predated 1989, they had lots of solar but also a back up, I can still remember the smell of the swimming pool after 12 months being unattended.

    It was in a valley bottom, summer sun not too bad, but wen we went past there in the winter there was no way you could rely on just Solar.

    Back on subject, it seems there is no objective solution, there could be two or more equally valid solutions to the same question.
  16. Gooserider

    Gooserider Mod Emeritus

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    One minor response / reminder for those thinking about putting the boiler / tank inside the house envelope is that if it's loosing heat, it does so year round (assuming you use the system for DHW), so any thermal gains in the winter will be offset by thermal loads in the summer - not saying this automatically means to move things outside, but that it DOES mean figuring that cost into the equation - will it mean needing AC or more of it? How much will the extra AC add to the costs - Even with a high efficiency unit, I think we pay more per day for AC than we do for the full on gas heat.

    OTOH, I think I saw NoFossil say somewhere that by doing a fanatically thorough job on insulating his tank, he had gotten his loss rate down to around ~1*F / Day - significant I guess, but not all that bad in the greater scheme of things...

    Gooserider
  17. jebatty

    jebatty Minister of Fire

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    Good point on the summer/AC effect. Not an issue for me as no DHW. My gasifier and tank are in my shop, and just talked to the Valentine Bride last week about building a new shop - she was open to the idea! In a new building I would build a separate boiler/tank room with venting into the heated area during the heat season and venting outside when heat not needed. I also like the idea of a buried septic tank for storage. Lots of storage and relatively inexpensive.

    Question for the chemists -- does a concrete tank present PH or other corrosion issues? I use a filter now on tank output, and would continue that for any grit, etc. that may enter the line.
  18. sled_mack

    sled_mack New Member

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    Regarding the cold start issue brought up eariler -

    I have a 3000 gal storage tank, non pressurized, filled to about 2700 gal. During cold startup (say first firing in the fall) the tank starts at 70 deg, I use manual valves to recirc hot water back to the boiler. Once the boiler is up to 100 deg, I have another valve I can open that allows water to go directly from the boiler to the house - bypassing the storage tank. This helps to get heat to the house while still charging the tank. Once the tank hits 160, I can close the manual recirc valves that put hot water to the return manifold of the boiler and push all water directly to the tank. Normally, this is the point where I close the valve that bypasses the tank and goes directly to the hosue, also.

    If I fire the boiler Sat morning, by Sun afternoon the tank is charged. The beauty is that it is very easy to keep the tank over 160, which means not having to worry at all about cold return to the boiler.

    My reason for the big tank was to carry the heating system through longer down periods when we are not home. We were just away for 8 days, and I was able to squeeze out 5 days off the tank. I caught a break on the weather, it wasn't that cold, but that was still a nice long time before switching to oil.

    There are other factors besides pure efficiency to picking your storage tank size. And, each time we remodel a part of the house and upgrade the insulation/windows/doors in that area, the tank lasts a bit longer too.
  19. jebatty

    jebatty Minister of Fire

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    I have almost the same setup, with the same results, except that I have the Termovar valve to insure that hot water is circulated back to the boiler. If my tank starts at 70, I have circ pump set to "on" at 150 from boiler supply. Boiler circ and hx supply tank circ start. Termovar at this point is mixing so water at 140 comes back to boiler, balance to storage or by manual valve shunted directly to heat zone. This lets boiler operate more efficiently. Note: due to Termovar, boiler return is always 140 or above.

    Actual data taken last week, all heat to storage:
    Boiler hx(boiler side) hx(storage side)
    Return Supply In Out In Out
    140 170 170 120 70 150
    140 162 162 120 75 145
    150 180 180 130 77 160
    148 172 172 130 82 155
    150 170 170 155 90 158
    160 185 185 145 97 170
    165 188 188 155 110 174

    I didn't stay to get data as storage rose above 110. I will do that. On this burn cycle I brought storage up to 165, with the bottom of the tank at 150.
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