Are Drum Traps less prone to siphoning?

I recently bought a 1930s fixer-upper and it has have a Drum Trap under the kitchen sink that serves the sink/disposal and the dishwasher. There is no dry vent and the stack connection is about 15 feet away. An inspector told me that drum traps are less prone to siphoning, but I can't find anything to back that up. Is that true? Everywhere I go (work, friends, public restrooms) I've been looking under sinks and haven't seen a drum yet. The only other fixtures in the house are within five feet of the stack, so I'm wondering if the builders just got lazy on this one or it wasn't a concern back then.

I'd like to believe we've learned a lot about what works well in those last 75-years or so. I'm surprised you haven't had problems with the trap and the disposal. The biggest disadvantage to a drum trap is that is it impossible to run a snake through, and is probably significantly caked up with crud over all these years. When it does die, if you can't vent it with an atmospheric vent, if allowed, consider an AAV. As to siphoning, can't say, but my gut feeling is, maybe.

P traps do not siphon if they are properly vented. Drum traps have not been used for many years for reasons previously cited. You would be wise to revise the drains to include proper venting and P traps.

In a normal configuration it is impossible to siphon a drum trap. The state of Maine required drum traps under all lav's up till a few years ago.

I think it might be better to ask how many people have seen a re-vented P-trap which was installed in the early 1900's. I have not. Plumbing codes have changed a great deal since than.

In 1930, WI had 3 plumbing inspectors for the entire state and did not require plumber licensing throughout until 1963.

Thanks Tom, I had no idea drum traps were ever required anywhere. If drum traps are siphon proof, why don't we see more of them?

With a siphon proof trap, you wouldn't need any vent until the stack because you would only have to worry about positive air pressure, right?

Drum traps are NOT siphon proof. They contain more water, in a different configuration, so when they do siphon, enough water usually remains behind to maintain a modicum of trap seal. It is the same effect that causes water to "fall back" into a toilet after it flushes.

tmpusr889 said:

Thanks Tom, I had no idea drum traps were ever required anywhere. If drum traps are siphon proof, why don't we see more of them?

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It was already explained that drum traps cannot be snaked...

Snaking has nothing to do with the ban on drum traps. In fact, since they have a removable cover they are EASIER to snake. But a drum trap, because of its construction, does not meet the requirement that "traps be self scouring" so they do NOT accumlate debris. Hair traps for beauty parlors, and granule traps for dentists, for example, ARE special purpose drum traps and ARE legal.

tmpusr889 said:

Thanks Tom, I had no idea drum traps were ever required anywhere. If drum traps are siphon proof, why don't we see more of them?

With a siphon proof trap, you wouldn't need any vent until the stack because you would only have to worry about positive air pressure, right?

Click to expand...

As far as I know, only Maine required them and again, only under a lav. As far as siphoning goes, I don't know what conditions it would take to actually siphon a drum trap. I have tried and can not reproduce any scenario that siphoned one. The big problem with drum traps is that crap accumulates in them and hair can buid up to the point that the hair hangs over the trap weir. Instead of siphoning the trap will then wick the water out. That and a lot of folks never cleaned them out so they become a maintainence problem. Traps should be self cleaning by code. Some inspectors will still allow them for free standing soaking tubs but a running trap would be a better choice.