[vaughnteamster]

Sewer gas small from upstairs sink when the fireplace is burning downstaires

[speedball1]

Simple! Smells raise with heat and sink with cold. Let's get the smell out of your lavatory. Most bad smells from a lavatory come from hair built up on the plunger rod and decaying down there. Every time you open the stopper to drain the lavatory you get a nose full of stinky.
The first thing I would look at is the stopper. Pull it and you will find a rod about 6" down in the drain. Shine a light down there and if you see a buildup of hair fish it out. If the stopper's tied down let me tell you how to free it up. Push down on the plumger so that the stopper pops up. Underneath the lavatory ahead of the clip you will see a knurled nut,(Leave the clip on)Now back the nut out until it's free but don't pull it all the way out. Now while you are pulling up on the stopper GENTLY pull the rod back until the stopper clears, stop! Now screw the nut back in place and just snug it up with a pair of pliers. When you clear the drain just drop the stopper back in. Some plumbers lock the stopper in when they install the fixture. I never have for it makes it impossible for the home owner to clean the drain. After you clean the hair pour a 1/2 gallon of bleach down the drain and let it set overnight without using it. Next morning flush it down with a pan full of boiling water.( This is important! ) That should take care of your smelly problem. Hope this helps and thank you for rating my reply. Tom