Page 1 of 2 12 LastLast
Results 1 to 25 of 47

Thread: What rod for fireplace heat exchanger?

  1. #1
    Join Date
    Jul 2020
    Posts
    445
    Post Thanks / Like

    What rod for fireplace heat exchanger?

    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).

  2. #2
    Join Date
    Mar 2021
    Location
    Near Roanoke, VA
    Posts
    530
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    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,,

  3. #3
    Join Date
    Dec 2008
    Location
    Lockhart,Tx (BBQ capitol of the world)
    Posts
    1,204
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    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!!

  4. Likes Kevin_Essiambre liked this post
  5. #4
    Join Date
    Jul 2020
    Posts
    445
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    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.

  6. #5
    Join Date
    Jul 2008
    Posts
    3,145
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    A couple thoughts:

    1. You could make your heat exchanger hugely more effective if you used some other material, such as aluminum or copper, instead of steel. Alum is almost 5X as thermally conductive as steel, and copper is almost 8X as thermally conductive as steel. This would also give your heat exchanger a lot longer life, since it won't rust out. And since it's so thermally efficient, you wouldn't necessarily need to put it right in the fire; if you had a wood stove for example, you could figure out ways to rig it to the outside of the stove.

    2. That said, you can also take TOO much heat out of your exhaust, particularly in a woodstove. This will cause creosote to build up in your stack much faster, greatly increasing the risk of a chimney fire that can burn your house down in a hurry. Normally you want the exhaust to be at least 400F at the top of the stack. Much less than that, and you'll need to clean your flue multiple times every winter, which can be a huge PITA when it's snowing and blowing.

    Personally, I would replace the fireplace with some kind of decently efficient woodstove. That alone -- without any add-ons -- would greatly reduce the amount of heat going up your stack, though it will also require you to clean your flue much more frequently than when it was just a fireplace, unfortunately. There's no free lunch here -- if you take more heat out of your exhaust, you're gonna have creosote build up much faster -- even if you burn only good, dry hardwood, and even if you're careful to always keep a hot fire going. It's just the nature of heating with wood.

    Whatever route you choose, if you plan to do much heating with wood, I would recommend reading a very good white paper on chimney fires that was a real eye-opener for me:

    https://www.csia.org/uploads/9/0/7/3...hite_paper.pdf
    Last edited by Kelvin; 10-23-2021 at 06:23 AM.

  7. Likes FlyFishn, whtbaron liked this post
  8. #6
    Join Date
    Jul 2008
    Posts
    3,145
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    Also -- hearth.com is a good place to discuss projects like the one you're thinking about...

    Lastly, if you change the setup much (for example by installing woodstove or whatever), be sure to have some professional (possibly including your insurance carrier) sign off on it, in case you ever have to file an insurance claim. We all know how insurance companies try to get out of paying claims.
    Last edited by Kelvin; 10-23-2021 at 08:31 AM.

  9. #7
    Join Date
    Jul 2020
    Posts
    445
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    Quote Originally Posted by Kelvin View Post
    A couple thoughts:

    1. You could make your heat exchanger hugely more effective if you used some other material, such as aluminum or copper, instead of steel. Alum is almost 5X as thermally conductive as steel, and copper is almost 8X as thermally conductive as steel. This would also give your heat exchanger a lot longer life, since it won't rust out.
    Thanks for the input. Aluminum is a possibility. However, I presume it will be much more costly also. The place I get metal from can source 6061. Would that work? Or will it soften up too much and melt in the heat? I imagine it will be exposed to direct flame.

    As to the chimney fire risk - you are absolutely correct. I am looking in to ways to minimize that and having a good chimney sweep brush is part of that. Ma says the stove by grandfather used at the cabins went in back in the '80's. We were up there a couple years ago in September with the snow flying and had it running all day every day. That was the same concern I had - the creosote build up in the flue. No one knows the last time it was cleaned. So I intend to sweep that, also. Although, I don't know what size the pipe is off-hand. We've never had an issue with it, but its better to do the work and make it good than to keep running on the "well its never been an issue..." thought. Up there the fire department shows up with ashes and coals left and says "yep, there was a fire here". They might roast a couple hot dogs and smores.

