Is my chimney vent causing my downdraft problems?

[newatthis]

I have posted before about my ongoing downdraft problem. The fireplace, and now stove (Oslo F500), just has a terrible cold draft that makes cold starts in the stove tricky (and smoky). We almost have to run the stove continuously to keep the cold air out. The chimney pipe has 1/4" insulation I just realized. Chimney pipe is at least 16' tall. Exterior chimney on a north wall in the shade.

The chimney guy is here now, and he is pointing out that the attic vent is just next to the chimney pipe, and maybe cold exhaust from the attic is just going directly down the chimney pipe. The intake for the attic is a gable vent.

He said that either covering the attic vent or setting up a metal divider in between the two holes might solve the downdraft problem. The photos will help clarify the problem. Attic vent is on the left, and stove pipe is on the right. What does Hearth.com think?

 

[stoveliker]

I think the attic is always warmer than the outside air in winter. Even for a vented attic.
So I don't think that's the cause.

A divider is good nonetheless to avoid smoke going into the attic vent instead.

And air sealing the boundary between living space and the attic will help a lot

 

[newatthis]

Stoveliker, can you clarify how to air seal the boundary between living space and attic?

 

[stoveliker]

I went in the attic, took all insulation out from between the joists, uses spray foam (cans from the box store) to seal all gaps between top plates of walls (don't forget outside walls) and use silicone caulk for any holes where wires go through top plates down to outlets etc. and around ceiling fixtures (silicone is not flammable).

Then I put the original insulation back (if good enough) and I added unfaced batts in top, 90 deg rotated. The latter step doesn't air seal but did increase my R value from the original R19 to now R57 (I added R38).

I did this before I started heating with wood. My heating bill was cut by 2/3.

See also here

Attic Air Sealing Project

www.energystar.gov

Air sealing helps because your home functions like a chimney too. Warm air rises. If it can escape up top, you create underpressure at lower floors.
Air then has to leak in to compensate (just like air has to enter your stove to replace what went up the flue).
You'll always have leaks at doors, bathroom fans etc. So the best first step is to stop the exhaust of air thru the top. Make sure your windows upstairs seal well too.

 

[stoveliker]

I didn't read any previous threads so maybe this is superfluous.

Of course any air exhausting devices (kitchen and bathroom fans) should be off during start up when you have issues like this.
Also did you try cracking a window near the stove to avoid any negative pressure at start.up?

It may be simply a matter of a cold column of air in the flue that is heavier than the air in your room. Open a path for that to flow down into the room and you're seeing what you see.
Did you try to use a propane torch aimed into the flue from r a minute or so (directly ) before lighting the stove?

 

[begreen]

Just a guess here. Is there a ceiling vent grille upstairs that connects to the attic? If yes, it will create a chimney effect in the house causing negative pressure downstairs. That's intentional, to vent heat out of the house and up through the attic. The house I grew up in had this setup. The solution is to block off the vent, at least in the winter. That used to be my job as a kid.

 

[brenndatomu]

I've never seen a "flue" used to vent the attic...is that common?!
There has to be other vents, no? I'd block that "flue/vent" off, if possible.
The attic should be drawing air in down low and venting up high, not drawing in above the peak! Does this home have gable vents?

 

[newatthis]

Just a guess here. Is there a ceiling vent grille upstairs that connects to the attic? If yes, it will create a chimney effect in the house causing negative pressure downstairs. That's intentional, to vent heat out of the house and up through the attic. The house I grew up in had this setup. The solution is to block off the vent, at least in the winter. That used to be my job as a kid.

Hmm, interesting. We have a whole house fan with a louvered closure. I've seen magnetic covers for use on the in-side of the vent. Is this what you were blocking off?

 

[newatthis]

I've never seen a "flue" used to vent the attic...is that common?!
There has to be other vents, no? I'd block that "flue/vent" off, if possible.
The attic should be drawing air in down low and venting up high, not drawing in above the peak! Does this home have gable vents?

I can say the other chimney (which doesn't downdraft, it's an interior chimney which makes it SO easy to start) has 3 separate flues- one for the wood stove/fireplace, one for the gas furnace, one for the attic vent. Yes, there are gable vents in the attic. I think of the gable vents as intake vents and the chimney outlet you see in the pics as outlet of warmer air. I thought that was "code". House was built in 1950.

 

[EatenByLimestone]

I see a cage around the chimney. Is there a cover on that normally?

 

[begreen]

Hmm, interesting. We have a whole house fan with a louvered closure. I've seen magnetic covers for use on the in-side of the vent. Is this what you were blocking off?

Yes, I used to have to go up in the attic and cover the grille from the backside with cardboard. A magnetic cover would also work as long as it fully blocks air passage.

