What rod for fireplace heat exchanger?

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I am looking at making a heat exchanger for the fireplace with steel pipe. Basically it would be a series of tubes that air blows through to get more heat output. Since the tubes won't hold any pressure or load of any measure, other than their own weight and what ever low fan pressure to move the air, I am thinking 6010 or 6011 as they are fast freeze, 7014 would have a cleaner bead appearance. I don't know how the heat will affect the welds. Over time I would expect the metal to rot - the heat is going to accelerate the rotting. I am not sure if there is a coating I can put on the pipes to prevent that, but I am not sure if it is worth it. The pipes would be exposed to direct flame so I would imagine most coatings would burn off anyway. I don't know how I would get the insides coated, either. The main concern is the pipes hold together and are sealed so I'm not getting smoke blown in the room. I am trying to think of ideas that would get the air inside as hot as possible. That would mean passing the air over as much hot surface area of metal as possible, I would think. That might mean tubes wouldn't be very effective. Maybe I need a box with panels to act as a larger radiator. We'll see. We are contemplating a stove, also, but the fireplace is not set up for one (insert style). It would be a lot less work installing a stand-alone stove and replacing the flue pipe, otherwise to get an "insert" to work we would have to rebuild the whole hearth. A stand alone stove and flue pipe set up is very expensive, for as much as that would be the best solution. The two better solutions would be a ducted wood furnace and the top option would be an outdoor wood furnace with heat exchanger - but neither of those are options in our case (both cost and other reasons).
Reply:I had one of those,, but it was commercially built.The result was that only 85% of your heat goes up the chimney,, instead of 95% without it,, almost a complete waste of $$$ to buy it.Mine was made out of formed tube, like exhaust pipe for a car.If you do make it, get the aluminized pipe, and figure out a way to simply clamp the tubes in place, no welding on the tube.The tube rusts at the exact place the weld is,,With all these kids driving diesel trucks, with GIANT exhaust pipes, maybe a muffler shop could bend the big pipe, like 4", into the shape you want.With the large pipe, you would get great air flow, lots of surface area, and you would only need about 4 tubes,,
Reply:If you do build your own be sure to do the first burn OUTSIDE not in the house!

There is no way you can clean it as well as fire will!!


Reply:Just thinking here... Maybe another idea is to perforate larger plates, rectangle plates almost the size of the square footage of the fireplace, with a hole saw. Then weld in a bunch of pipes sandwiched between the plates. Box off the plates and use the inside as the heating surface area for the air blown through it, and the pipes to pass the exhaust fumes on up the flue. That might work pretty slick. Maybe I could block off the air direction in the cavity so the air zig zags through for a longer distance as opposed to just blowing straight through. Thats an idea.