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?