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 furnace
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.