There are a million ways to plumb a sink drain incorrectly, and it’s almost always the result of ignorance. It’s not stupidity or malice; it’s just a lack of knowledge. And most of the time, these improper installations work pretty ok as long as they don’t leak. As home inspectors, we typically point out these improper installations and explain why they matter.
One common error we find under sinks is a double trap; this happens when water drains through two separate traps, as shown in the examples below.
In all of these cases, water has to pass through two separate traps, called a double trap. But don’t confuse this with two separate traps, which is fine.
Of course, those corrugated things in the photo above aren’t proper, but the configuration is correct.
Plumbing codes expressly prohibit the use of double traps.
But what’s the problem with this setup? I’ve been told that this will cause the sink to drain slower, increasing the potential for the trap to get clogged. I’ve also heard that you end up trapping a column of air between the two traps, which will mess with the drain performance. But I’ve never known for sure, so I decided to do my own test.
Testing a double-trap
To test the difference in performance between a standard p-trap and a double trap, I set up a pedestal sink in my backyard.
I filled the sink with water and let it drain twice, once with a single trap and once with a double trap. My results from the two tests were nearly identical. The water took the same amount of time to drain for both tests. So, there goes my theory of a double-trap draining slower, thus increasing the potential for clogs.
One interesting thing I found was that the second trap created a siphon, sucking some water out of the first trap. This would normally be concerning, but the second trap was still filled with water.
Conclusion
Sorry, I don’t have anything exciting to report. At this point, I’m not sure why the plumbing code doesn’t allow two traps. We’ll continue to report this as an improper installation, but I’m not sure what the big consequence is.
If any smart readers out there know the real answer, please comment. I’m genuinely curious.
