I’ve dealt with rust through issues like you mention about 4 times on other folks’ boilers and this is what I do to get a more permanent fix:
(a patch MAY work ok for a while, the rust has most likely progressed a good distance from the site of the leak.)
1. Remove all water, wood, ash and scrape the inside of the boiler, use a hammer to test for thin spots. (really swing hard, you don’t want a thin spot to begin leaking in February)
2. Remove the roof and any outer skin that will get in the way of the affected area from the outside of the boiler.
3. Determine the height of the inner drum and mark it on the outside of the outer drum. You’ll need to drain the water past this point (or do full drain)
4. I then use a plasma cutter (Oxy-fuel will work on non-stainless) to cut the lid off the boiler.
a. Cut around the rim of the chimney on inside of the boiler, (not into the chimney steel) removing the factory weld and leaving as little of a gap as practical
b. Cut around the rim of the chimney on the top of the boiler, (not into the chimney steel) removing the factory weld and leaving as little of a gap as practical
c. Lift or drop the chimney out of its location and away from the boiler.
5. Now you will be able to visually see the thickness and look for heavy pitting, etc. Clean this up with a wire wheel, hammer, etc.
a. If you find that you have thin or heavily pitted areas, (or baffles, like on Hawken, etc) cut away more of the top of the boiler using your preferred method (plasma).
b. I find it best to use a straight-edge to cut the line. If I need to cut a rectangular hole, I prefer to fully cut 3 sides and make a perforated cut one side (top or bottom) and bend the steel away using the steel that’s left from the perforation as a hinge.
c. Then you can get into the inner drum
6. Clean up the chimney with a grinder around the welded areas. At this point, you’ll see just how big of a pipe that the chimney is.
7. You may find that the portion that sticks above the outer drum is nearly as long as the part of the chimney that was inside the water jacket. You’ll see that the rust is most likely in the most upper portion of the chimney that was inside the water jacket. This is caused by the hot steel not being protected by water and constantly covered in a perfectly humid environment with very hot exhaust on the other side. (keep your boiler as full of water as possible)
8. At this point, I turn the chimney upside-down, putting the undamaged side into the water jacket and weld it all fully.
9. Fill boiler and check for leaks.
10. Run.
Use any mild steel for patching, etc.
Take care,
Phill
