Smoke gets in your....basement?

[Waccused]

I have a split-level house built in the late 50s, with 2 fireplaces - one on the main floor and one in the basement. They have separate chimneys, which are about 16 inches apart. When we first tried the main floor fireplace, after about 30 minutes it got smoky in the room. By pure chance I happened to go into the basement where it was MUCH smokier. We thought that the problem was caused by smoke exiting one chimney being drawn into the other due to negative pressure. We had our chimney sweep install a cap (that opens and closes with a chain) on the top of chimney connected to the BASEMENT fireplace. Nope........problem is still there. We've tried opening windows downstairs, still have the same problem. I just discovered that there is an ash trap from the main floor fireplace that leads into the basement AND that it has no cover (on the main floor).
Could this be how the smoke is getting to the basement? Can I seal the ash trap? If yes, how? Or any other ideas as to what the problem is?

 

[cmonSTART]

So just to be clear, you're still getting smoke in the basement with the basement flue top damper closed?

Have you had either flue video scanned?

 

[Waccused]

That's correct. I actually went up on the roof figuring that if the smoke was somehow still getting in with the cap closed I'd hear a whistling or feel the suction, but didn't.

No we haven't had either flue video scanned.

 

[Todd]

When the fire is out take a match and light it next to the ash dump and see if the flame draws in. The smoke is probably getting drawn down and out your basement ash dump door. You may be able to purchase a new cover at stove shop or just place a piece of sheet metal over it.

 

[humpin iron]

get the chimney scanned

 

[sksmass]

Do like Todd suggests, try an experiment with the fireplace ash dump hole. If that is your problem just cover it with an old license plate or something.