Bending 1 inch pipe for water heater coil in wood stove

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I'd like to pick your collective brains for info on bending galvanized pipe to make a square coil to put inside the firebox. Currently inside the firebox is a coil made out straight sections of galvanized pipe connected with cast 90 degree threaded plumbing connectors.

Problem is that my wife says she can smell antifreeze after the woodbox cools down, theres no obvious drips but since she can smell it that tells me that there is a leak in there, Im guessing that the cast connectors and the straight pipe expand at different rates and its caused a crack. Its only been in place for just over a year. The coil is connected to a thermal storage tank in the basement.

I want to replace it with a monotube heater so I dont have to worry about it leaking again. Im thinking of welding a square form out of 8 inch diameter pipe sections (so I get rounded corners), filling the pipe with paraffin wax to keep the bends from collapsing (the wax would be pumped into the pipe from the bottom to prevent voids) and then cold-formed around the form. After the coil is formed I'd heat up the pipe to remove the wax, rinse well with Xylene to remove any trace wax and then install it.
The back panel of the stove is bolted on with scavenged oven door seals to keep it smoke-tight so I can install it fairly easily, I made the stove that way to allow servicing of the pipes. I just didnt think I'd have to do it so quickly.

Do you think this is workable? I'd really appreciate any comments or suggestions.

Thanks!
Dave

-- Dave (AK) (daveh@ecosse.net), October 19, 2000

Answers

Why not use Soft Copper tubing, you could put that around your stove pipe instead.

-- Hendo (OR) (redgate@echoweb.net), October 19, 2000.

How about using soft copper tubing, no seams or joints?

-- Hendo (OR) (redgate@echoweb.net), October 19, 2000.

I agree with Hendo, on this one. I've done just that with soft copper, and it worked fine.

I've also heard of filliing the pipe with sand to help keep it from collapsing, but have not tried that. There are also hydraulic pipe benders, which have a channel which keeps the pipe from collapsing or crimping, and I've used them a lot, and they work well. I don't know about bending a one inch pipe into an 8" radius, though; that sounds awfully tight.

I also think you should make sure you even have a problem. If the pipe's leaking, you should be able to see it.

One other possiblity is to buy, or make, one of the flat plate heat exchangers that you can install in the stovebox, but you'd still have a pipe fitting ivolved.

I don't understand why a good pipe joint should leak in a wood stove. I've never had any problems like that with mine, nor has anyone else I know. Maybe your joints aren't tight enough, or you didn't use pipe joint compound. Can you tighten the joints?

Good luck,

JOJ

-- jumpoffjoe (jumpoff@echoweb.net), October 19, 2000.


I saw this in a book called "homemade hot water" at least I hope that is the title. The guys who wrote it ran a business for a long time selling these kinds of pipes for woodstoves and then spelled it all out in this book. Maybe the library could get it for you, I think it is out of print now but it used to be pretty popular.Anyway, all I remember is what was already said above; use copper pipe and put sand in it to keep it from kinking while bending.Good luck!

-- maureenb (firegirl102@hotmail.com), October 20, 2000.

Thanks for the responses, I didnt use copper pipe because I thought it would corrode quickly inside the firebox, and I didnt think it would stand up to the abuse of logs shifting around, etc. Now that I know that it does work I'll probably use that instead, it'd be a whole lot easier to work with.

Thanks again!

-- Dave (AK) (daveh@ecosse.net), October 20, 2000.



If it was me I would fill the pipe with sand and hot form, using black pipe. The galvanize will burn off anyway in the stove. Also, if you use galvanize pipe the zinc will crack and come loss from the steel in the bends. Good luck.

-- Rich (pntbeldyk@wirefire.com), October 20, 2000.

Xylene!!! nasty stuff. glad to hear you are not doing the wax thing. p.e.g. (polyetheleneglycol) in your antifreeze is dangerous when burned and inhaled. please be careful. -Brian

-- Brian (masterjoiner@webtv.net), October 21, 2000.

Dave, I agree that copper is not as strong as steel, and recommend that you avoid bashing it with logs. I have had two wood heaters that I installed copper coils in, and never had any problems with them, but I only used them for a couple of years each. The one that worked the best exploded-I'd been gone for a while, and the pipes (the water tank, even) had frozen. I built a big fire, then went outside to do something, and got to thinking that I probably shouldn't have built one, with the pipes frozen. I went back in to put it out, but it was too late. The water in the pipes apparently boiled before the pipes outside the stove melted, which resulted in a small steam explosion. It blew the door off the stove (a cheesy little "airtight" aka "tin stove").

