Can wood be used in a fireplace if the damper over the firebox has been take out, but has been replaced with a top sealed damper at the top of a chimney. Also, why is it ok to have a ventless gas fireplace in some applications, yet other gas fireplaces have to be vented? Is it solely the kind of gas logs used that determine if a gas fireplace should be vented or not? Or are there other reasons for determining whether a gas fireplace should be vented or not?

Top dampers are fine and most times preferable. They don’t have an effect on the performance of a chimney.

Log sets are listed as ventless, vented, or both. You have to use it and install it accordance with its listing. A top damper would not work for a gas fireplace.