  10. #8
    Join Date
    Mar 2021
    Location
    Near Roanoke, VA
    Posts
    530
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    Quote Originally Posted by FlyFishn View Post
    Thanks for the input. Aluminum is a possibility.

    Would that work? Or will it soften up too much and melt in the heat? I imagine it will be exposed to direct flame.
    You are right,, ONE side of the exchanger is exposed to flame,, BUT,,

    the OTHER side is exposed to the room temperature air going through the heat exchanger,,
    That cooling air would make it almost impossible for the wood fire kind of heat to damage the aluminum.

    The reason you are selecting aluminum is because it does a GREAT job of moving heat,, WAY better than steel.
    The heat gets moved, the heat can not build up enough to damage the aluminum.

    If you built out of copper,, the conductivity is so great,, the fire could be a "rose bud" oxy-acetylene torch as big as the fireplace,,
    The copper would just laugh at the heat,, and stay there,, like nothing is happening,,

  11. #9
    Join Date
    Jul 2008
    Posts
    3,145
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    Quote Originally Posted by FlyFishn View Post
    Thanks for the input. Aluminum is a possibility. However, I presume it will be much more costly also. The place I get metal from can source 6061. Would that work? Or will it soften up too much and melt in the heat? I imagine it will be exposed to direct flame.

    As to the chimney fire risk - you are absolutely correct. I am looking in to ways to minimize that and having a good chimney sweep brush is part of that. Ma says the stove by grandfather used at the cabins went in back in the '80's. We were up there a couple years ago in September with the snow flying and had it running all day every day. That was the same concern I had - the creosote build up in the flue. No one knows the last time it was cleaned. So I intend to sweep that, also. Although, I don't know what size the pipe is off-hand. We've never had an issue with it, but its better to do the work and make it good than to keep running on the "well its never been an issue..." thought. Up there the fire department shows up with ashes and coals left and says "yep, there was a fire here". They might roast a couple hot dogs and smores.
    If you ever hear a roaring sound coming from the chimney, that's a textbook sign of a chimney fire. Best way to put it out is to close the flue to deprive it of oxygen as well as you can. It's wise pre-planning to keep a big bucket of sand nearby to dump onto the fire in the fireplace/woodstove to smother it and prevent smoke from filling your house when you close the flue. Another way I've heard of fighting a chimney fire is to discharge a full water-charged fire extinguisher (these are cheap on ebay and you can recharge them yourself with water and compressed air through a Schrader valve) into the stove and flue ... it turns to steam which displaces the oxygen in the flue gases and (temporarily) snuffs combustion. The smart thing to do immediately after that is to call the fire department, because chimney fires can re-light, since the heat and fuel is still there -- only the oxygen is missing from the so-called "fire triangle."

    You should definitely have that chimney cleaned. I do mine at least every year, sometimes more than once in a heating season. From a small wood stove, I get about 2-3 gallons of black creosote every time.

  12. #10
    Join Date
    Jul 2020
    Posts
    445
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    Rough idea.

    The fireplace is like a fish tank and the top of it is a tapered "hood". My thought is to make the legs adjustable and the heat exchanger box sized to mesh with the hood shape/taper. Slide it in, then raise it to mesh. The intake and exhaust (cool air in/hot air out, not wood burn exhaust) ports may take some creativity to make work as the existing framing has a low opening where the glass door is. The ports would have to drop down to clear the opening.

    Thoughts on material? Again, I can get 6061. I will have to price things and see where that leads also. My concern with aluminum is it getting soft when hot.

    As noted with the tubes/pipes - it might help to get more opening (by diameter or quantity) so as to not restrict the flow of exhaust gasses from the wood burning too much.

    I also have ideas for blocking off the fireplace openings to add intake air control. Without it there is no way to control the burn. An aesthetic fire is not the same as a heat fire and using the most of the heat as possible from what is burned is important.

    Name:  20211023_113744 cropped small.jpg
Views: 318
Size:  68.9 KB

  13. #11
    Join Date
    Jul 2008
    Posts
    3,145
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    If you make that out of aluminum, you're not going to have enough heat left in the flue to carry the water vapor away (212F), let alone the creosote vapors (400F).

