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Andy Lambert
Andy Lambert, Building Inspector
Category: Home Improvement
Satisfied Customers: 162
Experience:  City building inspector 14 years. Contractor 30 years. RI Dept. of Health Certified.
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plumbing question

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What would be the reason that someone would drill a 3/16 hole through a plumbing cleanout cap in a vertical wate line?

Probably to drain a central air system evaporation/humidity line or a water pan that sits near it (but the pan needs to be above the 3/16" hole.). It could very well be that there was not enough air in the line to allow the plumbing system to drain properly so they introduced a small hole in the cap to allow more air to get in thus helping the waste water to move better. You need to  have a drain vent connected to the drain line. This is usually viewable over the roof line.

Hoped I helped :)

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Customer: replied 11 years ago.
Reply to Andy Lambert's Post: As this is a Florida house built within the last 10 years, it has all the required vent stacks as per South Florida building code. It just seems odd that there would not be enough air in the system to prevent a partial vacuum being drawn to empty the traps. Go figure!

You are correct. It probably served a purpose such as I mentioned in my earlier post.