Too much draft?

[j7art2]

This may be more appropriate for the Osburn forums, but I believe that this is likely to be more of a general question draft related question than an Osburn specific problem. My Osburn 3500 is struggling to keep up the heat in these negative temperatures and I can rarely get the house more than 72 now that it's wicked cold out. It's rated for 3000sqft and my house is only 1700, so it's oversized for the house to begin with and should have no problem keeping up. I know there are a lot of varying factors with this, but I think something else is going on. It was installed professionally this spring. When it's in the 30's, I have no problem keeping the main room in the upper 70's, which allows for the rest of the house to be high 60's or 70's also.

Some weird things I've observed. My top plate has never once gotten over 437 degrees regardless of where my air control is, even when my flue temperatures are at 1000 using a probe thermometer 18" above the pipe. MC of wood is 14-16% or less on average mixed with 10% occasional softwood, so I know it's not a wood/moisture problem. I've used about 8-9 face cord since October, and this winter all things considered has been extremely mild. If I cut the air control completely when I've got relatively full load or am accidentally over firing the flue, the fire dies down, but never fully dies out, simply slows down. Dollar tested, gasket is fine. Hardwood oak loads of 6-7 splits and rounds 4-5" in size loaded N/S burns from start to coaling phase last 4-6 hours depending. Blower is virtually always running on high, which may cool the stove top some, but I doubt it would cool it anywhere close to temps I'm seeing others report on their wood stoves.

I've never once packed the stove full like some of the pics I've seen here, as I'm very concerned I'm going to overfire the flue if I do, even if I'm running the air at 30% or less. Last night I loaded up 1/3 worth of coals in the stove with 4-5 hardwood splits and I was rocking 800 degree flue temps with my air at 10% and top plate running at typical 430's. I woke up 6 hours later to coals and running propane unfortunately.

Chimney if I had to guess is about 20-25 feet straight up (two story house), double wall uninsulated until it reaches the ceiling, then double wall insulated the rest of the way up. Double wall had to be used in the house because the install was slightly too close to the wall, which may potentially complicate my fix.

The stove manual recommends not having a flue damper if I recall correctly ( i need to double check this), and it appears that Duravent doesn't make a barometric damper in double wall, so if I did get one, it would have to be a manual as it appears that's the only option I have. I'm not sure how to remedy this other than going against manufacturer suggestions if this is indeed the problem.

I guess my questions are multiple:

Does this sound like an overdrafting problem?
What's the best way to fix it?
If the damper is my only solution, since I've drilled a 1/4" hole in the telescoping pipe for my flue thermometer, in order to fit the damper, it would shorten the length of the pipe to where the hole would be inside the next section once reinstalled. Is this going to be a problem or am I gonna have to drop a few hundred bucks to replace that section of pipe?

As always, thanks for the help

 

[kennyp2339]

Time for a pipe damper bruh

 

[njk4o5]

I looked up your stove and it says 3.5cu ft firebox which should get you a solid overnight burn i would i think, thats a pretty good size stove and shouldnt have much trouble getting the 1700sqft warm or going all night providing heat. Gotta get it totally stuffed with hardwood though. Then you have to figure out how to best run the stove so it burns all that wood evenly overnight, hopefully yours isnt as hard to figure out as a vermont castings.

Putting in a key damper is cheap and easy so you could try that. Straight up piping off the stove pulls good draft. I have the same thing and i put in the damper it made a slight difference.

 

[kennyp2339]

I had 3x to 4x the recommended draft - normal .05-.08” before the damper I was rolling .18 and up to a .23 during windy days, same symptoms as the OP, high flue temps, lower stove top temp and dreaded coals. It was as if the wood was coking itself and the heat had no time to transfer to the stove top, instead it rushed up the flue.
Installed a damper and things greatly improved, especially during extreme cold or stormy days. Best is to get a damper and a manometer, adjust the damper to keep the manometer between .05 & .08 for best results

 

[kcmclellan]

I don't think you're supposed to put a barometric damper in the flue, but a key damper is most useful for taking the edge off of a hot mess heading out of control.

 

[j7art2]

I had 3x to 4x the recommended draft - normal .05-.08” before the damper I was rolling .18 and up to a .23 during windy days, same symptoms as the OP, high flue temps, lower stove top temp and dreaded coals. It was as if the wood was coking itself and the heat had no time to transfer to the stove top, instead it rushed up the flue.
Installed a damper and things greatly improved, especially during extreme cold or stormy days. Best is to get a damper and a manometer, adjust the damper to keep the manometer between .05 & .08 for best results

Excessive coaling is also a symptom on super cold days? Check that box too then I guess. Lol.

I need to double check my manual when I get home from work. I could have sworn it said I shouldn't be installing a damper, but I dont know of any way to prevent over drafting other than installing one, so this may require an email to SBI solely to see what they suggest.

Is there any other way other than installing a damper? If indeed I read correctly, I'm hoping SBI will say its okay to do, if only for my own records in the event of catastrophic event.

