New house sewer backing up

[dthomas]

Hi - 1st time posting here!
I replaced ~33' of 3" house sewer ABS DWV in a friend's house back in September (inside of the house - suspended under the 1st floor). He has had repeated sewage back-up in this drain over the past three years. Once sewage exited his vent stack (~six years ago).
I installed a 1/4" pitch per foot on the new 3" drain, ran water through it for 1 hour, and it ran clear & effectively drained. My friend checked the drain about three weeks ago and there is a small quantity of sewage back up from the horizontal clean out.
A plumber drain-scoped the street-side/municipal drain two years ago and did not find any blockage/roots/etc. There is no backflow valve on the DWV.
Any insight/opinion/solutions would be greatly appreciated.
THX.
FYI: The drain drops about 3-1/2' to exit through the wall to the street DWV; the street DWV is about 3' above the basement floor. There is a Y connection from a sewage sump pit connected about 18" above the street DWV and an air cheater/cheater vent from the sump pit.

 

[Breplum]

by "cheater vent" you mean an AAV (air admittance valve) on the sewer pump. I can tell you definitively that the 2" vent needs to be thru to the roof. AAV Can't be used on a sewer ejector system.
A 1/4" slope on the main drain, supported at 4 ft. intervals rigidly as required should drain perfectly. I will leave to others to comment on the execution of horizontal wet venting.

 

[Sponsor]

 

[Reach4]

I replaced ~33' of 3" house sewer ABS DWV in a friend's house back in September (inside of the house - suspended under the 1st floor). He has had repeated sewage back-up in this drain over the past three years.

What is "this drain"?

Once sewage exited his vent stack (~six years ago).

Out onto the roof?

 

[dthomas]

What is "this drain"?


Out onto the roof?

(1) "What is "this drain" " = the 33' horizontal drain under the main floor of the bunlgalow.
(2) Yes - sewage backed up the full length of the 33' drain and spouted out of the 3" vent/stack onto the roof. He had this cleaned out and the plumber scoped the whole drain length. Apparently no blockage was found. There was a slight reverse "sag" in the original cast-iron 3" pipe, but that's what I swapped out and supposedly corrected.

 

[dthomas]

by "cheater vent" you mean an AAV (air admittance valve) on the sewer pump. I can tell you definitively that the 2" vent needs to be thru to the roof. AAV Can't be used on a sewer ejector system.
A 1/4" slope on the main drain, supported at 4 ft. intervals rigidly as required should drain perfectly. I will leave to others to comment on the execution of horizontal wet venting.

THX. I'll take this into consideration in this mystery.

 

[Reach4]

(1) "What is "this drain" " = the 33' horizontal drain under the main floor of the bunlgalow.
(2) Yes - sewage backed up the full length of the 33' drain and spouted out of the 3" vent/stack onto the roof. He had this cleaned out and the plumber scoped the whole drain length. Apparently no blockage was found. There was a slight reverse "sag" in the original cast-iron 3" pipe, but that's what I swapped out and supposedly corrected.

2A. What kept water from spewing out of the WC? Could it be that the vent that spouted water to the roof was not the 4-inch cast iron vent stack, but some other vent that was downstream of a backwater/check valve that you don't know about?

2B. Is this diagram just representing the basement plumbing? There is no shower or tub shown, so that cannot represent the whole house.

 

[dthomas]

2A. Don't have an answer for this first Q. It was the 4" cast iron vent that spouted the sewage on the roof; this is the only vent servicing the entire drain system of the home.

2b. The 3-d diagram illustrates (sorry - I didn't do well in art class ) the entire drain system of the house - it is a bungalow. The shower/tub drain is represented by the 2" T in the centre of the diagram (just after the 3" WC notation).

Again - sorry for the poor artwork & THX for your input.