writes:
Hot air rises unless it is opposed by a positive pressure coming
from above. The problem you are having is that there is a negative
pressure in your house. This may be cause by furnace combustion in
the house that uses inside air instead of out. It may be caused by a
bathroom vents. It may be caused by your clothes dryer ect.

There isn't an overall negative pressure in the house, because the same
model of gas fire is also installed in the living room above the den,
vented through the same chimney (but a different flue), and it always
has an updraft. In addition, the furnace and water heater have a
direct vent to the outside for combustion air. I can even open the
front door of the house, which is only a few feet away from the den,
and there is still a downdraft in that flue of the chimmney on some
days. It happens with no vent fans running, with the clothes dryer off,
and whether or not the furnace is running.

Having said that, the pressure in the basement room almost certainly is
slightly lower than that upstairs. The basement is cooler in the
winter, and the house itself acts as a sort of chimney. The real
chimney is outside the house, so the air in the vent eventually drops to
outside temperature and provides no draft of its own. There's an
explanation of this effect at

http://www.woodheat.org/chimneys/trichim.htm

in the "Put the chimney inside the house" section.

Now, *most* the time when I try to use this fire, there is
already an updraft, or the downdraft is weak enough that the hot gases
from the gase fire overpower it and the chimney develops a proper draft
within a few minutes. Sometimes a draft isn't established on the first
try, but running the fire for a few minutes, shutting it off, leaving it
for 10 minutes, and starting it again will give a proper draft on the
second try. But sometimes (maybe 10-20%) the downdraft is strong enough
that the hot gases from the fire can't overcome it.

If you recently put better siding up or installed more efficient
windows your house may be tighter and facilitate a negative
pressure more easily.

We moved in less than a year ago and this was our first winter, so we
don't know whether the fire has always done this.

The solution is to either use outside air for your fireplace or
install some sort of vent in the wall of the house to eliminate
your negative pressure. Maybe both?

There is already an outside air vent into the ceiling of the room.
There's another large vent directly through the basement wall to supply
combustion air for furnace and water heater. Even opening the main door
of the house, 10 feet away from this room and half a floor higher, does
not eliminate the downdraft on cold days. Even with plenty of access to
cold air from other outside sources, there's still a downdraft in the
chimney sometimes.

Dave