I noticed a sulfur smell coming from the basement a few days ago that I originally thought may be coming from the washer/dryer. Today, after taking a hot shower, I noticed a really strong smell in the basement again and it was very smoky. I noticed that the hot water heater had a substantial buildup of black smoke residue on the side of the unit.

I was very concerned about this and needless to say I turned off the boiler and hot water heater altogether.

Has anyone experienced this before and any idea what exactly caused it? Is it just a dirty filter, etc? I am calling a technician to come over tomorrow because at this point I am not trying to kill myself via explosion / CO2, but I am just curious if this scenario does indeed emit the smell I noticed.