[edwardc]

The "smoke" you're seeing is steam. Looks like your heat exchanger has two burst seams (freeze damage?). You can see the corrosion around both of them from the escaping moisture.

The upper seam could probably be brazed shut, but the lower one is more troublesome, as it is actually a seat for a gasket under the round end plate, which must seal tightly. It will be difficult to beat it back into round and still get it flat enough to seal. A machine shop could help, but honestly, by that time, you're probably better off just replacing the HX.