I open vents on both sides of my basement in warmer weather. This keeps the air pressure balanced between the basement and the outside to deter a draft reversal in the chimney. This is especially important if your house gets too warm and you decide to open windows in levels above the basement. You should have a manometer permanently installed so you can watch for low draft conditions
... Also, keep a more than normal amount of over the fire air (secondary air) coming into the combustion chamber during warmer temps outside.... This shouldn't attribute to much combustion, but instead the extra air will get heated and go up the chimney to also aid in maintaining draft in the chimney..