I think you misread TCMTech's post.
He said
Which would mean that just burning wouldn't cause tar formation. It would require short cycling of the fire to produce the tar. So if you had not been short cycling for the fire the past 20 years, then no noticeable tar deposits would have been formed in the past 20 years.
Sort of like how burning alone doesn't cause carbon monoxide to be produced. It requires burning in an oxygen defficient environment.
Also, you didn't seem to respond to the mention of explosive gasses or carbon monoxide in TCM's response.
The system HAS been short cycling for the past 20 years. It it has been running at low settings for extended periods. I think there is confusion over the exhaust gas temperature, which will be far higher than the that recorded on the flue (stack). Nonetheless, the surface mounted temperature WILL be a function of EGT. Perhaps I should have made that clearer?? Didn't think I needed to.