    If you made it out of steel, maybe. But even then. Seriously, it's not hard at all to cool the exhaust gases more than you want to. No fancy engineering needed.

    The thing to do is look at your stack temperatures now. That will give you an idea of how much more heat you can extract from the exhaust. If you're not at close to 400 at chimney top, there isn't much more you can get without gumming up your flue in no time flat.

    JMHO
    Last edited by Kelvin; 10-23-2021 at 04:42 PM.

  14. #12
    Join Date
    Jul 2020
    Posts
    445
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    Quote Originally Posted by Kelvin View Post
    If you make that out of aluminum, you're not going to have enough heat left in the flue to carry the water vapor away (212F), let alone the creosote vapors (400F).

    If you made it out of steel, maybe. But even then. Seriously, it's not hard at all to cool the exhaust gases more than you want to. No fancy engineering needed.
    Sounds like steel is the way to go. So back to the original post - what rod? 6011/6010 or 7014? Or something else? I'm thinking 6011 - it runs easy and is fast-freeze. Bead appearance is a moot point Id say - it will all be rusted over after a couple of uses anyway.


    Quote Originally Posted by Kelvin View Post
    The thing to do is look at your stack temperatures now. That will give you an idea of how much more heat you can extract from the exhaust. If you're not at close to 400 at chimney top, there isn't much more you can get without gumming up your flue in no time flat.
    We don't have a stove, just a fireplace. Hence the heat exchanger. If we went to a stove - wouldn't that alone be extracting a ton more heat from what goes up the flue than an "uncontrolled" fireplace? With that having been said - what is the difference here? For a given amount of wood burned lets say the heat output is uniform. Between burning the same amount of wood (with the same amount of available BTU's) - I would think a fireplace would allow the most BTU's up the chimney, the heat exchanger the 2nd, and a proper stove the least amount of heat up the chimney = most heat in the room. Or is there a chance with this design that I can actually extract more heat from the wood than a proper wood stove?

    As to gunking up the chimney - I am afraid of that and am willing to do the work to clean it routinely. That is par for the course with the overall idea.

    We have gas heat, but for certain reasons I am not wanting to put all my eggs in the one basket this season and wood heat is the only possible alternative - if we prepare for it - both with how to get heat and maintenance of it. Wood supply is the least of our concerns. Although, I suppose the weakest link in that might be my one gas saw, that is if we can't get wood delivered. I have a ton of chains and a bench sharpener - the saw has never let me down yet. When I am up north I've gone through 3 chains on a trip, all skip chains for a 24" bar, but I have 5 and another regular chain set for a 20" bar. I never really use the regular chains unless I want a cleaner cut. But if I am running the saw I'm very rarely cutting anything clean so no real point. The option is there, though, if I mess up a bar I guess. Theres a lot of other things on the saw that could bite, though, also.

  15. #13
    Join Date
    Jul 2008
    Posts
    3,145
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    Sorry, I may have spoken too soon. I was thinking you were proposing adding the heat exchanger thingie you drew to a woodstove, not a fireplace.

    Again, before I did anything, I would check your stack temp at the top of stack on a cold day with a normal fire.

    The main reason fireplaces are so inefficient for heating a house is because you need to leave the flue open when you go to bed. Once the fire gets small, there's only a little bit of CO, CO2 and H2O going up the stack -- but the flue acts like a venturi in a carburetor, or like a huge siphon or exhaust fan, and the warm exhaust gases going up the stack also suck out all the warm air in your house like a big exhaust fan. Cold air comes in from outside the house to replace that warm air -- and your house gets cold.

    The advantage of an "airtight" woodstove (not really "airtight," since it needs combustion air, but not as wasteful as a fireplace) is that you can choke down the combustion air when you go to bed, and it won't suck as much warm air out of your house. (A good woodstove will also have a temperature-controlled combustion air vent, which will automatically close down the combustion air as the fire goes out.) The trade-off is that your stack temperatures with a woodstove will be lower than with a fireplace when you do this, causing more/faster condensation of creosote in the flue.

