Trying to remember how to adjust one, it has been years since I had an oil
burner. The object is to prevent the draft or suction that the wind creates
on the chimney from sucking on the furnace and affecting its burning
performance, but to still pull all products of combustion up the chimney.
Until someone who knows better, this is the best I have.

If there is no wind outside it should stay closed. If there is a wind
outside, the draft should slightly pull itself open as the suction
increases. When there is a strong wind blowing you should see it open wider
as the wind gusts. When the furnace is running, it is the same way so that
it can remove the fumes.

wrote in message
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It's an oil furnace.