I am sure someone with more knowledge will chime in, but I think you only really need to be concerned with excessive duct heat when you have runs of flexible ducts. I Googled the question and came up with similar answers :
"Though there are many factors that will change as per the type of wood and moisture content, dry wood begins to char between 120 to 150 degrees celsius. It begins to burn at 280 degrees celsius. That is 540 degrees Fahrenheit. " (http://www.ask.com/question/at-what-tem ... catch-fire
). Please note the first three temps are in CELSIUS, not Fahrenheit.
Based on this I think you are fine.