Q:My livingroom fireplace smells like a burnt musty smell all the time. It stinks even worse when it rains or is humid. If I have a fire in the fireplace the smell goes away. However, I get smoke coming in through my basement fireplace. - Jake
A: Hi Jake, Well you have two different issues happening with your fireplace. Lets look at them one at a time:
1) The Smelly Dormant Livingroom Fireplace
This smelly fireplace is due to your clothes dryer, bathroom vent fan, and other appliances exhausting air to the outside. This creates a negative air pressure inside the home that is causing the fireplace chimney to draw in outside air (See Diagram). When a chimney breathes in like this it drags in all of the odor associated with the cresote and ash deposits in the chimney.
Step 3: When the Chimney Balloon arrives, you inflate the Chimney Balloon into place to seal the fireplace low and tight. This will force the home to find another location to intake air. If you have an ash cleanout in this fireplace it is important to tightly seal that off as well. (See Q&A below)
2) The Smoke Coming in the Basement Fireplace
You mentioned that when you have a fire in one fireplace you tend to get smoke coming in your other dormant fireplace. This is a common issue called "smoke crossover". Your two chimneys on the roof are close to eachother, so when the one chimney is exhausting smoke from the fire, the other chimney is drawing in air to equalize the air pressure in the home. When this happens the dormant fireplace drags in smoke as well.
Step 3: When the Chimney Balloon arrives, you inflate the Chimney Balloon into place to seal the fireplace low and tight. Once it is installed that fireplace will no longer draw in outside air and smoke. The home will be forced to intake air from another location.
Click on links below to see other Q&A about stopping fireplace smells: