F100 Should I add chimney height?

[Nate R]

Hello all, thanks for the help here in the past!

After installing my stove and chimney last year, I'm still struggling a bit with one aspect: I can't turn down the primary air very far and still maintain flames. Maybe the best I've been able to do is 2/3 of the way down..in stages, hot stove, etc etc,.... and I read about others cutting primary air quite a bit more in their stoves. This leads to overheating inside once our place is up to temp.
After finding a clogged chimney screen late last burning season, I thought some of my issues were that. But this year, I cleaned the chimney, cleaned the chimney screen, using some kiln dried wood, verified properly with a moisture meter, and I still have the issue. I'm starting to wonder if I need more draft and thus chimney height?

Specs on the install: Jotul F100 installed last year in a 520 SF single story cabin. Running the stove with the long leg kit, rear exit into a tee, into double wall stove pipe, straight up to the chimney.
Chimney is Selkirk Supervent. I've got 8 feet of chimney which starts 8 feet off the slab floor of the cabin. So there's about 6+feet of stove pipe, and 8 feet of chimney.
Jotul's manuals mention a "14 ft minimum" chimney, but never specify if this is from the floor or from the stove exit. From the floor, I have 16 feet to the top of the pipe, or about 14 1/2 feet from the stove exit.

Here's a few pics for reference:

Thoughts? Could this be my issue, that the stove doesn't pull enough air due to the chimney length? Since I already have the chimney braced, holes in the roof for that, etc....I'm resistant to add ANY more than I NEED... Do you think adding 1 foot would make any discernable difference? Should I add a 2 foot section? Or is there something else I should be looking at?

Thanks!

 

[bholler]

What moisture content is your wood at? How full are you loading your stove?

 

[stoveliker]

Can you turn down more if you crack a window (for testing purposes)?

 

[Nate R]

What moisture content is your wood at? How full are you loading your stove?

It varies between 10-18% this year.... I've burned framing lumber scraps, sawdust bricks, air dried pine and oak, and some kiln dried hardwood mix I bought from a local supplier. This fall, I got a delivery of kiln-dried oak that I found was NOT dry on much of it, (What was in the center of the kiln bin wasn't dry at all) so I got a second delivery from another supplier of a kiln dried hardwood mix...and have been very careful to check and only use dry wood this year.
I've loaded it one tiny split at a time after getting going up to as much wood as I could jam into the tiny firebox, depending on temps outside, and whether I'm trying ot load up for a partial overnight run, etc.

Can you turn down more if you crack a window (for testing purposes)?

That's a good question, I haven't tried that..... That would increase the draft limitations from building tightness? Or what would that indicate if it helps?

 

[stoveliker]

Yes. The pic could suggest this is not an issue, but it depends also on the house wrap tightness.

Draft is pulled by the chimney, but the flow depends on the impedance all along the path the air has to go. That also means the resistance against air entering the home.

If this is a problem (if cracking a window would alleviate the issue), an outside air kit might be an option.

 

[Nate R]

The pic could suggest this is not an issue, but it depends also on the house wrap tightness.

Not sure what you mean by this? Which pic suggests that? The lack of drywall, or the decent fire, or?

I did airseal very well.... All sheathing joints taped, and airsealing with caulks, foams, etc around where framing meets the slab, window openings, etc...and the ceiling has foam under the trusses with all joints and seams taped. My windows themselves aren't the greatest sealing-wise, though.
Hmm, so a higher chimney WOULD increase draft, but you're suggesting I rule out that I am allowing enough air to get in first.... Fair point... easy enough to try with a cracked window.. And if that helps, yeah, an OAK or some other source of fresh air inlet would help.

 

[stoveliker]

Yes, I was referring to the unfinished walls in the pic. Drywall will add to the barriers for air to move in.

You did pay good attention to sealing, so the window test is an easy thing to do.

Do you have a CO and smoke detector?

 

[Nate R]

Yes, I was referring to the unfinished walls in the pic. Drywall will add to the barriers for air to move in.

You did pay good attention to sealing, so the window test is an easy thing to do.

Do you have a CO and smoke detector?

Yes, the pics aren't current, so there is some drywall in place now.., and not using that as my main air barrier..,.but to your point, more drywall will just impede any air movement more.

Yes, a combo CO and Smoke detector is in place.

