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Use Cast Iron Radiators for heat exchanger in storage tank?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by scottinkn, Nov 17, 2012.

  1. scottinkn

    scottinkn Member

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    I am putting together, this weekend, my unpressurized storage tank, approx 960 gallons.

    Turns out I don't have a lot of money for copper

    So
    I can get some cast iron radiators for next to nothing from a client.
    (they are smaller, look thin and have like four tubes in each bank)

    I can maybe gang some together. Does anyone think that this will work? (efficiently)
    If so, boiler side or DHW preheat?.... It seems as if they would hold a lot of water!


    Thanks,

    Scott

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  2. maple1

    maple1 Minister of Fire

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    If you make sure they don't leak, they'd likely work in some fashion.

    They'd likely rust too.

    EDIT: I don't think I'd put them in my DHW stream - most are very ugly inside.
  3. scottinkn

    scottinkn Member

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    Good point about DHW Maple! I do think that they would last a very long time as long as I use "good" ones.

    I just wonder how efficient they would be.
  4. maple1

    maple1 Minister of Fire

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    I don't think I've heard of anyone even trying it - so you might be on your own, in a lone guinea pig sort of way. Sometimes that's the only way to find out if something will work.
  5. scottinkn

    scottinkn Member

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    Maybe so, be a loner, but I am leaning toward trying it.
  6. scottinkn

    scottinkn Member

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    Other than the DHW rust and bleeding the air, I don't see any potential problem with trying it.
    Is there anything I could do to measure the efficiency of them once installed?
  7. maple1

    maple1 Minister of Fire

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    Measure/monitor your temps going in & going out (difference = delta T). Not just once but constantly or periodically over time. Along with your tank temps. A more rigorous efficiency measurement would involve measuring flows too - but just seeing what the temps do over time with constant flow should be a pretty good seat-of-pants measure. If your delta T goes small after short time, that's not good.
  8. scottinkn

    scottinkn Member

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    What would be considered "an average" temperature drop? 10 degrees? 20? Just curious.

    Would it be better to just have gauges or is temp monitoring something I could do on my computer easily?
  9. Nofossil

    Nofossil Moderator Emeritus

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    One concern is that in a closed system, the oxygen in the water is quickly absorbed by creating a little bit of corrosion, but once that happens that's it. With DHW, you have a continuous supply of fresh oxygenated water, so I'm pretty sure that you'd get continuous corrosion. Now cast iron has much less of an issue than steel, but it would be good to hear from someone who has actual experience of using cast iron in such an application (unlike myself). Doesn't have to be radiators - any cast iron exposed to a steady supply of hot oxygenated water would do.
  10. ozzie88

    ozzie88 Member

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    I agree, whats really inside those things i dont think I would drink from it?? This is a first for me,never heard of it,but dont mean it wont work,just few concerns
  11. scottinkn

    scottinkn Member

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    Thanks Nofossil!

    That's another good reason for not doing it on the DHW preheat side... So THAT is out!

    BUT could it be feasible to do on the closed loop side?

    I mean it sounds like an easy, inexpensive, way to heat exchange on the boiler side...

    I am hoping that it is efficient, too!

    Asking these kinds of questions is why we are here I suppose!
  12. Jeff S

    Jeff S Feeling the Heat

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    In my 1916 & 1920 copies of "the Ideal Fitter" there is a small paragraph entitled "for heating liquid in tanks" where it is stated "American Wall Radiators can be used advantageously in hot water storage tanks in place of the ordinary wrought- iron or copper coils,insuring an ample supply of hot water for a very small initial outlay " they give a small chart of using both steam and hot water for heat sources.I could not find this info in the 1929 edition but all 3 editions did give a comparison chart of iron vs brass piping used horizontally in a storage tank to heat water with steam.

    So yes it has been done in the past but was obviously discontinued ,but why ?,probably many reasons perhaps the rust issue,the weight of cast iron compared to copper and eventually discontinued as a production item ,and also copper at one time was probably a better bargain price wise.

