The unvented S-traps under sink drains of the early 20th-century had a problem: suction, due to an air pressure differential between the front and back of the trap seal, often siphoned the trap water down the drain, which allowed obnoxious sewer gas to rise out of the sink. Not good. So a vent was added at the “crown” of the trap that equalized the air pressure and solved the problem. Unfortunately, that created a new problem. Heavy flow down the drain could briefly backup into the crown, and debris would collect there over time, eventually clogging the vent. The P-trap, with a vent separated from the trap, evolved to become a better design, and crown vents were banned by the building codes in the 1950s.

     Crown vents are sometimes unintentionally created today when a P-trap is installed too close to its vent. See our article What is the minimum trap arm length? for more on this problem. 

    Also What is a P-trap? and What is an "S-Trap” under my sink? Why is it a problem?

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Here’s links to a collection of our other blog posts about PLUMBING PIPES:

• What is the minimum and maximum distance a P-trap can be below a sink?

• How can I protect my pipes to keep them from bursting during a hard winter freeze in North Florida?

• How can I tell if I have cast iron pipes in my house? 

• Why can't a sanitary tee be used for a horizontal-to-horizontal drain pipe connection? 

• What is the difference between green and white sewer drain pipes?

• Is a washing machine drain hose required to be secured at the standpipe?

• What are the abandoned pipes sticking out of the wall in my house?  

• What are the code requirements for plumbing vent terminations?

• What are the code requirements for layout of drain piping under sinks?

• What causes a gurgling sound when a bathtub or sink drains? 

• What is a "combination waste and vent" in a plumbing system? 

• What is a building trap?  

• What is a galvanized nipple?

• What are the pipes sticking out near my water valves?

• How do you accurately find a broken water pipe leak under the floor slab?

• What is the difference between water pipe and sewage (waste) pipe? 

• Are plastic pipes (PVC, CPVC, and PEX) safe for drinking water?  

• If all the plumbing drains have water in them and you can still smell sewer gas, what's causing the problem?  

• How can I tell what type of plumbing pipe I have?

• Why is there a flexible accordion pipe under the sink? 

• What is the difference between PVC and ABS plumbing pipe?

• What is the difference between water service pipe and water supply pipe? 

• What are the pipes on my roof? 

• How can I find out what type of water pipe runs underground from the water meter to the house (service pipe)?

• Why is old galvanized steel water pipe a problem for homebuyers?

• What does polybutylene pipe look like? Why is it a problem? 

• Which water pipes are an insurance problem and possibly uninsurable?

• Can you connect CPVC pipe directly to a gas water heater?  

     Visit our PLUMBING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.