Thought I'd revive this thread with some observations I made today.. I had an opportunity to get some numbers with the mild daytime temps.
First picture, was at 4:00pm when I got home from work.
250 degrees over the load door, 158 degrees on the pipe and a -.015 of draft pressure.
I decreased the primary air at the ash pan door about 25% from where it was.
I increased the secondary air in the back of the furnace by 50% from where it was.
Second picture, an hour later at 5:00pm.
244 degrees over the load door, 160 degrees on the pipe and a solid -.02
My conclusion is that the decrease in primary air with an increase in secondary air with a low fire and mild temperatures outside gave me 2 favorable results.
1) Decreased the temperature of the furnace 6 degrees (favorable since its 75 degrees in the house) and increased the flue pipe by 2 degrees.
2) Improved my chimney draft by -.005
Though its a small improvement, I haven't yet had a draft failure since using this technique. The numbers show that more heat (which is being carried by the extra secondary air) is going up the chimney to help maintain draft..