 

[kennyp2339]

SBI won’t recommend a key damper because it will void the epa rating, stoves are lab tested in a controlled environment for emissions, just so happens the draft is what the national average home chimney length 15ft at .05”wc, if SBI puts key damper info into literature then the average joe installer with zero discretion will install one and now the stove can function as designed and won’t meet the clean burn requirements.

 

[EbS-P]

Adding a damper damper on my SBI insert. It was a good decision

 

[begreen]

It's rated for 3000sqft and my house is only 1700, so it's oversized for the house to begin with and should have no problem keeping up.

This is a common misconception. The stove is not oversized for the house. It may work fine for a 3000 sq ft home in western WA with winter temps in the 30s and 40s, but that does not mean it will or should be expected to heat an average 3000 sq ft home with polar MI temps. The fact that it is keeping the house at 72º in these conditions is excellent.

The high flue temps relative to the stove top temps do indicate that the draft is strong. A key damper will help tame them. To clarify, are the stove top temps reported with the blower on or off? Where is the thermometer located?

 

[j7art2]

This is a common misconception. The stove is not oversized for the house. It may work fine for a 3000 sq ft home in western WA with winter temps in the 30s and 40s, but that does not mean it will or should be expected to heat an average 3000 sq ft home with polar MI temps. The fact that it is keeping the house at 72º in these conditions is excellent.

The high flue temps relative to the stove top temps do indicate that the draft is strong. A key damper will help tame them. To clarify, are the stove top temps reported with the blower on or off? Where is the thermometer located?

72° in the main room, not the rest of the house unfortunately. That's fairly low for me, as I try to ramp the house to upper 70's before bed to load and coast through the night.

This wood stove should be plenty to heat me out of house and home here. In comparison, my best friend who lives 20 minutes north of me heats his 1500sqft home with a cheap-o Pleasant Hearth medium sized unit thats 1.5 or 1.7cuft (half the size box) and his main room often gets high 80's using less than optimal wood and he's cracking windows. Theres no way in hell that thing should out perform this, especially since I use better quality wood. His house footprint is longer, 18x40', mines 24x24, two story. One of my favorite northern Alaskan Homesteading Facebook pages has an Osburn 3500 heating about 1400sqft in a much harsher climate than I, and that was one of my deciding factors in buying this stove. My attic isn't amazingly insulated, but its not terrible. R-32 or 40 rolled insulation and R-10 rigid on the interior walls of the attic. My old 1975 dinosaur basement wood furnace had no problem heating the place, but I'm not exactly comparing apples to apples.

Flue temperature is measured with Condar Fluegard probe at 18", stove top is measured with laser thermometer.

Last night at 4°, it was 950° initially, deopping to ~800° in the flue and 437° on the stove top. Wood temperature was 741°. 1/4 of the stove was hot coals, loaded three splits of ash and two oak rounds 4" in diameter. Damper was closed down to 10% air, almost choked to nothing. Blower was on and going max speed. When I woke up I was on propane (65°) and had a 1/3 stove full of glowing coals that I just decided to scoop into a bucket and just toss rather than trying to burn them up since I've got limited time in the morning before I have to go to work and just start a new fire on some left.

I spoke with the wood stove company today that installed my unit and he said that the fire won't die out completely with the damper fully closed as it has a small hole in the slider and thats simply how its designed.

He also agreed that based on symptoms, I likely have too much draft and a damper is the only fix. They don't typically install them, but do in certain circumstances, and he agreed that this unit should be preforming much better than it is. They'd charge me cost of parts plus $100 to install, which still fits in the "professional install" required by my home owners insurance.

For the heck of it, when I got home today, (fire is going all day generally and loaded throughout the day) I checked the stove top with the load in it and with blower on, 361°, flue 800°. Once I turned it off for 20 minutes, checked again and it was 391°. Fire was 50% finished though, so I didn't get the best initial test.

I'll try it again on a fresh load and report soon. I'm not sure the blower is strong enough to cool it off a few hundred degrees, but maybe.

 

[TomMcDonald]

Have you checked your stove top thermometer works properly? topping out at 437 sounds very specific and sounds to me like the thermometer is broken.

 

[begreen]

It's not the stove. There are a lot of variables between house designs, construction, insulation, sealing, and glazing even if they are just a block apart. It sounds like the house may have higher heat loss than your friend's. Addressing the heat losses will make the house easier to heat (and cool).

Definitely try the damper. That may be all that is needed. This is assuming that the wood being burned is not part of the issue. Ok usually takes a couple years to fully season after it has been split and stacked. Has the wood been tested with a moisture meter on the freshly exposed interior face once it has been resplit?

FWIW, I usually close the air down all or almost all the way with each fire. We're on a 20' flue system.

 

[j7art2]

Have you checked your stove top thermometer works properly? topping out at 437 sounds very specific and sounds to me like the thermometer is broken.

Using a laser thermometer. Definitely goes past that.


Just tossed on a fresh load shortly after last post on fresh coals. Going well now, no blower 491°, turning on blower now to see what it drops to.

