Wait, you guys are actually spray foaming cracked heat exchangers and bragging about it? That's not a DIY repair, that's a ticking time bomb wrapped in a fire hazard. I guess if you're tired of having a working carbon monoxide detector and want to save $600 on a funeral instead, go for it.
Grab a can of spray foam and some high temp silicone caulk while you're at it. That's what I used to seal up a hairline crack on my old unit last winter and it held fine for two more years. Just make sure you tag it so the next guy knows what's going on.