Hello all,

First, I'd like to say that I am now working with a professional plumber and am willing to pay to solve this problem, but I could really use your thoughts and or suggestions.

Infrastructure:
City sewer
City water
Natural gas forced air furnance
Natural gas hot water heater

Problem Description:
In the evenings, I have been fighting a sour smell in my basement for the last three winters, which I presume to be some type of sewer gas. The strange part about this smell is that it is only present under the following conditions:

1) Cold night, Mid January to Mid-to-late March. 20 degrees F or colder
2) Night time, starting around 7PM and it is usually gone by morning
3) It is strongest is the basement, and sometimes permeates to the main floor as I have an open stairwell from the basement to the main floor
4) Rarely, when it is really bad, the smell will permeate to the main floor bathroom (through the walls?)

I have not been able to identify the source, and here is what I have done thus far to troubleshoot it.

1) Both floor drains have water in them
2) Basement sink drain p-trap has water in it
3) Basement toilet flushed regularly
4) P-trap for forced-air furnace has water in it
5) P-trap and clean-out cap for forced-air furnace is "tight/sealed"
6) Clean-out caps for all visible drains / etc are sealed
7) Roof vent is clean and not frozen
8) Professional plumber visited during the day when no smell was present with gas meter thing, was not able to identify source
9) Tried turning gas hot water heater off for a night, including turning off the source valve
10) Tried turning gas forced air furnace off for a separate night
11) Tried keeping the basement warm by opening all ducting vents
12) Tried keeping the basement cold by closing all ducting vents
13) All sinks / toilets / showers / tubs / washing machine / etc run regularly to ensure there are no dry traps

I am asking the plumber to visit during the evening when the smell is present, and hope to have this done this week.

At this point, I am willing to try anything and everything.

Please share your ideas!

Thank you.

You mention turning gas equipment off for a night. No effect on odor?

Correct, turning each off individually for their own night had no effect on order.

Have you considered a powered exhaust fan on top of your vent pipes? Have you considered barometric pressures ?
Best Wishes

Hello,

No I have not considered a powered exhaust, as I have not heard of them before, and I have not considered barometric pressures either.

If barometric pressure is playing a role, how would I go about solving the problem?

Thank you.



Edited 1 times.

Perhaps it's a nocturnal rodent, possum or skunk that doesn't like 20° or lower temps? It may not be sewer gas.

Double check all of your traps to be certain that water seal is not broken by siphonage when a toilet or something else is fllushed causing an inadequate vent to cause siphonage and therefore the odor. Just a thought. Do you have any source of fresh air coming into your heating system for your home? Also check your filters for your forced air heating if you have it. If clogged, you have an air circulating problem for your home. On the trap for your AC/Heating trap, make sure the drain is not clogged with that snot like goo that is caused by AC. Be sure the pipe is clear by flushing bleach water carefully down the drain with bulb like they used to fill car batteries water with. Good luck. Let us know what happens.

Determine the barometric pressure when the smell is at its worse. Powered vent are prototyped with each application. In the end they are forcefully sucking the air up n out. You will know right away if it is working....
Best Wishes

Update: The main floor washing machine drain did not have a p-trap on it, we found that out by issuing a smoke test, and the entire laundry system filled with smoke. We cut away the drywall to verify there was no p-trap and added a p-trap.

The side-plug (cleanout plug) in the basement floor drain near the furnace was leaking smoke. We replaced it with a galvanized plug with teflon and paste, and then siliconed around it.

Unfortunately the weather is getting warmer so I'm unsure if I'll be able to verify it's fixed until next January. We will see.