I have finally dug up some info on our furnace. What we have is a Lennox Elite G26 75000 BTU (not sure if it is Q3 or Q4). It has an igniter and is supposed to be pretty efficient at 92 AFUE (92.3 AFUE I think).
So the efficiency rating is really not bad considering the age of the unit. This model series has been discontinued, and is not highly rated based on some info I've seen online. Mostly people complain about constant repairs. That has not been our experience, which makes me wonder if a problem happened in later versions of this product.
Thank you for the advice and formulas! I suspect our system is not optimized (proper # and placement of ducts, duct work, heat capacity, blowers, etc.), but we don't have a lot of complaints so far so I plan to keep fixing it!
The repairman is here now, from what I am hearing, it may not be (just) the control board...... It is one thing to replace parts, but I am not in a good position to diagnose issues...