Sooting on heat exchanger
I have a Pentair Minimax 100 spa heater that's only a few years old. About a year ago I had to replace the gas valve, so that's even newer. Recently I've been blowing thermal fuses, and now I have a leak in the heat exchanger. When I pulled the heat exchanger out, there was sooting on the bottom. I gather this can be caused by problems in the gas line, and I'm going to have the gas company come out and test the line.
The heater is outside, and it has been working fine until it started doing this. I haven't done anything else to the heater that might have triggered this.
I'm not optimistic the gas company will find anything, particularly since the installation has been in the same place for some 30 years now. If that isn't the problem, what should I be looking for to keep this from happening again?