"Ray" wrote in message
news:BuKJf.916$0z.31@trnddc01...
Recently we got some (natural) gas logs which work very well, but I can
tell that probably more than half the heat goes up the chimney.

I remember years ago when we had unvented gas heaters in every room. There
seemed to be no danger then.

My question is, is there any danger to plugging up the chimney, at least
partially, so that we get some of that lost heat in the room?

I believe the danger is not carbon monoxide, but oxygen depletion. This is
a very old house and drafts alone should be enough to prevent serious
depletion.
At risk of bringing down verbal lighting I also have (semi ventless) gas
logs in my old wood burner. What I did was to close the new adjustable
damper (about 8x2 " when fully open) to maintain a minimal draft but keep as
much heat in as possible.(Probably about 30% open) We used a U tube water
air flow gage , but comparing it to just a piece of tissue in the air flow
into the fireplace the tissue as a air flow indicator was almost as
effective. Setting damper in chimney and using instrument was a tough job.We
did this 4 years ago and re set the damper every fall. Worst problem is with
family of wasps that live in the brick chimney can't drive them out with the
heat the gas logs deliver. Any help available?
Frank