Efficiency Tradeoffs of burning w/ max primary air

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7acres

Minister of Fire
Dec 5, 2013
653
South East USA
So if you close off the primary air to, say, only 1/3rd open you will achieve a longer burn time on a given load of wood. You will also be engaging the secondary burn fully too. This style of burning seems to promote maximum efficiency as well.

But... if you want to dump heat into your home quickly do you get the same efficiency (total BTUs given off), albeit shorter burn duration, from the same load of wood if you run primary air 100% open?

Either way the secondaries are engaged (giving you that added efficiency). So in my mind it seems the total BTUs released would be the same regardless of where the primary air lever is left for the duration of the burn. Am I missing anything here?
 
By leaving the air control open efficiency is greatly decreased. Incomplete combustion means more smoke and more heat will head up the flue. Flue temps can also get dangerously high. Closing down the air just 1/3 is a compromise, but our flue would still get too hot at that setting. As long as the wood is fully seasoned close the air down as far as possible while maintaining flame for the hottest stove top and lower amount of heat heading up the chimney.
 
Okay, so the balance is controlling what absorbs the BTUs. Your stove enclosure or the great outdoors. If the fire is rip roaring the heat gets sucked up the flue and your stove doesn't absorb it to radiate it back out.

If your flames are lazy the rate of reaction is slower meaning the heat generated has more time to get absorbed into your cast iron/steel. Is that the idea?
 
Yes, and combustion is more complete. You want the firebox to get hot. If I left the air wide open for more than a few minutes past when the fire is burning strong the stove pipe can get to over 1200ºF. That is beyond it's design range. Don't ask how I know this. ;em
 
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Over 1200F? Wow!. Was that the temp on the stove pipe's outside wall or inner wall?
 
Inside, that was probe temp. my spaceout. Fortunately it was brief and it cooled down quickly when the air was turned down. I will probably put a digital alarm on the stove pipe. It's too easy to get interrupted by a phone call, etc.. during startup.
 
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If I left the air wide open for more than a few minutes past when the fire is burning strong the stove pipe can get to over 1200ºF. That is beyond it's design range.
You can get that with the air controls fully closed also. Too many small splits that are too dry. Don't ask how I know that either!!!
As I have posted a few times, I doubt people have any idea how dangerously hot their flue pipe is unless they have a digital flue thermometer probe.

I have found that even large splits that are very dry (12%) can lead to overfires if you have a strong draft.

To get back to the question, IMHO your air control settings are very much dependent upon how strong the draft is in your system at a particular time. I installed a damper on my 8" double wall to help regulate the draft. Small changes will make a big difference in efficiency/flue temps.

If you really want to see how dramatic this is, get a digital flue probe and it will change your assumptions.
 
By leaving the air control open efficiency is greatly decreased. Incomplete combustion means more smoke and more heat will head up the flue. Flue temps can also get dangerously high. Closing down the air just 1/3 is a compromise, but our flue would still get too hot at that setting. As long as the wood is fully seasoned close the air down as far as possible while maintaining flame for the hottest stove top and lower amount of heat heading up the chimney.

I have not found this to be true and I actually run a noncat at high output as the OP is suggesting to "dump heat into your home quickly". Running air at almost closed is how you minimize the stove temperature and extend burntimes, I can't imagine why you would think that this provides the hottest stove top. Maybe I misunderstood your statement? I would agree that peak efficiency of a non-cat is at the lowest possible air setting that provides a clean burn.

To the OP: Efficiency of a non-cat at high output is poor. This is because to get high output you need a high burn rate and to get a high burn rate you are pushing lots of air through the stove with a raging fire. Lots of flow through the stove and very hot exhaust means a lot of heat is escaping up the flue instead of entering the room. This means low efficiency and possibly an overheated flue system. Now, if like wood furnaces do, you are able to scrub heat from the flue gasses then you can recapture some of that heat lost up the flue but doing too much of this can spoil the draft and overcool the flue gasses.