I'll try the window test the next time I'm there.

 

[Hoytman]

-Some people say height is measured from the top of the stove outlet.

-Others say height is from the floor of the firebox.

-Still others might say from the top of the fire box.

 

[begreen]

What was the outside temperature when you were burning? Was the cabin already warm? The draft strength will improve with the greater differential between indoor and outdoor temps.

As stoveliker suggested, try opening a nearby window 1" to see if that makes a notable difference. If so, an OAK is a solution. Considering this is a tight cabin that might not be a bad idea regardless. Be sure it is screened to prevent varmint intrusion.

If opening the window does not help a lot after the cabin has warmed up, then yes, you could try adding more chimney pipe. An additional 2-3 ft might make a notable improvement. As a test, you could remove the chimney cap and take some cheap 6" warm air duct and stick it into the chimney as a test extension. If that make a substantial difference then yes, add some chimney. If the chimney exceeds 10' it will need another brace at the 10' level.

Another solution would be to change the stove to top vent from rear vent. It will draft better without the 90º turn right out of the flue. This would require an offset in the stovepipe or proper NFPA 211 wall shielding.

 

[Nate R]

What was the outside temperature when you were burning? Was the cabin already warm? The draft strength will improve with the greater differential between indoor and outdoor temps.

This has been when the cabin is already warm, and outside temps anywhere between 10 and 40f. I understand there won't be a lot of draft at 40 degrees, but I haven't found the ability to turn air down greatly improved with cooler temps.

I'll definitely try opening a window first and see how that goes. Given I'm on a slab, an OAK is a bit tougher to do cleanly, but as you said, some sort of air inlet to the place might be a good idea anyway.

 

[bholler]

It varies between 10-18% this year.... I've burned framing lumber scraps, sawdust bricks, air dried pine and oak, and some kiln dried hardwood mix I bought from a local supplier. This fall, I got a delivery of kiln-dried oak that I found was NOT dry on much of it, (What was in the center of the kiln bin wasn't dry at all) so I got a second delivery from another supplier of a kiln dried hardwood mix...and have been very careful to check and only use dry wood this year.
I've loaded it one tiny split at a time after getting going up to as much wood as I could jam into the tiny firebox, depending on temps outside, and whether I'm trying ot load up for a partial overnight run, etc.


That's a good question, I haven't tried that..... That would increase the draft limitations from building tightness? Or what would that indicate if it helps?

So you are loading a split at a time instead of loading the stove up lighting it and letting it burn?

 

[begreen]

This has been when the cabin is already warm, and outside temps anywhere between 10 and 40f. I understand there won't be a lot of draft at 40 degrees, but I haven't found the ability to turn air down greatly improved with cooler temps.

I'll definitely try opening a window first and see how that goes. Given I'm on a slab, an OAK is a bit tougher to do cleanly, but as you said, some sort of air inlet to the place might be a good idea anyway.

The stove could be raised on a nice platform hearth. That might add some wood storage underneath, but it would reduce the overall flue system height.

 

[Nate R]

So you are loading a split at a time instead of loading the stove up lighting it and letting it burn?

This past weekend I was doing that when it was 40+ degrees outside to try and meter the heat output....but I certainly have tried turning down the air in various loading/burning scenarios without much luck getting very far....Putting 4 splits in a hot stove (500+ STT), letting them get established and then trying to ratchet down the air, only to find I have to open it back up soon thereafter to have some flame.


As I think through this more:
The F100 EPA test report contains a letter that the "Low" heat setting (7700 BTU, or around 1.7 lbs/hr of wet weight wood burned...you don't see many stoves that go that low now!) that was achieved with the air control 1.75 inches open from full closed. What's that 1.75" for!? I feel like there's only 2.5-3 inches of total travel!.... Is it to be able to regulate the air lower on much higher draft setups? (They used a "14.5 foot" stack height for this testing)

What I'm asking is, given that data...is there really nothing wrong, and that's all I should expect to be able to turn the air lever down on this stove unless I have a 30 foot chimney?

 

[begreen]

What I'm asking is, given that data...is there really nothing wrong, and that's all I should expect to be able to turn the air lever down on this stove unless I have a 30 foot chimney?

Possibly. You have a few things to try out the next time at the cabin. Bring a 3' section of 6" warm air duct pipe with you for testing.