The fireplace chimney as it wasn't introduced before the 13th century and fire smoke is a major cause of lung ailments.
But come to think of it, you could also make tons of money. An ordinary educated person today knows way more about medicine than medieval doctors did. So your optimal career goal should probably be to become a court physician. You could become as rich as minor nobility doing this, and (most importantly) without having to take sides in power struggles.
Unfortunately, another commentor recognized that firewood would be difficult to access, and cow poo doesn't fire hot enough.
Perish the thought that anyone might think either of those more important than making money, of course...
Still, there's an argument to be made that people can be better convinced of a need if there's something they think they have to buy.