Question: I have a conventional fireplace- built in 1967. It has two flues- rectangular masonry within the overall brick- all outside from the basement up 1 1/2 stories (split-level ranch).The fireplace does not have a clean-out door on the outside.Both flues take a 45 or maybe even a 90 degree turn before they go straight up. The left one is for the fireplace- the right for the gas water heater & furnace.Two weekends ago- we had a fire that started with a lot of smoke coming into the house during start-up (possibly downdraft)- but then burned fine for the whole evening.The next day- the entire basement smelled like a Skunk.The smell was there for the next week. After 4 days- I called a friend of mine who is an HVAC contractor and we looked down the chimney from the roof and placed a pipe up through the damper and could see plenty of light.The fireplace flue looked very clean- it had a smoky smell at the top- but nothing like the Skunk smell in the basement. No smell on the furnace flue.Since it's a split-level- there is a finished half basement and an unfinished half-height cellar where the furnace is. The smell is not in the unfinished half- nor is it anywhere else in the house or the attic.You smell it when you come down the steps from the main level of the house- but really don't smell it inside the fireplace.Do you have any idea what it could be? We had another fire this past weekend and again got the smoke in the house during start-up. The skunky smell seems to have been reduced somewhat but there is more of a charcoal smell in the air.Will repeated hot burnings in the fireplace get rid of the smell? Could a skunk have sprayed the outside brick- and would the smell come in when hot? I also thought maybe the wood was bad- but it didn't smell when I was placing it in the fire.We bought the wood from Home Depot- wrapped in plastic- looks well seasoned- but who knows? Ossipee Mountain Firewood from New Hampshire. Same kind of stuff that they deliver to supermarkets. Two or three packages last a couple of fires.Any help would be appreciated. Answer: This is a commonplace condition called "Flow Reversal" and takes place in many chimneys. Often- these chimneys reverse when not in use (and sometimes - as you found out- when starting them).. Anyway- you are smelling creosote and tars that are formed when you burn wood. This is very powerful stuff- and even a small amount (a few grams) could stink up a house big time.The following will help: 1. Read the articles about starting a fire and tending at https://www.hearth.com/what/specific.html 2. Make sure your damper is closed when not burning a fire. 3. A chimney cap on the top may help by keeping moisture out 4. Fireplace stores sell a "Chimney Deodorant" which is a small bottle with a wick that completely masks the smell.Also- you mentioned two flues... It is possible for smoke coming out of one to go down the other...it can sometimes help to make certain they are not terminated at the same exact level.