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I've welded many. Some of the newer furnaces burn out in five years. That is partly due to the cheap fire-blanket that is used. Once it falls part, the oil-gun will burn through the thin metal fast. Many older furnaces had fire brick or thick clay refractory liners and lasted much longer. I have a 1948 Firestone oil-hot-air furnace heating my barn and it still works fine. It doesn't even have an optic fire-sensor. It uses a stack heat sensor instead. Inside, it has a firebrick firestop where the gun-flame hits. My house had a new Onieda furnace that burnt out when it was three years old. My son had an Olsen that burnt out in its fourth year. With both, it was an easy fix. It's all soft steel anyway - so soft rod low-penetration rod like 6013 is fine, unless you use a MIG welder. I've also seen many with a stainless steel patch pop-riveted in and sealed with refractory cement. My neighbor did his in 1979 and it's still fine. Just use steel rivets and not aluminum. The main thing is - not so much the patch - it's the refractory protection over it. Don't use a generic "wet blanket" fix. Use a good quality solid refractory chamber built to fit. Or - make a custom chamber by cutting firebrick. Some better furnaces avoid burnout two ways - one with the gun flame not getting near the chamber wall, and some others using a thick clay fire-stop. I will also note that although furnaces have warranties - those are usually worthless - since the small print voids the warranty if burnout is caused by a failed refractory blanket or chamber.
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