Did you paint an exterior wall of your ash dump? If so, it could be that the paint heated up from hot coals through the wall and that's where the smell could be coming from.

But the hot ashes would be in the ash dump after I put out the fire, right? This starting smelling as soon as I lit the wood in the fireplace. And there's no smell upstairs at all. Which wall are you asking me if I painted; I am assuming the basement one. Either way, no paint has been added, just the existing paint on the cinderblock wall in the basement.

Oh, I forgot to mention that we are experiencing a snow storm and it is snowing like mad here. I'm not sure if that's a factor but if you say that ashes heat up something, maybe there making a smell from the cold air?

If this odor occurs after the hearth has a chance to get up to temperature some investigation around the newly finished ceiling is in order. It may be that the heat transferring through the masonry is reacting with some building material used for the finish work. Ceiling tile, glues, wood fillers, foam insulation, wiring, any number of things can begin to smell when heated.

That being said, if any odor smells like TOASTING MARSHMALLOS something could be TOASTING back of the wall or in the ceiling under the hearth. Better have a good look up in that area. Any burning smell "is not so hot" for the future! Don't waste too much time finding out exactly what this is and correcting it!

Under the hearth there is no ceiling, basement ceiling is unfinished. The heart is attached to the house, so it really can't be heating anything else up in the house. I think it was the cold temperatures and snow that caused it. The house is built out of plaster so I have nothing to worry about. We've been using the fireplace all the time throughout the winter for about 10 years now.

Are you certain the smell comes from the ash dump? Is there a 2nd flue next to your main fireplace that leads to a fireplace or furnace in the basement? If so, the flue liner is probably leaking.

If the smell is definitely coming from the ash dump, here's what's going on. Your fireplace is drawing air in. Some of this air is going through the ash dump into the basement. Since you probably don't have a stick-up air freshner in the ash dump, it doesn't smell so good.

Here's a test for this. With no fire lit, blow air through the ash dump using a hairdrier. Go into basement and see if you can reproduce the smell. If so, the smell has nothing to do with the fire but is the smell of air passing through the ash dump.

Seal off the ash dump in the main fireplace using high-temp caulk. You don't really need an ash dump to clean your fireplace and they cause more problems than they solve.

sounds like cresoate to me

credesote you mean?

Try: creosote