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In the back of my house I see two external dryer vents. I am sure one isn't for the dryer.

I live in a townhouse and all the townhouses have these two vents so its not a reroute job.

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up vote 4 down vote accepted

Vents could be

  • exhaust for dryers, bathroom exhaust fans, stovetop exhaust hoods;
  • intake (fresh air supply) for gas- or oil-burning appliances (furnaces, water heaters, fireplaces);
  • or plumbing vents. (Although plumbing vents would typically be through the roof.)

Can you follow the duct inside to see where it goes?

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One thing that can be done is to turn on various items, such as @HenryJackson has enumerated, one by one and see of you can detect sound, air flow or warmth out of the vent exit at the external location. – Michael Karas Oct 2 '13 at 4:37
It is most likely a bathroom or laundry closet vent. – wallyk Oct 2 '13 at 8:26
could also be intake/exhaust for a heat recovery ventilator (though much less common) – mac Oct 2 '13 at 20:25

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