Air Handler Vent Cap

I just noticed that the cap that prevents water from going through my air handler vent in my roof has blown off. And we have had some rain lately. How bad is this? Does this mean that water has gone straight to the unit? Apart from putting the cap back on, what do I need to do?
Thanks in advance for your advice.
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Oh MY! Run for the hills
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Put the friggin' cap back on, and hope for the best. Tom
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Air handler vent cap, do you mean chimney-exuast cap.
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It's the cap that covers the vent in the roof from the air handler (in the attic, 2nd flr).
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Im sure someone thats knows more will help but my Spacepack air handler has no exterior air vent, I can`t see why an air handler needs a vent. Furnaces exuast outside, but air handlers? . Fresh air Exchangers vent out but this is a different type of unit. If a cap fell off put one on, water is getting in, and maybe other critters. But what is venting and why.
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