Perplexing chimney downdraft problem


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
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Dave Martindale wrote:

I am going to take a SWAG. Could it be due to the difference in the upper level vent having access to warmer lighter air and the basement vent cooler heavier air?
--
Joseph Meehan

Dia \'s Muire duit
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Joseph Meehan wrote:

Agree with this line of reasoning. I think this could account for the problem during warm or moderate outside temps. Then you have basement air which is cold competing with outside air, or intially air in the chase, which could result in little temp difference. The air from the upstairs heated space is warmer and also has less distance to go. In the summer when it;s hot out, I've seen this problem with chimney entrance points in the basement. They exhibit very poor draft on startup. Is the problem less when it's very cold outside?
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snipped-for-privacy@optonline.net writes:

I'll have to pay more attention when it's below freezing outside again.
Ironically, as the outside temperature drops, the furnace runs more often, which tends to keep the basement warmer (the living room temperature remains the same, since the thermostat is adjacent to it). So the desire to warm up the basement room comes mostly when it's cool but not really cold outside.
    Dave
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Joseph Meehan wrote:

My downstairs fireplace is in family room next to furnace room. Furnace needing air can cause drawback and I need to keep door to stairs open when burning fire. Otherwise, even when fireplace is heated, smoke can come out when furnace comes on and I open glass screen to add wood.
Frank
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I don't think that's my problem. The furnace is located in a closet of sorts that has a direct vent through the wall to the outdoors, at least 6 inches in diameter, so it should have lots of combustion air without depressurizing the house. And the basement room has a hot air register but no return air register, so the furnace should pressurize the room slightly if it does anything.
    Dave
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Yes, that could be a large part of it. The main floor is heated to 20 degrees C (68 F), but the basement is often around 17 degrees C (63 F). The outside temperature is usually above freezing.
    Dave
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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.

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To add to thread, I just got through with an inspection of my fireplace this morning. My chimney takes a 90 deg bend and he was recommending a draw fan on the roof to assure draft and keep down cresote buildup. This would certainly solve the op and others' problems with down draft.
We decided to ditch the whole thing and have an electric insert installed. My wife never liked the wood smell from the downstairs unit.
Frank
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Yeah, in my case it would likely only need to run for a minute or two in order to start the chimney drawing hot gases from the fire. Once the vent gets warmed up, the natural draft is fine.
I suppose I could mount a small compressed-air jet in the entrance to the vent. That would probably get the draft going in the right direction for long enough to start warming the vent. Unfortunately, I don't have compressed air on tap.

We've lived in houses with proper wood fireplaces and liked them. But I'm happy with the Valor gas units. Except for the downdraft problem, you just turn them on and push a button to ignite the flame, and they operate unattended indefinitely. No splitting wood, no cleaning up the ash afterward. They're pretty to look at (the main flame burns blue as you'd expect from natural gas, but there are also decorative flames set rich so you get nice yellow flames up through the fake logs). And they heat the room quite well because of the built-in heat exchanger.
I also like the fact that I can heat a few rooms of the house even when electricity is out. We've had a few windstorms lately that have disrupted power for some customers for days at a time. The Valor units need nothing more than a natural gas supply to work: the igniter uses a piezo sparker with a button, the safety systems work off the millivolts of electricity produced by the thermocouple, the heat exhanger uses convection (no fan) and the flue draft is natural.
    Dave
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writes:

I can remember people lighting a piece of paper and holding it up in the fireplace to start a draft. You might be able to do something similar to warm the chimney. If it has to be automatic it could be some sort of electrical pre-heater. Even a pilot light might solve the problem.
Don Young
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Dave Martindale wrote:

If I had a natural gas line, that is the way I would have gone. As far as eliminating a heat source when power is out, I have a back-up generator which keeps oil furnace on line. The incident that finally precipitated me getting a generator was a cold day's power outage where house was down to 40 deg. F in spite of having both fireplaces burning.
The fan on the roof solution is pricey - over $1.000 - and I have good draft too. I always started a wood fire by burning paper first. Maybe you can get yours started by using a separate propane torch first.
Frank
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Unfortunately, it's quite difficult to access the bottom end of the vent. It's about 6 inches below the top and a foot in from either side of the gas fire unit, attached to the draft diverter on the rear surface of the unit. But the gas fire is screwed to the metal cover that partially blocks the old fireplace opening, and the cover is permanently attached to the inside of the old fireplace.
I can *see* the vent duct through the diverter vent openings on the side of the unit, but I can't get anything larger than about the size of a pencil in there without somehow modifying the gas fire.
A thin tube with a right-angle bend at the end might fit in there, but it would have to be custom-made.
Interestingly, for the couple of evenings since I wrote my original posting, the fire has lit up fine with no downdraft problems. It's been warmer these last few days. That suggests it isn't a problem of inside-outside temperature difference (because that's less now), and more likely to be related to wind direction or the air in the vent getting cold and dense on cold days.
    Dave
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Dave Martindale wrote:

The problem is that cold air is heavier than warm air, and the stove is in the basement. The cold outside air is trying to siphon in through the flue and fill up your basement, displacing the warm air. I have the same problem with a basement wood-burning stove.
I buy E85 (gasoline and ethanol blend that's mostly ethanol) and use it as lighter fluid to get the fire going. It burns *much* cleaner than kerosene or other petroleum fuel, and it doesn't flash nearly as bad as gasoline. Even this is often not enough and I have to open a basement window just until I get the updraft going.
Best regards, Bob
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I've also read several times about techniques for building fast-burning torches from newspaper to get the draft started. Those probably work well in a fireplace, where the fire is right under the flue. But this is a permanently-installed natural gas heater, with a metal flue duct attached at an inaccessible place behind the heater (inside the original fireplace space). There's no easy way to get something burning back in there, right at the entrance to the duct, downstream of the draft diverter.
    Dave
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