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Oversized Storage Tanks

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by smangold, Feb 17, 2008.

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  1. smangold

    smangold Member

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    Hi, my first post. What a awesome forum. I have been researching wood boilers for a while, but only recently found this site. In December I went to see the Garn @ the RI dealer. Very cool but I don't think its for me. I like the idea of a pressure boiler. I got info from Tarm and have looked at one, seems well made. I have read quite a bit on here about the EKO and it seems that people really like them. I would really like a system that works as conveniently as possible. I currently have a masonry heater which I love. I don't mind wood chores, but my biggest dislike is bringing a 50 pound bag of wood up the stairs to the living room and the mess. I end up using it 3 or 4 nights per week. And while it works well it doesn't heat my bedroom (its to far away). I plan on building a shed that is attached to my garage for the wood boiler, outside will be a lean-to for wood storage, class A chimney. I think I will do a exterior tank insulated and partiality buried next to my house foundation. Like everyone else, I have sizing questions. My plumber tells me a 100,000 BTU unit is plenty, and he doesn't buy the storage thing. It seems to me a larger unit with a bigger tank is the way to go. A 60 isn't much more than a 30 and if your building, or scavenging a tank 500 gals isn't much less money than 1500 gals. Is there a disadvantage to bigger? I do like the idea of the Eko 60 which will take bigger log lengths. Most of my wood is 18" but I randomly end up with some longer pcs. which I stand up in the masonry heater. The tarm lit. recommends 600 ,800 and 1000 gals for the 30, 40 and 60. So if I get a 60 with a 1500 gal. tank and only really need a 30, wouldn't the increased gallonage make it work size wise and therefore make the times between firings less frequent? Say once a day only when its real cold and typically every other in the 30's? Or should I stick to the smaller size system. I notice everyone here has different size tanks for simaler boilers. What gives? Is it just they found a certian size factor or is there a method I'm missing? Thank you Scott

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  2. Eric Johnson

    Eric Johnson Mod Emeritus

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    Well, smangold, welcome to the Boiler Room.

    Most people will tell you that more is better when it comes to hot water storage. With a big tank (1,000 gallons or more), the size of the boiler becomes less critical, because the tank gives you enough buffer to operate it at capacity. With a bigger boiler, you need to run it less often. Hot water storage is a relatively foreign concept to most heating professionals, so it's not surprising that some people who you'd think would know all about it, don't.

    I really like the EKO. Our Tarm members all seem very happy with their boilers. I think they're both really good choices.

    The Garn is a great choice in some applications, but not for everyone. Some people have trouble believing that, but it's true.

    Hard to say how often you would have to load any boiler under certain circumstances, but I can assure you that you get a lot more heat out of the wood you burn than you would with an OWB or conventional indoor wood-fired boiler.
  3. normandp

    normandp Member

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    If your house has a heat loss of 40,000 btus/hrs, lets say for 24 hours (not accounting for sun or other heat gain) then you would need 960,000 btus for that period. If your distribution system can accomodate it, you could have storage initial at 180F varying down to 125F, that would bring the needed initial storage to about 2,000 gallons before heating the water back up to 180F.

    Storage tanks gives you the advantage of burning wood without smoldering cycle. You can burn your wood non-stop at your boiler's full capacity.

    Without storage tank it is not easy to achieve the right balance with wood fire and heat loss...
  4. sdrobertson

    sdrobertson Minister of Fire

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    On the subject of over-sized storage tank what would be the point of going overboard on a tank. I was planning on a underground tank with a inside diameter of 7' x 7' by 10' deep which I figured I could use 9' for the water which will give me almost 3300 gallons. My thought is it would not be much more expensive to go 12' deep and use 11' for the water which would give me 4000 gallons. I will be using a EKO 60 so at what point will I be "spinning my wheels" in terms of going overboard?
  5. Eric Johnson

    Eric Johnson Mod Emeritus

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    The only person I've ever heard caution about going too big with storage is our member and occasional contributor master of sparks. He knows what he's talking about, but everyone else, including others who know what they're talking about, say that more is almost always better.

    Think of storage as a battery. The bigger the battery, the longer you can go between chargings. But it takes longer to charge it up.