Also, the upper chimney flue should extend 12-18" above the other flue pipe to eliminated siphoning. Homes with multiple fireplaces often experience negative house pressure issues. Negative pressure pulls smoke from the upper floor chimney into the lower floor's. When the upper-level fireplace height is increased, it prevents smoke from being drawn down adjacent flues.

 

[newatthis]

I see a cage around the chimney. Is there a cover on that normally?

Yes, it was taken off for inspection/cleaning.

 

[EatenByLimestone]

Try leaving it off. If you stop the smoke from rising out of the way, it has more time to get sucked into places you don’t want it.

 

[vatmark]

Have you tried using a propane torch to warm up the chimney before starting the fire? I do this with my oslo. Sometimes I can feel that there is a downdraft of cold air. If I do not prime the chimney the firebox fills with smoke and it starts leaking out into the room. Ever since using the torch I have not had any problems.

 

[newatthis]

Yes, I used to have to go up in the attic and cover the grille from the backside with cardboard. A magnetic cover would also work as long as it fully blocks air passage.

Also, the upper chimney flue should extend 12-18" above the other flue pipe to eliminated siphoning. Homes with multiple fireplaces often experience negative house pressure issues. Negative pressure pulls smoke from the upper floor chimney into the lower floor's. When the upper-level fireplace height is increased, it prevents smoke from being drawn down adjacent flues.

Begreen, is siphoning when the attic flue (the one on the left in the pic) sucks in smoke from the stove flue (on the right)? Our house has two separate chimneys, which are far apart. The other wood stove is on a lower floor on the other side of the house, and it draws well- but now I wonder if it is causing negative pressure on the difficult chimney.

 

[newatthis]

Have you tried using a propane torch to warm up the chimney before starting the fire? I do this with my oslo. Sometimes I can feel that there is a downdraft of cold air. If I do not prime the chimney the firebox fills with smoke and it starts leaking out into the room. Ever since using the torch I have not had any problems.

How exactly do you use the torch? We have a T connection at the back of the Oslo. When we have used the electric heater to reverse the downdraft, we put it inside the stove, but it takes awhile for the heat to transfer from the stove baffles horizontally to the back and up the pipe.

 

[stoveliker]

I mentioned the torch too, above.
I use it too. My stove pipe goes up 2 ft, then horizontal for a few feet (!) then up 27 ft or so.
I am the torch into the flue, I.e. the first 2 ft up. 30 secs is enough for me.

 

[newatthis]

I mentioned the torch too, above.
I use it too. My stove pipe goes up 2 ft, then horizontal for a few feet (!) then up 27 ft or so.
I am the torch into the flue, I.e. the first 2 ft up. 30 secs is enough for me.

You aim the torch directly onto the pipe? Does it harm the pipe?

 

[stoveliker]

No, from the stove into the pipe.
Not at the pipe from the outside.
In your case, I'd aim above the baffle towards the exit to the flue.

 

[brenndatomu]

A hair dryer aimed "up the pipe" from in the stove (as best you can...some models have a more direct access to the flue than others) works really well to jumpstart difficult cold chimneys too...just don't put in on high n aim it at the ashes...POOF!

 

[stovelark]

For cold chimney syndrome at start up, you can do the gel alcohol on your starter kindling trick, gel alcohol is fantastic, no smell, no smoke, lights gently and gets the fire started and chimney heated up quick, establishing quick draft. Have used it for years, worked well with my Oslo I had years ago. Living in a raised ranch, stove was downstairs, shorter chimney (15'), in marginal cold weather, a little extra oomph helped her get going. The Enviro stove I have now, no need to, very easy breathing stove, it'll run fine in summer if desired. Enviros, PE, Quads, all of the Pacific Northwest designed stoves tend to be free breathers. I'll add too, that is an unusual setup for venting the attic, haven't seen that before either. Good luck resolving it.

 

[brenndatomu]

gel alcohol is fantastic, no smell, no smoke, lights gently and gets the fire started and chimney heated up quick, establishing quick draft.

Sounds like a slick trick...I'd worry a bit about the CO its making, if the draft didn't take off.
I guess you don't use a ton of gel though...

 

[EatenByLimestone]

I used to pack the stove with crumpled newspaper when I had a marginal draft. It’d heat up the flue and start the draft quick! And it only cost a match.

 

[EatenByLimestone]

I have a friend who had to get rid of a couple gallons of expired hand sanitizer. Starting the campfires was real easy for a bit! Lol

 

[brenndatomu]

I used to pack the stove with crumpled newspaper when I had a marginal draft. It’d heat up the flue and start the draft quick! And it only cost a match.

I did that years ago when trying to start up the masonry fireplace after the furnace konked out at my then girlfriends house...made the whole house stink like burnt newspaper (I tried more than once too) never did take off for me...I gave up, defeated.
Years later it occurred to me that I never made sure the damper was open probably wasn't...I still feel like a dork even admitting it, all these years later