Open Source Hydronic Wood Stove
Prototyping from 2015
Link to static picture - 
2016 Early Spring
- Joints had leaks.
I tried brazing the entire setup because of leaks, and made it even worse. Pinholes all over the place, which means that ultimate care has to be made for cleanliness and braze quality, meaning remelt after first pass to melt out any pinholes. I am finding that because of geometry, it is impossible to clean the pipes well - so with gunk on the exchangers from the first firings, I think that brazing is a dead proposition from the start unless one disassembled the whole assembly and made sure to clean all the joints thoroughly. Could it also be that brazing pipe to cast fittings creates pinholes just like in welding - where non-cast fittings must thus be used instead? Did you use regular cast fittings or were they non-cast special order? Please comment on any of my above assessment.
Anyway, I will get new black pipe and fittings and redo the experiment, since some of the joints appear to be overtightened and may have thread damage. I agree with the placement of the heat exchanger on top - you are right that the creosote will self-clean only when the pipes are at the roof of the stove, not on the sides
I will get the 400F pipe dope. Do you know which one you used?
Sorry you're having trouble. It can be difficult to braze or weld cast iron fittings. Sometimes you just end up chasing the holes around and making them worse, such as your experience seems to show. I would disassemble and then reassemble, unless the threads have been over-tightened to the point where they're going to leak. Of course, with brazed or welded fittings, it's time to start with new piping. I had one tiny leak that I could hear on my large stove, and I simply let it clog up on its own. It's no longer audible, and there never was any sign of leakage. However, if you have a number of leaks, you'll have to fix them as I wouldn't count on more than one or two tiny leaks fixing themselves through sediment clogging the pinholes.
You might pressure test with a limited air supply, perhaps 60 psi, as you build the heat exchanger, just to learn more about how tight to get the pipes to prevent leaks without deforming threads to the point of creating leaks. Assuming, of course, that 60 psi is well above the pressure you intend to normally use in your system.
As for fittings, I simply used standard threaded iron pipe from Home Depot. I can't recall the thread sealant I used, but I know it was for use with oil and gas, and it was workable from well below zero to many hundreds of degrees. There are all kinds of thread sealants on the market that you can try, some with very high heat tolerance. I would suggest staying with something that remains flexible so it won't crack on you in service.
Another option is to use copper or stainless steel tubing that can be coiled into a heat exchanger. That would require a dielectric fitting external and away from the stove so the dielectric material doesn't melt (at about 180 degrees). The dielectric fitting keeps the dissimilar metals from eating a hole in one another over time, induced by water flow. The main drawback here is the coil can't be cleaned because it's nowhere near as robust as the iron pipes, so it wouldn't take the abrasion associated with cleaning, and holding it in place is also challenging because it's thin and this isn't conducive to soldering, welding or brazing.
I hesitate to even mention leak stop as an alternative, simply because I've never tried it, but it would seem that interior water temperatures would be similar to that of a water jacket on an engine, it's just the external temperatures would be much higher than a radiator.
By the way, this summer I'll be building another black iron pipe heat exchanger for the outdoor furnace I'll be building, and I'm going to refit the large wood stove that sits in my living room. So, I'll be having a similar adventure as you (and taking my own advice too.)
I hope things go well for you,
Thanks for the feedback. I'm confident in, perhaps, permatex, and primarily getting the right tightening force.
Any suggestion on overtightening/undertightening? I let someone else do it, and they appear to have killed the fittings with a 3 foot pipe wrench by overtightenig.
Do you count the number of threads remaining, or go more by tightening torque?
Do you have any designs or back-of-envelope sketches for your stove design that you can share?
I think the motivation here is clear: we are near a $500 hydronic stove, where the cheapest on the market is $5k. We need to nail this, and run build workshops on it for others to learn.
I'm sure permatex makes a fine product...they've been in the business for many decades. A 3-foot pipe wrench is about twice as long as you need. Those monsters are for working with 2 and 3 inch pipes. Looking at the remaining threads is an indicator because they should all be nearly the same number of threads exposed, but this could vary a bit. I would suggest you get it tight to where you're using upper body strength only. If you start leaning into it, you're likely to overtighten and that deforms the threads. Deformed threads have less surface area to meet up with the surface area of opposing threads. It's like the hydraulic lines on my equipment, I haven't had any leaks because a get them tight, but take care not to overtighten. Remember, the seal is made by the surface area of the threads, so don't over do it. If they get deformed, they'll be much more likely to leak because that puts too much fluid pressure on the pipe dope that is supposed to seal up microscopic cracks only. When the fittings seem like they're plenty tight, they probably is. Pressure test with water and a soap solution occasionally to make sure.
No diagrams at this end...I make rough pencil sketches, then finish my design as I build. Almost everything I do it one-of-a-kind.
I agree with the $500 versus $5,000 reasoning. Also, heating is one of your most expensive fuel usages, so it's wise to concentrate there.
Happy new year. We are continuing the stove adventures, this time adding a TEG unit and household hot water. You can see the presentation and past genealogy here - http://opensourceecology.org/wiki/Hydronic_Stove
We just built this new off grid expandable eco home - this is where the new stove is going - https://www.kickstarter.com/projects/622508883/open-building-institute-eco-building-toolkit/posts/1750160
Qeestion: should I design a side port on the stove, outside of the front door, for cleaning the pipes? I am moving to pipes overhead only as you suggest - you're right on about the creosote buildup. Do you ever clean the pipes, or do you leave the creasote to burn off by itself? I am tending towards self-cleaning operation - that must happen, or otherwise, maintenance will be a killer.