here is a different scenario.
Your flugard says "too cool" right there. Seems pretty obvious.
When you read about what people say about flue temps you will notice that they often forget to mention whether they are talking about surface temps or internal temperatures which we know causes a discrepancy of 2x.
It is only logical to understand that you don't want flue temps below the condensation point of water anywhere in your flue system. What do you think will happen to all of that water and smoke juice?