Sewer back-ups blamed on backflow valve?

[Horse Heaven]

Called out at 3 AM for backed up sewer on ground floor of a two-story. This is the fourth occurrence in ten years. Third was exactly three months ago. Single occupant home. Following third occ., homeowner has flushed older 3+GPF toilet 2-3 times/week, and flushes nothing other than tissue. Minimal use of disposal. House built in 1975. 3" ABS outflow ~45' from C/O to 10" city sewer. Layout from clean-out (under stairs) is: Cleanout + 2' to shower drain tee + 2' toilet flange tee + 4' 90° ell + 8' backflow preventer + ~30' to 45° ell down 18" into 10" concrete city sewer. The house is slab on grade, and the BFP is under the slab with no access.

In August, city utilities ran a camera between manholes that bracket the tie-in, and found nothing at all, before the blockage was cleared. After the professional rooter-guy pushed 100' of power snake with a root knife three times (which should be at least 50' into the city pipe), the clog cleared. The knife got a little bent up from getting past the BFP both ways, but no sign of root. He ran a camera as far as the BFP, and was able to bump the flap open easily. The scratches from the rooter were obvious, but not significant. It swung freely and closed fully. He did not push the camera further, for obvious reasons. City's camera could only see up the 45° and nothing showed but smooth black plastic. The pro blamed the BFP and low-volume toilets.

This morning, lacking a power snake, I opted not to crawl back under the stairs, and instead pulled the toilet. I had a 40' sewer rod, and pushed it easily to the BFP. I could feel the tip hit the metal. It took a few tries to bump the flap open, most likely because I was hitting the collar with the small barb. Once the flap opened, I expected the clog to clear, but it didn't. I pushed the rod on out, and the clog cleared ten to twenty feet past the BFP. Ran copious water through and the flow was excellent. Had the expected difficulty getting the barb back through the BFP, but got it out and continued water flow while I cleaned up.

So, this setup worked fine for about 30 years. It backed up around 2008, resulting in about $20,000 of renovation and new flooring. Backed up again in 2017, but was caught before it reached carpet. Again in August of '19, and now the fourth time in November of '19. Seems to be most problematic when the plumbing is used the least. Never a problem when multiple people are home. Now, with just the one person home (last three times) it's too common. Occupant is running lots of water, doing stuff like draining the bathtub about once a month, and the toilet flushing mentioned above. This morning, we struck carpet, so have some work to do, and cost involved.

Is it the BFP? Why didn't it clear for me until I was well past it? Not enough slope? Seems to flow like white water once it's clean. The BFP and half of the sewer outflow line is under the slab, the rest is about 8' below grade. What's a guy to do about it?

 

[WorthFlorida]

The city sewer could still be the culprit. The BFP could be blocking passage back into the house when the city side of the sewer backs up leaving solids behind at the BFP or down hill a little past the BFP such as a dip in the pipe but cameras seem to dismissed any of it. It has happen where the city sewer backs up and when weight from the blocked water builts enough pressure it clears the blockage but solids are left behind on the main drain for this home. When the backup occurs again, a camera is needed first to see what it might be before trying to clear the blockage. This is a tough one.
Since the one plumber cleared a blockage well into the city sewer pipe it sound the city workers missed something.

 

[Sponsor]

 

[Reach4]

The classic flapper valves usually don't stop the backflow after they have been in a while. Those can be called normally closed. There is a "backflow valve" that is normally open. Those are much more likely to seal off an incoming sewer backup. They likely are easier to rod thru, and don't look like they would want to accumulate stuff.

They do need some altitude drop between input and output. If you do some searching, you could find discussions. I am not a plumber.

 

[Mliu]

Wow. First off, the idiot who installed the backwater valve under the slab (with no access) should be shot. I don't know what the plumbing code said back when this house was built 30 years ago, but 2015 UPC 710.6 states: "Backwater valves [...] shall be located where they will be accessible for inspection and repair [...]" etc. Furthermore, a backwater valve installed in a building drain or building sewer must be of the normally open design to allow the free circulation of air. Specifically, UPC 710.6 states: "Such backwater valves shall remain open during periods of low flows [...]" etc. Also, there should be a cleanout in the drain immediately after the backwater valve.

Three recommendations:

1. Cut open the slab and expose the backwater valve. Build a pit with a removable cover for future access to the valve.

2. Replace the existing backwater valve with a full-port normally-open valve. The "Mainline Fullport Retro" (ML-FR4) would be my choice as it requires minimal fall so less concrete breakout to install. Install a cleanout after the valve. I'd also encourage the homeowner to install a backwater valve flood alarm.

3. Outside the house where the drain exits from under the slab, excavate down and install a cleanout. If local code permits, at the top of the cleanout riser install a backwater overflow device (either a "mushroom" or a "popper"). In some jurisdictions, these are required and the city will deny liability if their sewage backs up into a house without a backwater overflow device installed. I think the mushroom style is better, but the popper is inconspicuous so more esthetic if the placement of the device will be readily visible.

Once you do all this, there should no longer be a danger of sewage backflow into the house. It may seem like a lot of work, but far less expensive than what they've paid so far due to sewage damage. Once you have all this in place, it will also be much easier to diagnose and address blockages or other sewer drain problems. Like WorthFlorida discussed, I suspect the origin of the problem is the city's sewer system. Ask the homeowner if each of these backups has coincided with heavy rains.