Sewer gas in brand new restaurant

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Tygon, Jan 24, 2006.

  1. Tygon

    Tygon New Member

    We just built a brand new franchise restaurant according to national specifications and hired an experienced plumber for the job. Not everything works too well (the hand washing sinks drain very slowly), but the main problem is that we have a huge amount of sewer gas in the revents and seeping into the bathrooms. This problem is only on the bathroom lines, since the sink lines go to a grease trap which effectively keeps sewer gas out of that system.

    I'm also a licensed contractor with a lot of experience with plumbing. I have looked everything over, including the revents and made sure that they are all working properly. I have been keeping the numerous floor drain p-traps filled with bleached water and have caulked around the toilets. I've done everything I can think of, and still the bathrooms smell of sewer gas, even in the morning. The gas has to be coming from the city sewer, since we don't have any reservoir that would support decomposition.

    I asked the plumbing contractor about installing some kind of p-trap to keep the gasses from the city sewer lines from entering our system, and she said nobody ever does such a thing and that it is impossible for city sewer gas to get into our lines, since the city sewage is flowing.

    That doesn't make sense to me. I think the gasses from the city system are coming up and overwhelming our vent system. Am I wrong?
  2. plumber1

    plumber1 Plumber

    From where I come from, sewer lines used to be trapped where the sewer left the building, but it's been outlawed for a long time.

    Could there be negative air pressure inside the building and drawing air from the sewer system?
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    New England
    plumber1 has a point...restaurants usually have some significant exhaust vents. If these aren't supplied with something like an air-to-air heat exchanger for make-up air, you could be running a fairly high negative pressure which could suck things in from lots of places...including from a roof vent.
  4. Tygon

    Tygon New Member

    I believe that adding a trap right on the main line where it meets the city sewer would be considered "double trapping" and would be prohibited by the municipal code; though in effect, the grease trap on the sink lines does just that! We have no gas problem on those lines.

    The negative air pressure inside the building is indeed a factor, since the building atmosphere is vented. The bathroom atmosphere is passively vented--no power vents.

    I can't figure out how the sewer gas is entering the men's bathroom. I might take up the toilet and put in a new wax ring, but I doubt that is where. I think the water in the floor traps just gets totally saturated with hydrogen sulfide and start effervescing it into the room.

    I'll go up on the roof and see if there is some way there could be gas coming back down the passive atmospheric vents. I would like to have an air-exchange system installed, eventually. But I'm still looking for a plumbing problem/solution.
    Last edited: Jan 24, 2006
  5. dubldare

    dubldare Plumber/Gasfitter

    I do not understand how the bathrooms can be ventilated without a fan. Are you sure that there is not an in-line fan that serves the bathrooms, with only grilles in the rooms themselves.

    If they are truly passively ventilated, I'd like to smoke some of the stuff the clown who designed that has.

    While you definately have a breach in the waste system, most likely a bowl wax, adding a building trap would only be putting a band-aid on a bad situation.

    If there is an atmospherically vented water heater, check for air being drawn out of the draft hood. That is a sure sign of a negative pressure condition. Using a punk or a cigarette is a good visual indicator.

    A properly functioning waste and vent system (without a building trap) will ventilate the city sewer. The sewer gas should definately not be overpowering even an undersized vent stack.

    Causes for a negative pressure condition (if it's cold where you are) are plugged furnace and make-up air filters, incorrect economiser settings, make-up air set to summer rather than winter (or turned off all together) and incorrect design.

    In my personal opinion, almost all new construction is built so 'tight' that it works on paper, but not when something as simple as a $.79 filter gets plugged.

    Rather than taking this all on yourself, why not have the companies involved in the construction find a solution for you.
  6. Tygon

    Tygon New Member

    Thanks for your replies!

    Over the phone, the plumber contractor just flat out denied any responsibility for such a problem, without even having made an inspection. I'm being fairly easy on her at the moment because that might be true and I'm trying to coax them into a friendly visit to share any ideas they might have.

    The mechanical contractor is out of state.

    I do hope for a faulty toilet ring--at least that would be the easiest fix.

    The ventilation system balance is frankly a mess, with power vents on the cookers, and makeup air coming from doors and the takeout window besides where it is supposed to be coming from. (And, yes, the Paloma hood when the hot water isn't on.)

    I haven't checked the filters or the settings, and that's one of the jobs I'm doing today. I don't even know if our system is a true air exchanger. I assumed not. But I do have the manuals here to go through.

    One thing I should mention is that I have had a charcoal filter on the 3" main revent at times due to complaints by drive-thru customers, but so far there seems to be no difference inside the men's bathroom with it on or off. I've tried both ways. I think the smell near the drive-thru was improved by the filter though.
  7. Bob NH

    Bob NH In the Trades

    New Hampshire
    Get or make a manometer

    You should eliminate the guesses about pressure differentials. You can buy a manometer at Grainger to measure the differential pressure between inside and outside. You can also make a simple U-tube manometer using Tygon tubing.

    You can determine the effect of different operating conditions and exhasut systems, and can check different locations. If the differentials are too small for a standard manometer, you can tilt it and use it as an inclined manometer.
  8. hj

    hj Moderator & Master Plumber Staff Member

    Cave Creek, Arizona

    Most of the time, when I have encountered that problem, it is sewer gas from outside the building being drawn down the bathroom's exhaust fan duct. Since the bathrooms do not have "return" air to the heating/air conditioning system, the odors pool there and accumulate to the point of being objectionable. Most restaurants have a flat roof with a perimeter wall and the sewer gases from the various vents create a pond of sewer odors above the roof, except when there is sufficient air movement to disperse them.
  9. Tygon

    Tygon New Member

    I worked on the problem almost all day and found two things:

    Under the toilet, the toilet flange and its plumbing was still in the sand box! They had tiled up to the sand and never filled in the box nor fastened the hold-down ring. The result was that the metal ring was all bent up from the overtightened toilet bolts and the wax ring was apparently leaking and soaking the sand. I replaced the toilet flange and cemented the box in and put the toilet back. That seems to have helped.

    The other problem was that the revent was very close to the bathroom air duct and at roof level. I extended the revent up a few feet above the air duct.

    This evening, there was no odor in the bathrooms!

    Thanks for your advice, everyone. I'm still learning about the air system and plan to get it all balanced.
Similar Threads: Sewer brand
Forum Title Date
Plumbing Forum, Professional & DIY Advice Sewer gas from garage floor drain Nov 24, 2014
Plumbing Forum, Professional & DIY Advice Plastic chips in sewer vent Nov 22, 2014
Plumbing Forum, Professional & DIY Advice Sewer cleanout cover right next to pine tree Nov 18, 2014
Plumbing Forum, Professional & DIY Advice Intermittent & horrible sewer smell Nov 2, 2014
Plumbing Forum, Professional & DIY Advice Long sewerline run... lift needed? Sep 30, 2014

Share This Page