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Making a copper coil for the wood stove

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by SteveKG, Jun 23, 2009.

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  1. SteveKG

    SteveKG Minister of Fire

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    Our current water heater, one of those AquaHeaters sold until the mid 80s, an 8 gal. tank with a firebox built into the bottom and a stove pipe running up through the center of the upper cylinder which heats the water, is rusting out. I've brazed it and have it secure again, but I know the time has come to replace it. Such heaters are no longer sold or even imported in the US, I've worn out the keyboard trying to find something similar.

    My most current idea is to cut off the old water tank portion and keep the firebox, which is in good shape. I would then build a copper coil to fit into the firebox [which is a cylinder about a foot or so in diameter]. Thereby heat the water from a storage tank, perhaps an insulated one like the Vaughn, 30 gal.

    Anyone have an idea how I could figure how big a coil I'd need [or, to put it another way, how many feet of copper pipe]. I realize it makes a difference whether it is 1/2 inch or 3/4. I don't want to have to run a wood stove for 7 hours or something in the summer to heat up the storage tank supply. The current stove takes 20 minutes to heat the 10 gal. it holds and the firebox is surrounded by a thin water jacket so it puts out only little heat into the room.

    I would be planning to use a circulating pump and the storage tank would be on the same level on the floor about six feet away, under my current scenario.

    Thanks, this is my first visit to the forum. I did not find my question answered using the Search....

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  2. Gooserider

    Gooserider Mod Emeritus

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    Welcome to the forum... I'd suggest that some pictures of the existing unit might be helpful if you can manage that...

    My initial thought was that if you are happy with the way your current setup works (and it sounds like you are) then I'd look at trying to figure out a way to reproduce it - possibly doing something like cannibalizing a gas water heater, since that's what your description reminds me of...

    Gooserider
  3. jebatty

    jebatty Minister of Fire

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    Am I reading correctly that the copper hot water coil will go inside the firebox? If so, isn't boiling a concern? Steam, pressure build-up?
  4. SteveKG

    SteveKG Minister of Fire

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    Well, pressure and steam are always a concern with wood-fired systems, same as with oil or other fired systems. There are many stoves with coils built in, or water jackets, and have been for many decades. [There are tales of these blowing up, which is maybe why we can no longer buy the stove like I currently have, maybe.] But if used correctly, they are fine. Our current design is really no different, physics-wise, than a copper coil inside the firebox. One must not allow the water to sit in the coil [or the water jacket or whatever] as, yes, it'll superheat and cause trouble.

    I've never had or been around a coil-in-the-fire setup. What I've read over the years is that you must either set up the holding tank from which the water in the coil comes and where it goes when heated so that thermosyphoning takes place. Or, you put a pump in line that moves the water thru the system. This is common sense, of course, and what we planned to do.

    Our current water heater needs about 20 minutes to heat the water, and it gets up to probably in the range of 175 degrees F. I am guessing as you can hear it begin to "burble" a bit, and boiling pt. up here is somewhere in the mid-185 degree F. We have a pressure-relief valve plumbed in, in case we let the fire go too long and too hot. It hasn't happened to us.

    What I don't know how to figure, without trial and error and lots and lots of time and effort, how large a coil [how many turns, etc.] I should try to get a reasonable temp. increase in a given time. It would depend on heat of the fire, temp. of the incoming water, diameter and length of the copper, all of which put together make parameters which preclude exact calculations, I just hoped for a general idea where to begin.

    There are companies selling copper or stainless steel coils and loops for installing in various styles of woodstoves. The trouble for me is that my proposed stove, the salvaged part of my old setup, is a small diameter round gizmo so I need to make my own coil. I don't have the ability, or think I don't, to work with stainless, so copper is probably the way to go for us. In our very small abode, space is premium, so my small, salvaged stove box would be great to use. It sits now right next to a kitchen stove/range [wood] so I could put a loop or one of the commercially available stainless ones in the range, but to heat it up summertimes to get water for a couple showers would heat up the kitchen a lot and keep it hot a long time.