 

[j7art2]

It's not the stove. There are a lot of variables between house designs, construction, insulation, sealing, and glazing even if they are just a block apart. It sounds like the house may have higher heat loss than your friend's. Addressing the heat losses will make the house easier to heat (and cool).

Definitely try the damper. That may be all that is needed. This is assuming that the wood being burned is not part of the issue. Ok usually takes a couple years to fully season after it has been split and stacked. Has the wood been tested with a moisture meter on the freshly exposed interior face once it has been resplit?

FWIW, I usually close the air down all or almost all the way with each fire. We're on a 20' flue system.

Yep, average is 12-17%. My softwood slab is marginally less, about 10-11%. Covered with tarps. I don't test every piece, but batch test every few splits per downed tree I harvest. Generally the base is most wet, but not always. Anything 20-27% goes in next years stacks, anything over 27% goes in the year after. This is my first year with this unit and new chimney, but I think it was actually you that got me going back in 2014 on my old furnace, and admittedly you and the others here taught me well.

I dont ever harvest anything living, just dead/down and live in a forest, so I normally get a little bit of a head start most of the time. A lot of the dead standing and dead down ash here is 14-17% right off the bat.

 

[j7art2]

Oops. Temps still climbing. 515° with blower going now, flue temp 900°. Air is 40%ish.

Turned blower back off to see what it gets to.

Something to note of potential relevance: yesterday was 4°. Tonight its 28°. At these temps everything works great.

Seems the colder it gets outside the colder the stove gets.

Sounds like a damper is the fix.

 

[begreen]

Seems the colder it gets outside the colder the stove gets.

Sounds like a damper is the fix

Draft will increase with the greater differential between interior and exterior temperatures.

 

[j7art2]

Draft will increase with the greater differential between interior and exterior temperatures.

Thats what I gathered looking at previous posts in the search, so thank you for confirming.

Within the next year or three I'm going to be forced into a new roof, so when I do, I'm gonna get more insulation in the attic too I think. There are likely a few other points of heat loss, but that's generally the best place to start it seems.

 

[j7art2]

Staying about the same without the blower at this time. We'll see what a damper does. Worst case scenario it does nothing and I just remove it.

 

[begreen]

Within the next year or three I'm going to be forced into a new roof, so when I do, I'm gonna get more insulation in the attic too I think. There are likely a few other points of heat loss, but that's generally the best place to start it seems.

Be sure that the chimney support box is cleaned after the roofing work is done. It they are also replacing the roof decking, a lot of sawdust and scrap can fall down into the attic space. Also, be sure the attic insulation shield is in place before the new insulation is added.

Staying about the same without the blower at this time. We'll see what a damper does. Worst case scenario it does nothing and I just remove it.

Yes, in milder weather you might end up leaving it always open.

 

[ckr74]

I'm pretty sure that stove has a top heat shield inside the stove so the temps won't be as high as a normal stove without it. My Drolet Baltic II has one and I"m just guessing that the top ls about 100 deg less than a stove without this shield.

 

[j7art2]

I'm pretty sure that stove has a top heat shield inside the stove so the temps won't be as high as a normal stove without it. My Drolet Baltic II has one and I"m just guessing that the top ls about 100 deg less than a stove without this shield.

Big ceramic solid piece on top of the secondary burners? Sure does. I figured would help with heat retention if anything!

 

[ckr74]

No, it's under the top. You can't see it unless the board over the secondary tubes are removed.

 

[begreen]

The current generation is slightly different. It has 2 layers of insulation at the top. This is to keep the firebox hotter and combustion more efficient. It shouldn't be messed with.

 

[cahaak]

I run a drolet myriad here in MN. When it's cold, I have to run it with the primary air completely closed if I want longer burn times. The secondary burn is fine. I virtually never burn with my damper more than 1/8" open. I have about 27' straight up combined stove pipe.

 

[j7art2]

I'm a little embarrassed to admit this, but I think I've found the solution to my problem and it may have simply been user error. This wood stove is new to me, and I've yet to figure out all of the nuances of it. In my struggle to find a solution and not heat with propane, I decided one night when it was -11 to simply pack my stove to the brim like I've seen others do here and run my air almost completely off. I've yet to do this at all since getting the stove, as I was concerned I'd be way over firing given my chimney heat with 5-6 cross stacked splits. My flue temps were running around 900 degrees or so, but lo and behold, at the peak of my fire (and nearest to the flue) I recorded a temperature of 581.

I also managed to get by far the longest burning fire I've ever gotten, and went to bed with the main room at 77, waking up to 71.

I'm noticing every 2-3 back to back fires, I'm still getting a fairly good amount of coaling (current wood on rack is ~15-16% MC), so if I'm not home to rake it to the front of the stove and crack the door for a few hours to burn them down, I simply shovel them into a stainless steel bucket and dispose of them in my larger steel bucket outside.

Maybe a damper is still warranted, but at least I know I can "make it" in the negative temps now.