    If you have a good way to bypass the tank so that you can heat your house when the tank is depleted, I think 3,000+ gallons with an EKO 60 would be really desirable. I can't think of a downside, other than it costs more to insulate a big tank and you're going to have more standby heat loss.
  6. Eric Johnson

    Eric Johnson Mod Emeritus

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    If I had a 3,000-gallon tank, I'd try to keep it as close to 180 or 190 as much as possible, but you make a good point, Pook. From what I've read, your 150-degree guideline is a good one.
  7. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    Depends on your radiation. If you are using radiant-floor heating, and can run that on 100-degree water, then having the option letting the storage tank heat to 200 degrees, then drop to 100 degrees would be very nice. That's 100 btu's per gallon of storage capacity.

    As long as it is being repeatedly brought back up near the 200-degree point, it will kill most critters. Chemical treatment can kill the remainder.

    Joe
  8. sled_mack

    sled_mack New Member

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    You have to define what you want from your tank. I wanted it to carry for a few days when we are away. 2700 gallons does that for me. But, you've got to use more insulation (bigger tank surface area) to keep the losses down. Heating the tank and the house at the same time, I can recover the tank from 120 to 180 in a bit over 24 hours. And that includes bringing the house temp up from 55 to 68 at the same time, so the load was pretty high.

    For most efficient use, sizing the tank to the min size to get you from one day to the next is probably the way to go. But I wanted a few days of storage.

    I have learned that I use a lot less wood keeping the tank between 160 and 180. I only push the tank to 200 before going away for a few days.

    Is bacteria in the tank a big issue in a non-pressurized tank? I'm not circulating that water anywhere, so I didn't think I needed to worry about it. It certainly hits the 200 deg point a few times a season, and there is absolutely no light getting to the tank.
  9. sdrobertson

    sdrobertson Minister of Fire

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    Would a chemical treatment hurt EPDM liner?
  10. sdrobertson

    sdrobertson Minister of Fire

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    So I guess the ideal tank size would be between the 2000 and 3000 gallon size. Sled_Mack, when it takes 24 hours to bring it back to temp is that a constant burn, or I guess my question is how many loads are you talking about. I can figure out the btu's I need based on what my CB is using but I'm not sure on figuring out the recharge of the tank. I'm trying to figure this out as I can build it to the size I want but afterwords it will be really hard to change - I don't want the "I wish I would have..." after I get this done.
  11. sled_mack

    sled_mack New Member

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    It's not a constant burn. I've got a timer on my boiler so it burns for 4 hours then shuts down. So overnight there is a pretty good down period. I'd say probably 4 or 5 loads of wood for that recovery. But, remember, that was also bringing the house up 13 deg. There is a lot of water in my dist system in the house to be heated up as well. And it was working hard for 8 or 10 hours, too.

    Normally, I load it twice a day, morning and night. With the timer shutting it down, all I do for the next burn is throw more wood in the fire box and reset the timer.
  12. SciGuy

    SciGuy New Member

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    Even better yet at 834 BTU's per gallon with a 100 degree delta T;)

    Hugh
  13. smangold

    smangold Member

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    Hi, thanks for all the comments. I have been thinking in simle terms,as my math is my weakness. However if tarm recommends 660 gals. for the thirty, and size wise this is appropriate for me ,wouldn't oversizing by twice the BTUs make me want a 1220 gal. tank? Is tarms sizing figured at what it takes to charge a tank when you have no house heat demand. So in the summer if I had no heat demand the 60 would charge the tank from 95 or so to 175 ,on one loading of fuel. I guess maybe there are infinite variables of usage needs over a given time. It makes my head ache a bit. Maybe all of us that are new are in the same boat.I know its hard for me to spend 10K with out being sure I'm doing the best thing. Perhaps I should just start with out storage although this would almost definitely make me stick with a 30.
  14. Nofossil

    Nofossil Moderator Emeritus

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    Unfortunately, math is the only reasonable way to figure out what you need.

    Bigger is generally better, but heat loss to the outside increases as the size of the tank increases. It's also true that temperature matters more than total heat energy. If you're heating with baseboards, you'll be happier with 1000 gallons at 170 degrees than with 2000 gallons at 140 degrees.

    Water stores roughly 8 BTU per gallon per degree. You'll need to consider your normal heat load and the lowest usable temp for your system. You don't want a tank that's too small to carry you from one day to the next. Two days would be nice, but more than that might start getting too big to deal with.

    You can probably plan on something in the neighborhood of 5000 BTU per pound of dry wood, or about 15 million BTU per cord, at least for back-of-the-envelope calculations. YMMV.
  15. sled_mack

    sled_mack New Member

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    Purely guessing?