    Maybe your proposed heat exchanger will help -- I don't know. My hunch is that it will make the room with the fireplace HOT when the fire is going well, and then have little or no effect once you go to bed and the fire burns out. Either way, I suspect your house will be cold when you get up in the morning. But give it a try...it can't hurt!

    ETA: Regarding materials, yeah, I would probably use steel for the heat exchanger, and if it's going to be a a "firetube" design, I wouldn't expect it to last long. It's probably going to warp all to he!! given how hot it's going to get, how large the temperature differential will be between the air intake and exhaust ports, and how thin the metal will be (I assume something like 1/8" since anything much heavier than that is going to be a biotch to move around and install) and in the "off" season -- summer -- it's probably going to rust pretty badly. But it might last a couple years...good luck. I suspect a wood stove is in your near future...might want to start researching them now...

    What rod? No matter...I'd use whatever I could run best. Probably 6010 in my case.

    One other thought: If you have a fan on the HE, I would blow cold air IN rather than sucking hot air out, since the HE will eventually rust enough to get holes in it. When that happens, you want positive pressure in the HE to prevent sucking smoke out of the fireplace and blowing smoke into the house...also, you want cold air going over the fan, not heated air...

    OK, last thought (didn't realize I was writing a book here!): If your flue hasn't been cleaned in many years, as you say (I'm assuming this is an OLD stack) you really need to be careful. Old masonry stacks were often laid with "sand-lime" mortar, and over the years, the acid vapors in wood smoke will "eat" or dissolve the lime in the sand-lime mortar, leaving the mortar very thin or even crumbling or gone altogether, leaving "holes" in the stack hidden behind the wood walls in your house. In the case of a chimney fire, this can be very bad juju. Just FYI...

    Good luck and let us know how you make out. I've been kicking around the idea of making an outdoor wood-fired boiler "smoke dragon" for a few years now...one of these days...
    Last edited by Kelvin; 10-24-2021 at 11:44 AM.

  16. #14
    Join Date
    Jul 2008
    Posts
    3,145
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    I just looked at your drawing again. If it were me, I would either use much larger fire tubes, or just use one or two much larger firetubes to make it easier to make. Remember that you really don't need (or want) to pull all that much additional heat out of your exhaust, so you don't need all that much surface area to do it. You're not making a boiler for a condensing oil burner. You could probably just make a box with one or two 4" diameter "fire tubes" in it, and allow the smoke to go around the sides of the box, and extract all the heat you want or need.

    Or just make a box with NO fire tubes in it, and allow enough space for the smoke to go around the HE on all sides, and weld some baffles inside to make the air travel all throughout the box (rather than taking a shortcut directly from the intake to the exhaust ports) ... and call it macaroni. Like this:



    I wouldn't spend a lot of time making it, since I suspect it will rust out or "burn out" within a few years, and you'll be making another one soon enough...
    Last edited by Kelvin; 10-24-2021 at 12:23 PM.

  17. Likes whtbaron liked this post
  18. #15
    Join Date
    Jul 2020
    Posts
    445
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    Thanks @Kelvin.

    A few thoughts:
    - Metal flue pipe, 8". It is gray inside now, looking at it from the bottom. My intent is to sweep it anyway and from the top - at which point I can see what is coated on it up top.
    - Chase is framed mostly with metal studs
    - Yes on the fan blowing air in to the box, not pulling hot air out.
    - As to material and weight - you are correct on weight adding to the challenge, however I have a couple ideas on how to deal with it. I was thinking 1/4". We'll see.

    Lots of ideas. We may very well go to a wood stove. The install would require removing the existing flue pipe and putting in a higher rated insulated system (double or triple wall).

    I like the idea of the outdoor wood furnaces. It won't work here, but some time down the road that would be pretty cool.

  19. Likes Kelvin liked this post
  20. #16
    Join Date
    Jul 2008
    Posts
    3,145
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    Not sure of the current codes, but if you want to install a woodstove, and if you have an existing masonry stack, you can probably get away with putting a single-walled SS liner into it (I'd use 316, not 304 SS), then pouring a vermiculite/concrete insulator around it. This will dramatically reduce the costs of upgrading your stack. Also, I would avoid the "flexible" (corrugated) stainless steel liner pipes...with all those corrugations in them, I suspect they are a nightmare to clean the creosote out of. We have a smooth SS liner, and the creosote basically falls out of it, leaving it clean at the top when you go to clean it (but most creosote builds up at the bottom of the flue on most chimneys, anyway, I believe)...

