I looked into a chimney cap that encourages more of a draft last year. I never ended up getting one cause I figured out how to use my stove

Anyway, if you do a google search, there are a few different designs out there. Most will say a cap of any sort will hinder draft, other seem to even a regular old varmint cap will help conditions. I ended up using a regular caps on my chimneys, didn't seem to change conditions either way.

I would suggest for safety sake having it inspected and/or swept. Couldn't cost much and might be cheap insurance if there it is indeed in unsafe condition.

 

MikeD, are all the flues the same height? If so then that could be the problem. I used to lay brick and we had to make each flue a different height. I can't remember what the differential was but I think it was about 8 or 10 inches. Somehow when they are even one or more of the flues may not be able to sustain a draft and the house fills with smoke. What also happens is that when they are equal in height the smoke may rise but then be sucked down one of the other flues if they are level.
The other thing it could be is if your chimney is not as tall as the roof line. I remember that we had to be 2 feet above the roofline on the chimneys here in Virginia. If not, then it was possible to not get a draft....
You can go to the masonry supply and buy a flueliner and they will probably cut it for you. then get some mortar and put that baby on there. It's amazing how something so simple can cause such big problems....