Only serious damage was a lot of soot everywhere, but it could have been worse.

My very best wood water heater was one I built from 1/4 inch plate steel. I made one entire side of the heater double layer, with a one inch air space. I welded a 1" galvanized steel coupling near the top of this "water jacket" and another near the bottom. From these I ran galvanized pipes up to where I had removed two fittings on an old electric water heater (the two heating element, that is) The heating elements thread into a 1" female threaded fitting on the water heater. The pipes connect in this manner: top pipe on water jacket to top heating element fitting on tank; bottom pipe on water jacket to bottom heating element fitting on tank.

I used this system for over twenty years; it is still being used three years after I sold the house, by the new owners. It heated water so efficiently that we had to fill the bathtub up with hot water once or twice a day during cold weather to avoid the water boiling. (This heater also was our space heater).

There are at least caveats: this heater, like all efficient wood powered water heaters, is capable of boiling the water in the heat exchanger/water jacket. Code here requires two temp/pressure relief valves-one on top of the water tank, in its normal location, and one right near where the water comes out of the heater (I'm not positive where they want this second one, as I have never used but the one relief valve.). Make sure the water which comes from these relief valves is directed to go where you want it, because once it opens, you'll likely be draining a large part of the tank, whether you want to or not!

Second, my stove was rectangular. I don't believe the water jacket was built strongly enough to stand the pressure of a "standard" water system. I had a very low pressure system, as it was gravity flow, and the springs were not very much higher than the house. I did have a few friends who "copied" my design, but avoided this problem by making their heaters out of old tanks, or utilizing their existing heaters which were cylindrical (e.g. "Fireview" brand) They then took another tank, cut off maybe a third of it, longways, and welded it onto the heater. This resulted in a fairly ugly heater, but one which worked as well as mine.

The third caveat is that any wood powered water heater causes the fire in the firebox to burn cooler than I would recommend. I would not use one again, unless I had no other way to heat water, unless I only heated water with it, rather than using it for both hot water AND space heat. You'll burn a lot more wood space heating with one. On the other hand, during early spring and late fall, when I didn't really need much space heat, but still needed hot water, I coud build a very quick, hot fire and have enough water for one or two days in only an hour or two.

Hope this helped.

JOJ

-- jumpoffjoe (jumpoff@echoweb.net), October 21, 2000.


My husband who has read about and played with these systems says DO NOT use galvanised pipe in the fire box it gives off toxics when heated. It appears as if you had been talked out of it already but just in case you were still thinking of useing it.

-- Terri (terri@tallships.ca), October 21, 2000.

Thanks Joe! I'd been thinking of that route, but you hit upon the exact point that convinced me to keep from using a flat plate collector, the firebox would be too cool to burn the wood efficiently.
I knew that the zinc would boil off inside the firebox since it vaporizes at something like 760 degrees and the temps inside a firebox can commonly reach 1300 degrees, but I hadnt thought of any structural weakining caused by the loss, so you're right I should use black pipe. I also hadnt thought of the toxicity issue, thanks Terri.
I think Im going to go with black pipe for sheer strength and durability after all, I would like to use the hot-form-and-sand route (dont have to worry about cracking the pipe then), but I cant figure out how to get the sand all the way into a 30 foot section of 1" pipe, much less how to get it back out again after forming (a thin plastic tube with pressurized water maybe? how would it make it through all those turns....)
Another thought that occurs to me is that instead of using standard cast plumbing connectors I could weld the pipe sections together, welding pipe and making it airtight is kind of a pain though.
By the way, if somebody else is thining of doing this the loops of the coil are 5 inches apart and the only real drawback is that they collect a LOT of soot on the coils for the first half hour or so of the fire, after that most of it burns off. I also made the thermal storage tank too big - it takes forever to heat up, on the other hand I can have a fire going for one day and night and use the stored heat for about 3 and a half days, after I get the solar heating panels built and installed (some day...) it should work a lot better though (I hope).
Joe, you mentioned pressure relief valves, there is one on the main 5000 gallon thermal storage tank and the drain goes to a lined sump hole underneath the basement floor, the 15 watt 10 GPM pump is an impeller type so when the storage tank is heated up to capacity (200 degrees, its never got there yet) the pump automatically turns off and vapor pressure empties out the coil.
There is also a heat recovery coil around a section of flue pipe with a antifreeze mix going through it, its copper and it feeds my hot water tank (also in the basement, its a 5' high by 3' diameter section of steel pipe I scavanged from the scrapyard). It does hit 200 degrees now and again and the shutoff works fine and the vapor pressure does empty it out to prevent steam explosions with no problem so Im pretty sure Im safe.