  21. #17
    Join Date
    May 2014
    Location
    Manitoba Canada
    Posts
    5,270
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    The only experience I have with a heat exchanger was one in a local church that was in the top of a very old oil furnace. That one was actually cast steel but after several decades of use, it was also rusting out. There are 2 ways of looking at this... try to make it indestructible (HTP high heat coatings come to mind) and expensive, or just accept that it's eventually going to be a casualty so make it cheap and replaceable. I'm leaning towards the 2nd option. Rods can be whatever you feel comfortable with welding because it's going to rust out in the HAZ first anyway.

    I've burned wood for almost 50 yrs and the best protection you have against creosote fires is to look after the wood you are going to burn. I burn Manitoba maple (box elder) a lot and everyone will tell you it's the worst for chimney fires. The trick is to cut a year to 2 yrs ahead so it's seasoned, and keep it DRY. I make sure it's split and piled inside before fall. A roof over it is a minimum, I store mine in an old abandoned grain bin so it's out of the rain and snow. Snow melting down through the pile in the winter is almost worse than a summer rain since it will affect the wood as you're burning it. If you want to see the nastiest chimney block ever, just burn a bunch of half green wood that hasn't been kept out of the weather.... I've seen it block so tight I couldn't drop a foot of railroad iron through it... had to poke through it with a long piece of rebar. You don't want that.... trust me.
    Last edited by whtbaron; 10-24-2021 at 12:45 PM.
    The harder you fall, the higher you bounce...

    250 amp Miller DialArc AC/DC Stick
    F-225 amp Forney AC Stick
    230 amp Sears AC Stick
    Lincoln 180C MIG
    Victor Medalist 350 O/A
    Cut 50 Plasma
    Les

  22. Likes FlyFishn liked this post
  23. #18
    Join Date
    Jul 2020
    Posts
    445
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    Quote Originally Posted by Kelvin View Post
    I just looked at your drawing again. If it were me, I would either use much larger fire tubes, or just use one or two much larger firetubes to make it easier to make.
    ...
    I wouldn't spend a lot of time making it, since I suspect it will rust out or "burn out" within a few years, and you'll be making another one soon enough...
    I agree with your last comment there on easy to make.

    As to the tubes - I did the math and the flue is 8". For the same square inch area with 3 tubes the ID's would have to be about 2.6". If I round up to 4" OD tube with 1/4" wall (should be easy to source) that would give me 3.5" ID - that would be a lot more open area than the flue.

    As for holes - this might be a good project for my cutting torch - something I have yet to use. I've used it for heating truck parts (not the cutting torch assembly though) with a rose bud and welding tips.

    The baffle idea is good. The longer the path the air blows across hot surfaces the more heat it picks up. I like the idea of the fire and exhaust heat/gasses passing up through, as opposed to around, the box. That will draw flame in to the tubes and get the exchanger hotter I would think.

  24. #19
    Join Date
    Jul 2020
    Posts
    445
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    A couple additional thoughts:

    - Making a sealed fire box appears to be a better idea - that makes it possible to control the fire with air control and damper.
    - I am thinking getting the exhaust down to a single tube that meshes with the flue pipe is preferable. Originally I was thinking the rectangular heat exchanger would mesh with the tapered hood before the flue pipe, but after measuring that would be pretty hard geometry to fabricate to. Tapering down the exhaust ports through the heat exchanger to a single port that meshes with the flue would be a ton easier.

    In regards to the latter point - getting the exhaust ports through the heat exchanger down to a single pipe - the question is how to do that. I could put any number of ports through the heat exchanger, then cap them off on the top with another box that goes to a single larger pipe. That option appeals to me more than just running the single pipe through the heat exchanger. The reason being it would allow the hot gasses to pass over more metal inside the heat exchanger on which baffles can be placed to pass air over said hot metal surface areas. If I did a single pipe through the box it would make baffling, uh, baffling. I could put baffles in, but there woudn't be very many paths for the hot gasses to go up "through" the heat exchanger baffles - only in the one spot. That would limit the amount of heat able to be stripped off and sent to the air blown through the heat exchanger.