    Thanks for the responses.
  5. Gooserider

    Gooserider Mod Emeritus

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    As I mentioned before, if you are happy with the current unit, I'd try to replicate it... Not sure if they make one small enough, but it would sound like if you took a very small gas water heater and pulled the burner guts out of it, the remaining tank with a chimney stack running down the center sounds like the same kind of unit you have now, so it might be able to set on top of the salvaged firebox with minimal hassles...

    One thing I'm not quite clear on about the current setup, is whether you are using the tank on the unit as hot water storage, or if it is functioning as a heating element to put hot water into a separate storage tank?

    You mention this is a summer-use heater, that you don't want to have heating up the house - Thus I assume the firebox and heater exterior is well insulated?

    If it is an element that heats a storage tank, then it would seem to me like the critical factor is that the coil extracts heat efficiently from the fire - I would look at how I could design the coil to extract as much heat as possible so that the stack temperature is reasonably low (say under 500*F) while the firebox temperature stays high enough to maintain good secondary combustion...

    Similar thoughts would apply if the unit is functioning AS the storage tank, except to note that you presumably would want the tank / coil volume to be the same as what you currently have in order to avoid water shortages.

    If you've been lurking on the site much, you will notice that we place a lot of importance on burning cleanly, both to reduce pollution and to get the most "work" out of every chunk of wood, so that WE don't have to work as hard at making lots of said chunks :lol: Smoke is essentially wasted fuel, so we want to burn the smoke in order to get the heat out of it, and to do this requires an adequate air supply, and keeping the firebox temperature high enough to maintain secondary combustion. (Unless using catalytic converters, above about 1200*F)

    Generally speaking, it is best to avoid having water jackets / coils actually IN the firebox. They have a tendency to lower the firebox temperature below the secondary combustion point. What is better is to insulate the firebox to control the heat flow out of it, and extract the heat from the stove exhaust...

    How are your welding skills and equipment? If they aren't up to fabricating something fairly fancy, do you have anybody that you can make deals with to get it done cheaply?

    If I were starting from a clean sheet, I would probably want to do a well insulated firebox with a good air supply, set up in such a way as to pre-heat the air before it went into the box. Possibly I would want some sort of baffles in the firebox to increase the "residence" time of the combustion byproducts, with additional air added above the fire. Probably it would be a good idea to have the inside of the firebox lined w/ firebrick or other high temperature refractory (secondary combustion temperatures are high enough that they can be hard on bare metals) I would run the exhaust through a heat exchanger of some sort, ideally a multiple "firetube" type with some sort of turbulators in it to increase thermal transfer, so that the exhaust leaving the heat exchanger was in the 3-500*F temperature range, possibly less...

    The challenge would be to see how close to that you can get while staying w/in budget, making use of your existing materials, and so forth...

    Gooserider
  6. Gooserider

    Gooserider Mod Emeritus

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    now make a comment about a MAGIC HEAT extractor,eh?[/quote]
    Different critters... A MODERN wood stove is designed to have controlled radiation that is supposed to emit all the heat that the manufacturer felt was desirable, with the balance needed for flue temperatures to drive the appropriate draft. The heat extractors pull some of this heat out, and as a result may cause draft issues and / or excess creosote buildup.

    On older stoves, they often did send more heat up the chimney so the extractors did pull more heat out, but again at a cost of possible draft problems and more creosote buildup.

    In this case, I am talking about increasing the insulation on the firebox so that more heat is deliberately being sent up the flue, with the intent that the heat exchanger (which BTW should be as close to the firebox as possible) extracts most of it, but still leaves enough for the proper draft to operate...