Thanks again!!

Dave

-- Dave (Ak) (daveh@ecosse.net), October 22, 2000.



Dang it!
The whole point of that last paragraph was to point out that I dont keep any potable hot water under pressure but I forgot to type it in. A feed from the cold water pressure tank goes into the hot water tank, through a bunch of coils near the top and then out to the hot water feed for the whole house. The water gets so hot that we've scalded ourselves a few times and I had to put in a temp control mixer valve to keep from serious injury.
When we are away during the winter its simpler this way for draining the water from the potable water pressure tank, dont have to worry about the heat storage tanks freezing -its all antifreeze mix in there.
I also forgot to mention that our space heating is wood stove and hot water baseboard.

I really appreciate all the ideas and help, I can't afford a stupid mistake that would waste money or even worse - injure us.

Dave

-- Dave (Ak) (daveh@ecosse.net), October 22, 2000.


I'm often thought that a small radiator (like an aftermarket transmission cooler from JC Whitney) would work well in this type of application, expecially since it's flat it could be hung below the baffel. These types of radiators are exceptionally thin, too.

Just an idea I've toyed with.

- Justin

-- Justin Shelton (justinshelton@netscape.net), October 24, 2000.


Regarding using sand in the pipe-- I have used sand in a length of half inch copper (20 feet long, maybe) that I bent into a coil to wrap around a stove pie (used a cement filled 3 pound coffee can as a form). I got a bag of fine sandblasting sand, nice and uniform and fine and very dry, and it worked well. Pourd in and out almost like water. Not sure about bending one inch steel pipe, though. Let us know how it goes.

Jim

-- Jim (jiminwis@yahoo.com), October 24, 2000.


Thanks Jim, Thats the perfect solution! I hadnt thought of using very fine sand, I was stuck on the idea of beach sand.

-- Dave (AK) (daveh@ecosse.net), October 26, 2000.

why would you use steel pipe or copper has anybody thought of using stainless steel.go to home depot and buy a pipe bender or a hickey and take your time bend it slowly.if you fill it with sand you wont colapse the pipe .remember thinwall stainless steel will outlast thickwall steel .good luck

-- dirk klein (dklein@funcow.com), December 22, 2000.


I'm all for stainless steel pipe. It eliminates the corrosion problem I use 1" o.d. .120 wall type 304 stainless steel. Cut it to mitered corners on a chop saw, and tig welded into a serpentine coil. properly welded the coil will withstand thousands of psi . The stainless will withstand the heat of the stove even without water in it. Freezing is about the only risk, so a drain provision should be made. I will build these type coils custom order for folks who can't do it themselves. Contact me.

-- Kevin Coopee Welding & Fabrication (practicalman45@yahoo.com), May 02, 2001.

This is actually a question. I am planning to retrofit a modified indoor wood burning hot air furnace (now located outside in a shed)with a coil to heat water that will store in tanks under 0 pressure (they will be vented). This water will be circulated through a water-to-air heat exchanger in the outflow plenum of my existing hot air furnace, which is located indoors. Circulation will be continuous, and the hot air furnace blower will turn on and off via its own indoor thermostat. My question is: what GPH flow do I want to create in this loop that comes from the coil in the woodburning furnace and on through the heat exchanger? I am trying to optimize the flow, so that I do not create water that is either too hot or too cool.

Can anyone help me with this?

Thank you.

-- Hal Houghton (houghton@empireone.net), May 18, 2001.


I have a small bolt on tank for a woodstove. Will keep you with hot water for quite a while. Tell me if you are interested.

-- Robert J, Rolleston (RRolleston@netzero.net), January 17, 2002.

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