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Fin tube as heat exchanger?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Reggie Dunlap, Oct 21, 2007.

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  1. Reggie Dunlap

    Reggie Dunlap Feeling the Heat

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    Has anyone used 3/4 copper with fin tube as a heat exchanger? This is what is commonly used as baseboard heat. Are there any potential issues with using aluminum and copper in a large tank.

    I have a Tarm Solo 40 and a 750 gallon tank, just need to figure out the most economical way to heat the water.

    Thanks,
    Reggie

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  2. Eric Johnson

    Eric Johnson Mod Emeritus

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    I've heard that there could be a problem, Reggie. I've also heard that you can treat the water to mitigate it. Also, they use very thin copper tubing for those elements. It's a lot thinner than Type M copper. So corrosion will kill it a lot quicker than standard copper coil or rigid pipe. Sounds like you and I are in the same boat, exchanger-wise.

    Did you just put in the Tarm, or have you been using it?
  3. webbie

    webbie Seasoned Moderator Staff Member

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    It may be that a pre-made (replacement) Domestic Hot Water boiler coil is the lowest price thing made out of copper - because they are made in quantity and out of a finned copper......

    something like these:
    http://www.boilersupplies.com/tankheaters/burnham-america.html

    With a little luck - or a friendly HVAC man, you might be able to get them wholesale - probably 50% off those prices. You could also ask them about defective coils or overstock....

    these coils are usually rated quite high.....

    it's a lead, anyway...
  4. Nofossil

    Nofossil Moderator Emeritus

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    I was mightily tempted myself, but I can't evaluate the risk of galvanic corrosion. I'd love to get some guidance from someone who knows what they're talking about....

    I'm toying with the idea of forming rectangles of copper flashing over a steel bar so that they have a 'U' shaped channel u=in the middle, then bending them a little so that they clip onto the outside of the copper tubing - poor man's fins.

    I'd love to find a source of finned stainless.
  5. Reggie Dunlap

    Reggie Dunlap Feeling the Heat

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    Eric- The boiler and the tank are both new. The boiler is built and ready to go whenever, just waiting for heat exchangers and a pondliner for the tank. The tank is 8" concrete block insulated with 3 inches of Celotex on the outside, so the block is part of the heated mass. The bottom also has 3 inches of Celotex and I will also build a hinged insulated lid. It is in the basement on top of the slab. The house is still under construction.

    The boiler/tank set up will heat approx. 3000 sq. ft. of radiant in concrete and 1200 sq. ft. of baseboard hot water as well as DHW. I have a Trinity propane direct vent boiler as back up.

    I'm a builder, but my plumber did all the boiler work and piping. He is pretty sure the fin tube will work using less copper than a conventional coil exchanger. My only concern is corrosion or some type of reaction with aluminum and copper submersed in 170 degree water.
  6. Eric Johnson

    Eric Johnson Mod Emeritus

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    Slowzuki addressed that concern in another thread. He mentioned chemical treatment, but not specifically what would work best. Maybe you can PM him for details. I'd like to build a house around a hot storage tank.
  7. Nofossil

    Nofossil Moderator Emeritus

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    If you have an EPDM lined tank, I'd go for it. I think that aluminum and steel can't coexist in the same body of water (or steel and copper), but I think that you'd be OK. Could you set it up so that failure would not be catastrophic? For the performance vs. cost, it's pretty attractive.
  8. Corey

    Corey Minister of Fire

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    Here is a link to a pretty good galvanic series list.

    http://www.eaa1000.av.org/technicl/corrosion/galvanic.htm

    Basically, metals on the anode end are most easily corroded while the cathode end are the most 'noble' or corrosion resistant. Any time you have two metals in electrical contact with each other, in an electrolyte (water) this is a galvanic cell and corrosion can occur. You also need to be aware of geometric effects. For example - one aluminum fitting in a copper system - The aluminum will corrode, and since there is a relatively small amount, it will corrode in a hurry. If you had a mainly aluminum system, with a copper fitting, the aluminum would still corrode, but the effect would be spread over the entire system.

    You can also use this to your advantage by using a 'sacrificial anode' - basically a piece of metal that is anodic to the metal you are protecting - magnesium is always a good choice. So you install the magnesium in a place that is easy to get to - it corrodes, but protects the rest of your system - and you replace the magnesium every now and then as opposed to a leaky fitting in the middle of the night.

    Also note that sometimes stainless steels don't perform well in boiler service. Stainless steel needs oxygen to build and preserve the oxide coating on the steel. Boiler water is notoriously oxygen depleted - especially in a boiler that is circulating water as opposed to one that is always heating fresh water. So the stainless cant build its oxide coating and is really no better than normal steel with a good corrosion inhibitor in the boiler water.

    Corey
  9. Reggie Dunlap

    Reggie Dunlap Feeling the Heat

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    Corey- Thanks, that is interesting. I'll try a sacrificial chunk of magnesium. The water in the tank never leaves the tank. It is just a BTU dump heated by the boiler. The heat is then transferred to the house via flat plate heat exchanger.

    If the aluminum corrodes the worst that will happen is some loss of heat transfer. If the copper corrodes there could be some leaking, but just into the tank. The tank will be sealed with a .045 EPDM pondliner.
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