    I hope not, because now you have me concerned. I see about 15 deg rise with a 4 hour burn. That's with the house removing heat from the tank at the same time. I've never weighed the wood going in, but I'm never starting with an empty firebox. The bed of coals is usually just 3 or so inches below the bottom of the door.

    I could probably squeeze more out by burning the load completely, but the convenience factor of burning weeks without having to restart the boiler is pretty high.
  16. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    Sorry. Yup, although I like to round down and call it 833 :)

    That will teach me to post in a hurry...

    Joe
  17. wdc1160

    wdc1160 New Member

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    Joe, I saw it, but didn't have the heart to post.

    Also, Sled. I figure 7K btu (usable) per pound of wood. So your not hallucinating

    60 LBs of wood 420K
    To move 3000Gals up 100F = 2500K
  18. wdc1160

    wdc1160 New Member

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    Pook, What???
  19. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    Please do... we don't need new folks reading things and making bad plans. I won't take offense - we all make typos and such. That's why we use "rules of thumb" to double-check our numbers when we do heat losses and such ("hmmm... 75 btu's per square foot for this house... I think I should re-do the math...")

    Joe
  20. Jimxt88

    Jimxt88 New Member

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    I'm just wondering if another medium might be a more efficient heat tranfer/storage medium than water. What about oil? Say, parafin? Have you ever heard of anyone filling a tank with oil instead of water and heating the oil to two hundred degrees farenheit? I suspect it would take more energy to raise a pound of oil one degree than a pound of water but I'm not sure. Also, how much longer would that pound of oil hold onto that one degree of energy and how easily would it give it up? But I'm just wondering if 500 gallons of oil might not be a more efficient energy storage medium than 2,000 gallons of water.
  21. SciGuy

    SciGuy New Member

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    The specific heat (amount of energy needed to raise the temperature of a given mass a given amount) of liquid water is the highest of all earth materials.

    see http://www.engineeringtoolbox.com/specific-heat-fluids-d_151.html

    Oil's specific heat is only .4 of water's so it will only store .4 times as much heat/lb/degree temp change. Given that oil is ~ .8 times as dense as water that makes its storage capacity even that much less than that of water.

    Hugh
  22. Jimxt88

    Jimxt88 New Member

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    Thanks Hugh. I am thinking in terms of a fast hot gasified burn of cord wood - that is a fixed amount of energy right? The amount of energy released during a burn like that isn't affected by the exchange medium. If I give the burn a number, say 100 energy units, you are saying water will absorb 10 units of when oil will absorb onlly 4. Do I have that right?
  23. steam man

    steam man Minister of Fire

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    I am wondering if anyone has attempted to use a PCM or Phase Change Material in their application yet so as to not have an "oversize storage tank" . I have been researching it but haven't found anyone that can say for sure how it has worked. Glauber's salt (the most frequently mentioned product) seems to change phase at a pretty low temp <110 deg or so. That doesn't seem to work for DHW though it could work for low temp radiant loops. I find that the supply seems to be limited in the US. I don't even know what it would cost to use a PCM and to have a custom temperature one made. For a unit volume it seems PCMs can hold a lot of heat.

    I probably will go with a home built wood storage tank though I am looking at factory made stuff. I have some experience with skate park ramp building. Why is that relevant? 1-I think I am going to build a 1200 gallon or so cylinder to get the maximum structural integrity. 2-I can't wait to see the look on my son's face when he sees me tearing apart his 16ft wide 25ft long half pipe for the materials I am going to need.
  24. Nofossil

    Nofossil Moderator Emeritus

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    The only way to beat water for heat storage density is to use a phase-change material. Paraffin, for example, will store and release a decent amount of energy as it passes through the melt/freeze boundary. However, it's expensive, flammable, and really hard to work with. And it doesn't convect worth a damn, especially in its solid state.

    If water cost $5 a gallon, we would all think of it as the holy grail of heat storage, and we'd use it if we could figure out how to pay for it. We don't give it as much honor as it deserves because it's so cheap.
  25. steam man

    steam man Minister of Fire

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    I suppose if the space was at a premium it would weigh in on the decision to use PCMs. I have the space and access I need but there has to be a practical limit on size for each person. I am guessing PCMs will be more common as solar heating becomes more mainstream. Just a thought.
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