    The down side to the multiple exhaust ports tapering to 1 is the possibility of turbulence restricting draw. The less draw the more time for flue gasses to cool and the less effective the draft through the flue is.

    Thoughts?

  25. #20
    Join Date
    Mar 2018
    Location
    Clovis California
    Posts
    3,543
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    You do know heat exchange should made from stainless steel.

    Dave

    Quote Originally Posted by FlyFishn View Post
    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).

  26. #21
    Join Date
    Jul 2020
    Posts
    445
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    Quote Originally Posted by smithdoor View Post
    You do know heat exchange should made from stainless steel.

    Dave
    Thanks for the input. I am not sure overall the extra cost vs expected life expectancy of it would be worth it to go to stainless steel.

    Speaking of which -

    I've been looking at stove paints etc and there are paints/coatings that are spec'd for direct flame applications.

    Would one of these flame-resistant coatings prolong the life of mild/carbon steel welds with normal welding - IE - not "low hydrogen"? Or will carbon steels that aren't low-hydrogen (let alone the weld metal), regardless of the coating, have accelerated rot due to the high temperatures?

  27. #22
    Join Date
    Jul 2008
    Posts
    3,145
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    SS is one of the least thermally-conductive materials you could make the HE out of (less than 1/3 as conductive as MS)... which is why it would also probably warp all to He|| in the firebox.

    The heat will also likely cause carbide precipitation in SS, which (along with the corrosive vapors in smoke) will probably lead to it rusting faster than you might expect. Yeah, they make flue pipes out of SS, but they probably don't get as hot as your HE will...

  28. #23
    Join Date
    Mar 2018
    Location
    Clovis California
    Posts
    3,543
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    Most codes say stainless steel it will last a long time.

    Dave

    Quote Originally Posted by Kelvin View Post
    SS is one of the least thermally-conductive materials you could make the HE out of (less than 1/3 as conductive as MS)... which is why it would also probably warp all to He|| in the firebox.

    The heat will also likely cause carbide precipitation in SS, which (along with the corrosive vapors in smoke) will probably lead to it rusting faster than you might expect. Yeah, they make flue pipes out of SS, but they probably don't get as hot as your HE will...

  29. #24
    Join Date
    May 2014
    Location
    Manitoba Canada
    Posts
    5,270
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    Yes, and the steel v.s. stainless debate takes us back to the same old question, expensive or easily replaced? Those high temp paints have been used on headers for years (or at least they were in the 70's and 80's) with mediocre performance. It's why header companies have gone to high temp coatings like HTP... again, do you want to buy a couple extra years for a significant investment or just make a couple cheap ones you can change out easily?
    The harder you fall, the higher you bounce...

    250 amp Miller DialArc AC/DC Stick
    F-225 amp Forney AC Stick
    230 amp Sears AC Stick
    Lincoln 180C MIG
    Victor Medalist 350 O/A
    Cut 50 Plasma
    Les

  30. #25
    Join Date
    Jul 2020
    Posts
    445
    Post Thanks / Like

    Re: What rod for fireplace heat exchanger?

    Quote Originally Posted by whtbaron View Post
    Yes, and the steel v.s. stainless debate takes us back to the same old question, expensive or easily replaced? Those high temp paints have been used on headers for years (or at least they were in the 70's and 80's) with mediocre performance. It's why header companies have gone to high temp coatings like HTP... again, do you want to buy a couple extra years for a significant investment or just make a couple cheap ones you can change out easily?

    There was a coating I came across - Thurmalox (link below) - that is about $70/gallon. I don't need nearly that much, but a quart is 1/2 that at $35 so I'd be better off getting a gallon for the same price as 2 quarts.

    Other than the added cost of the paint/coating - what am I missing in your perspective of "significant investment for a couple more years"?

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Page generated in 1,638,512,785.85094 seconds with 13 queries