I have the MasterTemp 400 heater, and, after failure to ignite, I found the tub full of water. I know the general answer to this is that the heat exchanger is shot, but the problem is that I replaced this one brand new 5 years ago. Is it possible for this to fail after 5 years? Or, is there a chance that the gaskets on the manifold could be causing the leak? I'm not certain if the gaskets would let water into the tub. If it's absolutely the heat exchanger, then I'll just replace it, but I wanted to ask if there were other possibilities just in case. Thanks for the help!