Water Heaters - Thermocouplers don't last and rusty chimney pipe

My gas water tank is going through a thermo coupler every 3 months (I am now on my 4th one). Also, a lot of rust is forming inside the chimney pipe inside the tank - every time I change the thermocoupler I have a lot of rust to clean up off of the burner. Is the rust caused by my chimney pipe entering into too big of a flue pipe? (2 years ago I insulated the and finished the basement, so maybe the tank isn't getting enough air to take the moist exhaust the whole way up the chimney.)

I seriously doubt you are actually finding bad t'couples each time. Are you taking voltage reads each time. Possibly the pilot needs cleaning. Just not enough heat on t'couple tip. Only 3/8" at tip must have flame impingement. The rust you mention is a product of moisture. Appears you have a venting problem. The products of combustion are not being vented to the outside. Yes you may have to large a vent. Too large a vent would take longer to heat and start to draw, leaving moisture to drip on burner or around the draft hood. Most water heaters 30,40, & most 50 gal use a 3" vent. As far as the combustion air in the basement, if you have the heater in the same open room you occupy than that would be enough. Is it in a small enclosure?

I don't know how to check a thermocoupler - I just replaced them after the tank wouldn't stay lit.

Each time I changed the thermocoupler, after vacumming all visible dirt/rust, I blew through the pilot gas supply tubing with my compresser and it works fine for three months. Maybe it is all of the new rust from the moisture problem that is messing up the thermocoupler.

First, I'll make sure only 3/82 of the coupler is exposed to flame, as you suggest. Then, I'll have to find someone to address the moisture problem then see if it fixes my other problem.

Thanks!!

mbk3 I believe has pointed you in the right direction with venting. I have experienced poor venting with the newer water heaters causing excessive heat build up in the burner area causing premature thermocouple failure. I have some customers I go out every few months to replace the thermocouple because of this.

One thing I failed to ask about your venting. Does your water heater have 2 draft hoods installed? I have seen hundreds of installs where the new tank was to short and the draft hood from the old heater fit perfectly on top of the new draft hood. When doing this venting is affected and products of combustion spill instead of venting to outside