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My wife and I just bought a house and have been smelling sewer gases from our tub. After taking a look at the piping, the tub doesn't have a p-trap. My original plan is to install a trap before the sink's tie-in, but I have two questions:

1) With the current piping, is a tub trap all that I need?

2) What is up with the vent being after the toilet tie-in?

House Plumbing laid out Attached is a simple drawling of the plumbing, and here's the description:

  • We live in a rancher house, with a main floor and basement underneath (Where all the piping connects)

  • The main air vent (2" diameter / Purple line in the drawling) runs from the roof down to the basement ceiling

  • The tub's drain connects directly to it's overflow pipe, and then runs straight down to the basement ceiling (Total length of pipe to the basement is about 2ft)

  • The tub's piping in the basement runs straight into the sewer line (Orange line in the drawling / Total length of the basement run to sewer line is an additional 5ft)

  • The bathroom sink has a trap underneath, which empties into the tub's line in the basement

  • The Toilet (with built-in trap ties directly into sewer line 6 inches down from where the tub's line connects into the sewer line

  • The Air vent line ties directly into the sewer line 6 inches down from where the toilet ties into

Does this make sense that the air vent would be so far down the line (After the tub, sink and toilet)?

Thanks for your time! :)

P.S. Here's a few notes:

  • This house was built in 1972
  • When the house was built, it originally had a septic tank, but since the early 2000s, it has been on city sewer (Not sure of exact date)
  • The tub, sink and toilet drain without an issue (No gurgling or slow draining)

Edit: I had some time to take pictures of the actual plumbing, see below:

1) Basement Piping Layout
1) Basement Piping Layout

2) Tub - Overflow and Drain Connection

3) Tub - Piping to Basement

4) Tub - Piping in Basement

5) Sink - Piping to Basement

6) Sink - Piping in Basement

7) Tub and Sink - Connection to Sewer Line

8) Toilet and Air Vent - Connection to Sewer Line

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    You can just call it a trap. Save yourself some typing. :P – isherwood Dec 4 '17 at 3:15
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    You might mention the era of the home. It can shed light on typical practices of the day. – isherwood Dec 4 '17 at 3:17
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    In that diagram the possibility exists that the Air line / sewer line could be partially be blocked. Your sewer gases could easily follow the path of least resistance to the tub, the fact the tub has no trap means nothing blocks those gases from going back through the tub. So regardless it should be fixed. Now as for the air vent / sewer in your situation if the toilet backs up a little in your situation your tub may get the overflow as it is path of least resistance. Install the Tub Trap. – Ken Dec 4 '17 at 8:43
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    Looks like the sink has an S-trap, which is not allowed by code. Seems like whoever did the plumbing, had a bunch of extra elbows they wanted to use up. Code allows a bathroom group (sink, toilet, tub, shower), to be vented with a horizontal wet vent. So the venting for this bathroom is acceptable. – Tester101 Dec 4 '17 at 14:19

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