Ideally your flu temperatures, when there is active flame, don't drop below 400. Mine stay between 400-500. Stove top temps when cruising are around 650-700.What do you mean by burning cool, as is not getting up to a hot enough temperature during burning? I have a thermometer on order and just ordered a moisture meter as well.
What you should see is a fine dust. As mentioned by others, you have some flakes which are usually related to low temperatures.
Andrew