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Plumbing Traps

By In Plumbing Traps On August 21, 2012

Have you noticed any foul odors in your basement that you just can’t get to the bottom of?  It might be sewer gases coming in to your home.  Every plumbing fixture needs to be equipped with a trap, which is basically a dip in a pipe that water fills up.  This water sitting in the trap is what prevents sewer gases from coming in to your home.  The photo below shows a “P-trap” – this is the type of trap you’ll find below sinks, showers, and bath tubs.

P-trap explained

Toilets have their own built in traps, and so do floor drains.  The trap on a floor drain is located below the surface of the floor – the photo below shows a floor drain as seen from the side.

Floor Drain

The problem that home inspectors often find in basements is that floor drains or other plumbing fixtures in the basement never have any water flowing to them, so the water in the trap eventually dries out and allows stinky, hazardous sewer gas to come in to the home.  Because of this, abandoned or shut off plumbing fixtures are always listed as a hazard or required repair on Truth-In-Sale of Housing evaluation reports in Minneapolis, Saint Paul, and the rest of the surrounding cities.

P-trap with no water

Floor drains are the most frequent offenders.  If a floor drain doesn’t have water flowing to it on a regular basis, the water in the trap will dry out.  A few common things that regularly drain to floor drains and help prevent the traps from drying out are AC condensate drain lines, high efficiency furnace condensate drain lines, humidifier drain lines, dehumidifier drain lines, HRV drain lines, and water softener discharge lines.  If you don’t have anything draining to your floor drain on a regular basic, the water in the trap may evaporate.

rv antifreeze
One fix is to pour some RV antifreeze in to the drain.  RV antifreeze is cheap, sold everywhere, safe for the environment, and it won’t evaporate.  It’s made just for this kind of thing.  Another option is to periodically pour some water down the drain; you’ll obviously need to do this on a regular basis, but it’s free and easy to do.

Basement toilets are another frequent offender.  These are typically found in old Minneapolis and Saint Paul homes, and it consists of a toilet sitting out in the middle of the basement, with no privacy offered.  These toilets don’t get much use, and the water in the bowl eventually dries out.

If you have an abandoned toilet in your basement, have it removed and have the opening to the sewer capped off.

Abandoned standpipes can be another source of sewer gases.  A standpipe is a stand-alone trap that typically receives the discharge water from a washing machine.  If the washing machine gets moved and is not longer discharging to the standpipe, the water will eventually evaporate.

Standpipe diagram

The fix for an abandoned standpipe is to cap it off or remove it.

Infrequently used bathrooms are the final common offender.  In larger homes with guest bathrooms that never get used, the water in the sink, toilet, or tub / shower can evaporate.

As with floor drains, the fix is to pour some RV antifreeze in to the fixtures, or remember to run some water through them every few months.  Easy.

Reuben Saltzman, Structure Tech Home Inspections


About the Author


Reuben is a second generation home inspector with a passion for his work. He grew up remodeling homes and learning about carpentry since he was old enough to hold a hammer. Reuben has worked for Structure Tech since it was purchased by Neil in 1997, and is now co-owner and President of the company.


  • Bonnie 3 YEARS AGO

    I live in an apartment project. The city flushes pipes every year around September... I smell sewer odor at that time coming up through my lower level floor heat registers (no heat on) at that time. It smells outside from the water drainage holes also. After the outdoor stink goes away, my lower level register still gives out a horrible smell for almost a month. My neighbors do not see a problem. It makes me sick, personally. How can I politely ask the housing authority to address this issue? Could it be a problem just in my middle section of a 4 apartment unit?

    • Reuben Saltzman 3 YEARS AGO

      Hi Bonnie, I don't have any experience dealing with a situation like yours.

  • Patricia 4 YEARS AGO

    Wow thank you I've been in fear if turning my ac in for fear of this mysterious water that wouldn't drain in my floor drain...I though it was clogged and was afraid it would back up if I left the ac on over night thankssss!!!

  • Klaus Reichardt 4 YEARS AGO

    Great site and glad to see it;without the common plumbing trap, humanity would not be advanced and as populous as it is. With the threat of traps drying out and sewer gases entering the building, try EverPrime, a biodegradable sealing liquid;only $2.30 a shot;cheapest service call you made in a long time;also good for HVAC traps, empty building and vacated sites.

  • Just a Security Guard 4 YEARS AGO

    You saved a multi-million dollar building with this information tonight. I can't thank you enough. Filled the abandoned bathroom toilet and sink with water using a bucket, and recommended RV Antifreeze to my bosses. That section of the building is only used once a week, but I imagine that the girls had symptoms, the odor was so severe. You deserve an award. One spark from the light switch, and this place would have gone down. You have my eternal gratitude, and will be in my thoughts often.

  • Roseville Plumbing 4 YEARS AGO

    WOW !! What a nice post and great sharing to Plumbing , cheers author for you nice site

  • Roseville plumbers 4 YEARS AGO

    Nice blog and rocking sharing to plumbers, thanks author for your nice tropic..