    Gooserider
  7. SteveKG

    SteveKG Minister of Fire

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    Thanks for the attention to my original question. To clear things up a bit, this water heater is used year round. The water tank built into the heater is our only "storage." We heat the water, about 20 minutes gets it very hot, take our two showers, wash the dishes, then the fire is out for a couple or three days til we do it again. Very simple. That's all we use hot water for.

    The current unit is insulated, the firebox, in that the firebox is surrounded by a thin water jacket which is contiguous with the water tank, the way it's made.

    My first idea was the gas water heater tank welded atop the old firebox [cut off from the original unit and that water tank tossed to recycle as it's rusting out]. The gas water heater tanks, with their exhaust pipe running up thru the water tank, is the same idea that this old unit uses. The drawback for us is size. We live in a very small cottage and I'm attempting to keep associated tanks and stoves and etc. to the smallest footprint possible. For reasons beyond me, gas heaters don't come in small tank sizes, only electric units do. So for now I'm setting aside that idea. We don't store any hot water nor do we need to. By the way, I ran across a YouTube series by a guy who uses old propane tanks to make a firebox atop which he attaches one or the other type of water tank. This wouldn't give me any better firebox than the one I can salvage from my old unit...but it's a cool idea for someone else who has welding ability.

    If I salvage the firebox, I'm in business, I think, for a firebox...I may attempt to set a small storage tank somewhere else in the house [not too far] and so on. Building my own new water tank, with a stove pipe up the center, is daunting. To go to the effort and expense, it would need to be stainless, beyond my abilities and beyond my checkbook to have done in a machine shop. Or that's my thinking.

    The winters here, we have wood stoves going a lot of the time. It can average 0 F. a lot, or less. Summers, for a few months, it can be 90 F. to near 100 F. So, I don't want to do the easy, relatively simple thing of a heating loop or coil or whatever in one of our current heat stoves. Plus, my 20-minute burn currently is gonna go up to a lot longer for a larger tank.

    So...you've given me some more info' to mull over, and I thank you. [Solar would be elegant, but again, lots of stuff to house for the drainback/closed loop system we'd need and lots more money even if built by us. Plus, we'd still perhaps need, or wish we had, a backup system for those cloudy spells.]

    Thanks again, everyone.
  8. ManiacPD

    ManiacPD Member

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    My neighbor up here in the Sticks of Maine is a stereotypical tinkerer, hacker, etc. Many times he's admired my 25 year old Memco wood boiler that you can buy for $500 all day long but won't "splurge" to buy one. To each their own I guess.

    He's had a wood stove in his garage (go easy Code Enforcement police) that he's been trying to put a coil on and tie into his oil fired boiler for two winters now. Wants to save oil. You know the type because we all have the same goal.

    Two years ago it over pressurized and continuously blew off.

    Last year one of his soldered joints failed and he cracked his stove as water poured onto it. It did steam clean everything in his garage I will admit.

    He's got a new stove this year and is still at it. I've found three Memcos nearby and he won't even look at one.

    When you figure it out I'll be over for a visit with my neighbor.
  9. JustWood

    JustWood Minister of Fire

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    Find some Amish. They use these type of water heaters and could hook you up with a phone # or address. Might take a week or 2 as I'm super busy right now but I could prolly get you a phone #.
  10. tom in maine

    tom in maine Minister of Fire

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    How about something a little different?
    Use the copper coil, but run it into an unpressurized tank. I know this sounds self-serving, considering who it is coming from (www.americansolartechnics.com LOL)
    but if you use a bronze pump and pump tank water right into the coil and back again, it will not explode.
    I would still install pressure relief valves on both legs, but you will not build up pressure. The worst thing that can happen is you push steam back into the tank--a very cool thing, since it sounds like a steam engine.

    You do need a heat exchanger to get domestic hot water out of an unpressurized tank.

    We did a lot of these types of systems years ago. The beauty of them is that whether the coil is in a stove or a mini-boiler like the Aguaheater, there is no scale buildup which is a big issue when trying to heat DHW in a relatively controlled device like a wood stove.

    Tom
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