This is a common problem with basement fireplaces/stoves. I have an
air-tight wood burning stove in my basement family room and it does the same
thing. If I open the door, I can feel a strong draft of cold air falling
down the chimney and it gets worse on damp rainy days, less so on clear cold
days. I have often lit the stove only to find that I fill the house with
smoke.

I have learned to open a close by basement window slightly and this seems to
stop or at least minimize the downdraft enough to light the stove. Once it
is running it can maintain its own draft with the window closed. I also make
sure that any exhaust fans for the kitchen and bathrooms are off to
eliminate any mechanically produced negative air pressure that can add and
worsen the downdraft effect.

"Dave Martindale" wrote in message
...
Our house has two identical gas heaters that are vented to two flues in
the same chimney. One of them *always* has a positive draft and we
never have any trouble lighting it. The other often has a downdraft,
and this makes it difficult or impossible to establish proper venting
when we light it. Yet the two installations are nearly identical, and
I can't figure out what the difference is. A few hours of searching
and reading haven't revealed a likely culprit.

Some background: The house was built about 17 years ago (on an older
foundation; the original house burned down). It has an outside chimney
near one corner of the house, which was designed to have two
wood-burning fireplaces, one in the living room upstairs and one in the
rec room in the basement. Each room has a combustion air supply vent
to bring air from outside the house. The vents are in the floor of the
upstairs room and the ceiling of the basement room.

At some point, a previous owner decided to convert the wood fireplaces
to gas fires. The gas fires are Valor "Homeflame Super" units. They
are not fireplace inserts; they sit on the hearth in front of the old
fireplace. The fireplace opening has been partially closed off with a
metal plate, and the old fireplace chamber is now empty except for the
gas and vent plumbing. The Valor units are vented through 4 inch
flexible aluminum duct run inside the original fireplace flues, so
there will be some dead air insulation around the duct. There is a
draft diverter built into the Valor units.

The Valor units have heat settings that range from 5000 to 20000 BTU/hr
gas input, and they are designed to deliver most of the heat from
combustion into the room, not send it up the chimney. The flame first
heats some fake logs which radiate heat through the glass front of the
combustion chamber. Then the gasses pass through a heat exchanger
which heats room air flowing through the unit by convection. So the
flue gasses that exit from the back are probably fairly cool.

Now, the vent for the upstairs gas fire works perfectly. Even when the
fire is off, a candle held near the draft diverter vents on the side of
the Valor units shows that the chimney has a small draft upwards. And
when the gas fire is lit, the chimney always handles the flue gases
with no spill into the room.

But the vent for the downstairs fire often has a downdraft. With the
fire off, I can feel cold air coming out the sides of the draft
diverter, and a candle flame confirms the outward flow. There's
sometimes enough downdraft to blow out a small birthday candle. When I
light the gas fire with a downdraft present, the draft diverter spills
the combustion gases out the vents in the side of the unit into the
room, and they don't go up the chimney.

Sometimes, if the initial downdraft was mild, warm gasses will find
their way through the draft diverter to the vent and start flowing up
it. Then the chimney starts warming up and is soon drawing properly.
Once that happens, it will accept the output of the fire on "high",
plus display a positive airflow from the room into the draft diverter.
So once the chimney establishes a positive draft, it works fine.

Other times, there will be a downdraft during the initial light-up, and
the Valor unit will eventually shut itself down (after about 5 minutes)
because of this. If I leave it off for 10 minutes, sometimes the warm
gases will find their way to the vent and start it drawing, and a
second attempt to light the fire works fine. But sometimes, the
downdraft seems to be strong enough to prevent the warm combustion
gases from ever getting to the vent, so it never warms up and continues
to have a downdraft. (I wish the draft diverter could be disabled for
a few minutes, to force combustion gases to go up the vent to warm it.)

And sometimes the vent doesn't have a downdraft at all. On these
occasions, the venting works fine on the first try.

The vent for the basement gas fire isn't blocked anywhere I can see.
The vent cap on the roof is undamaged and unobstructed. I've removed
the cap, and the vent duct is clean and unobstructed for as far as I
could see down it (about 10 feet).

The problem can't be depressurization of the downstairs room. It
happens whether the furnace is running or not (and the furnace closet
has its own combusion air inlet). It happens even if the front door of
the house is open to supply cold air (the door is only about 15 feet
away from the basement room).

In summary: I have two vent pipes in adjacent chimney flues. One (the
shorter one) always has at least a small updraft, while the other
taller one often has a downdraft. Sometimes the downdraft seems to be
sufficient to prevent any warm combustion gasses from getting to the
vent at all, so it never warms up and never starts drawing. Without
proper venting, the safety equipment on the fire shuts it down in a few
minutes.

So: any idea why the one vent has a downdraft? How can I establish an
updraft for long enough to heat up the vent and have a natural updraft
take over?

Dave