Using the primary air control properly

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Nate R

Member
Nov 5, 2015
81
Wisconsin
Getting a little more familiar with using my Jotul F100 now that my cabin is insulated. Newish to wood stoves overall. Thankfully I have a good amount of red oak and red pine stored and DRY, verified with a meter.

When trying to get a slower burn (Say when going to bed) to try and extend the heat production and coals, I'd just turn the air control down from 100% open to say 25% open, in one fell swoop..Often after reloading and letting the load get rolling flames. Usually after turning air down, flames would die down to just lazy secondaries, and then soon ( a few minutes) go out completely to just a "cigar smoking" type of burn. Sometimes I'd wake up with the door glass blackened, sometimes not.

Doing more reading here now, it looks like I should turn the air down in stages? Is that correct? Say I have a good fire going....turn it down to say 50% first for several minutes, and then down to 25%? What am I looking for? Should I always see flame in the firebox until the load is at coaling stage? How do I know if I've turned the air down too fast/too far?

Just looking for a bit more specific guidance at reduced output burns, getting them established at lower burn rates, but cleanly.
 

jotulf45v2

Member
Sep 22, 2021
55
CT Shoreline
With my (limited) experience, it's hard to get long consistent burns without a thermometer of some kind. If you don't have one, a flue thermometer is ideal to make sure you're burning cleanly and not wasting excess heat.

I'll close in quarter increments once the flue temp is >400F. Then when it's >450 I'll close another quarter amount. By this time the STT will have caught up with the heat in the flue and I'll let it climb to ~550 in the flue before shutting it down more if it gets there. Depending on the type of wood (hard vs soft) it'll either continue climbing or hover around this range before dropping. If it continues rising I'll close it to ~80% closed. On the hottest load, it would rise to ~750 before backing down and cruise between 550-650 for an hour or so.

You can see in the 2nd graph between 17:00-23:00 where it rises and plateaus before cooling back down. The air was only closed a little more than half and just hovered there for an hour before it started dropping. That load last night was a mix of maple, ash, and maybe hickory. 2 big splits and 3 smaller in the box.

Screen Shot 2022-01-05 at 11.46.55 AM.png
 
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begreen

Mooderator
Staff member
Nov 18, 2005
92,752
South Puget Sound, WA
Yes, there should always be some flames present until at the coaling stage. They can be lazy or just at the top under the baffle, depending on the stage of the burn. Turn down the air as much as possible, as soon as possible, without smoldering. How soon and how much depends on the stove, the wood, the outdoor temps, etc.

And yes, instrumentation is good.
 
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Nate R

Member
Nov 5, 2015
81
Wisconsin
OK, so I should turn it down in stages? Wait a bit, see if I still have flames before going further?

Yeah, got the instrumentation there....flue temp thermocouple, and a magnetic stove top thermocouple, and a Jotul magnetic STT. Which reads about 100 deg higher at high temps than the thermocouple... J
 

stoveliker

Minister of Fire
Nov 17, 2019
4,353
Long Island NY
Your secondary air is what's cleaning up what goes into your flue. So unless you're really at coaling stage, you should always have some flame - otherwise what is smoldering goes right up your flue (the way your window looks might be the way your flue looks in that case).

(Secondary) Flame is your friend in making heat, being efficient, and being clean.
 
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begreen

Mooderator
Staff member
Nov 18, 2005
92,752
South Puget Sound, WA
OK, so I should turn it down in stages? Wait a bit, see if I still have flames before going further?
Yes, you can also use the flue temps for guidance.
 

Nate R

Member
Nov 5, 2015
81
Wisconsin
Your secondary air is what's cleaning up what goes into your flue. So unless you're really at coaling stage, you should always have some flame - otherwise what is smoldering goes right up your flue (the way your window looks might be the way your flue looks in that case).

(Secondary) Flame is your friend in making heat, being efficient, and being clean.
That makes a lot of sense to me....hadn't thought of it that way....if the seconary is burning the smoke...there should be flame. DEFINITELY have been times with no flame that I wasn't anywhere close to coaling stage yet. I'll keep an eye on flue temps and flames and ratchet the air down in stages, and see how that goes. Hopefully no more black glass in the morning, then. ;em
 
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firefighterjake

Minister of Fire
Jul 22, 2008
19,592
Unity/Bangor, Maine
I've found that relying on the flue temp provides me with a lot of guidance.

Once the flue temp (and stove temp for the record) are up to temp (I forget the exact figure -- above the "Below this temp will make creosote" and well below the "Above this temp you can set creosote on fire" setting) I will shut down the air in stages. Move the air control lever a quarter mark at a time . . . wait 5-10 minutes and make sure I have good secondaries and then shut down it a bit further.

I will confess however that after over 13 years of becoming intimately familiar with my stove and my fuel source I now routinely cheat . . . once the temps are up I can often shut down to the quarter mark or sometimes even completely "closed" with no ill effects . . . although I still wait 5-15 minutes to make sure I have a sustained secondary flame.
 
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