    I have also heard that once cast has a thin layer of rust on it that actually protects the inner layer, the example I have often heard is take a rusted piece of cast and break it and only the outer surface will show signs of rust,of course you could always coat the radiators for extra protection.

    In the above example brass vs iron,brass did have the edge requiring less square footage but if the radiators are free,use more of them.
  13. scottinkn

    scottinkn Member

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    Wow! Thanks Jeff!

    I would think reason they stopped using them is because copper was lighter, did not rust and was not cost prohibitive. (at the time)

    My main considerations are now:

    Will rust inhibit heat transfer, enough to diminish efficiency?
    Do they require higher temp in order to operate? ( I know that you can use them for a hot water system instead of steam)
    How many will I need?
  14. tom in maine

    tom in maine Minister of Fire

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    If you are doing an unpressurized tank, you should probably treat the water when using iron in it.
    There will be oxygen getting into the tank water regardless and eventually you will see corrosion.

    A radiator is not going to work well within the tank in terms of maintaining stratification.
    It is thicker walled than copper, which is an issue for heat transfer. Rust is not going to enhance heat transfer.
    If you are using a liner, you need to protect it.

    One leak in a radiator is going to ruin your day. Taking a rusty radiator out of your tank is not going to be fun.

    At least you can pull a copper hx out easily.

    As the Car Talk guys say, "it is the stingy man who spends the most".
    Copper coils work well and are well documented.
    I would save radiators for in room heating. They are excellent for that.

    If the cost of hx was just too much to bear, I would treat the water in the tank with corrosion inhibitor and run the boiler and
    heating loop unpressurized. It works and eliminates the space/boiler hx.

    my 2 cents worth.
  15. ozzie88

    ozzie88 Member

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    Amen!!!!
  16. Eric Johnson

    Eric Johnson Mod Emeritus

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    A properly installed ci rad in a closed (i.e., hydronic) system won't have any corrosion or rust inside. Usually, it's just a fine coating of black residue which turns to dust when dried. Steam rads are a whole nother matter. They're full or rust, corrosion, cat hair, etc. As to whether or not the radiators would corrode in a non-pressurized tank, I think that if you seal it off properly (the tank), the ox levels will drop over time and you'll wind up with very little potential for major corrosion issues. Yes, a leak would be inconvenient, but a radiator in good condition that is properly installed and pressure tested is not going to leak. If it does, a little Boiler Stop Leak would probably fix it. Cast iron rads throw out A LOT of heat, thick walls be damned. They're designed to efficiently transfer heat. I don't see what difference there is between air and water in that regard, other than you'll get a lot more transfer (measurable BTU's) with water.

    All that being said, I've never done it, so I can't endorse the idea without reservation. However, I would think that if you installed the rad (or rads) in a way that they could be relatively easily inspected and monitored, you should be able to anticipate and compensate for any potential problems if you keep an eye on them (like once or twice a year). I would keep the number of rads to a minimum, however, as the more piping you incorporate into the installation, the more potential for problems. But I would consider stacking them so that you get some stratification in the tank.

    Let us know if you decide to move forward with this project. I'm not aware of anyone else who has tried it, and I'm dying to know how it would work.
  17. scottinkn

    scottinkn Member

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    Thanks Tom!

    I really needed to see both sides and I DON'T want to to it over again.

    AND I was worried about adequate heat transfer.

    Thanks again!

    So maybe going to have to be unpressurized, with a rust inhibitor, can you recommend one?

    P.S. I will have a final copper tally in a few days, then I will know what I am dealing with.

    (so far I have - 20' of 2", 20' of 1-1/4", 50' of 1", 20' of 3/4")

    I really want to do it right, so I may just have to do DHW preheat later.
  18. scottinkn

    scottinkn Member

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    Thanks for the support Eric!

    I really DO want to try it, but I DON'T want to fail miserably!

    Sucks to be me right now & I am so on the fence with this.

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