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There is a large PVC pipe (3 actually) which vent air intermittently. I started noticing I could see steam rising over my deck in the morning and then I found that these pipes vent air at least a couple times per hour.

What is the purpose of this venting? I'm concerned about efficiency.

Bonus: When the pipes are venting it is fairly noisy. When we're on our deck we can hear it and it's annoying. Any way to muffle this noise without impacting the operation?

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Well, you CAN turn your furnace off, but it seems to me that the cold house will then be a problem. I suppose that will be up to you. It will be VERY efficient though. – user558 Nov 3 '12 at 14:36
maybe put some lattice around the vents some kind of barrier between the deck and vents – jberger Nov 3 '12 at 19:22
For some reason DIY is plagued with users who are not familiar with the provide a comment justifying your downvote etiquette. – kingdango Nov 4 '12 at 13:58
up vote 3 down vote accepted

Do you have a high efficiency furnace? If so this is the exaust and air intake for your furnace. Unlike older furnaces, HE furnaces do not exaust up through your chimney and instead use PVC or ABS pipes.

Not sure about the third pipe though - do you have a heat exchanger?

Best way to know for sure what they are is to try and trace them back to the source.

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