Some installations require 1/3 primary air just to burn clean and some burn clean with the air fully closed. This is mostly due to draft but could also be due to fuel characteristics. If I fully shut the draft on my NC30, with a 19 foot stack, it will snuff and smoke.
 
We have a 20ft stack and can close the air all the way off once the fire is burning briskly. The difference could be that our stove has a double-wall connector and insulated chimney starts much sooner. A long run of single-wall cools the flue gases and slows draft.
 
We have a 20ft stack and can close the air all the way off once the fire is burning briskly. The difference could be that our stove has a double-wall connector and insulated chimney starts much sooner. A long run of single-wall cools the flue gases and slows draft.

Yeah, I could do that too with my hearthstone and the 12 foot chimney above that. The OP didn't ask about the lowest possible burn, he asked about the high output scenario.
 
As noted, we get the most efficient burn and highest stove top temps by turning down the air all the way (once burning well). Could your experience be anomalous?
 
As noted, we get the most efficient burn and highest stove top temps by turning down the air all the way (once burning well). Could your experience be anomalous?

So you've got one of those stoves that when you turn it down it gets hotter? Could your experience be anomalous? The instructions for your stove likely indicate higher intake air settings for higher output.
 
I couldn't imagine running with the air wide open even after a cold startup. I'm usually closing my air down at least 50% withing 15-20 minutes of starting a fire. Within half an hour my stove top is usually over 500 with the flue around 550 and the air 75% closed. If I leave my air open my stove sounds like a freight train.
 
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So you've got one of those stoves that when you turn it down it gets hotter? Could your experience be anomalous? The instructions for your stove likely indicate higher intake air settings for higher output.

That kind of threw me too. If I leave the primary air wide open my stove top temps can get 800+. With primary air closed down it would never get that hot.

My understanding has been that closing down the primary air gives you a more complete combustion combined with prolonging overall burn time. Leaving primary air open will stoke your fire to the max but that reaction won't be happening in the secondaries.
 
I understand stoves differ. Here are the instructions from the Hearthstone Clydesdale:

Burn Rate
There are four basic burn rates: high burn, medium burn, low burn, and overnight burn. Each has its own purpose. High Burn Rate: We recommend a high burn rate once or twice a day to fully heat the stove and chimney; this also helps keep the amount of creosote in your stove and chimney to a minimum. After the fire is established and both the stove and chimney are hot, create a high burn for maximum heat output. Pull the primary air control out so that it’s fully open or almost fully open. Fully load the firebox with wood on a bed of hot coals or on an actively flaming fire. After you have attained a high burn rate, monitor the stove temperatures frequently to ensure that the stove is not over-fired. The surface temperature on top front of the insert (firebox body under grill) should not routinely exceed 600° Fahrenheit (316° Celsius). Once you understand how your insert burns and heats in your particular setting, you can easily create a high burn rate without risk to the stove or chimney. When your room’s temperature is comfortable, maintain your fire by loading smaller quantities of wood. Burning smaller amounts of wood at a high burn rate creates the least amount of creosote in your chimney, emits the lowest amount of pollutants, but provides shorter burn times. Medium to Medium Low Burn Rate: In general, you would usually create a medium to medium low burn rate, especially if your stove is left unattended periodically. Set the primary air control to a mid-range setting, appropriate to maintain a comfortable temperature in the heated area. This setting produces the most efficient overall burn. Low Burn Rate: Close the primary air control for a low burn rate. This setting provides the longest burn time. However, set a low burn rate only periodically. Over extended periods, a low burn rate promotes the accumulation of creosote. If you consistently maintain low burn rates, inspect your venting system frequently. Overnight Burn Rate: An overnight burn, as you might expect, allows you to keep an area heated while you sleep. To create an overnight burn, first ensure the stove and chimney is hot from an established fire. Next, pull the primary air control fully open, and then completely load the firebox with wood. Remember, larger pieces of wood burn longer. Allow the fire to burn intensely for 20 to 30 minutes. Finally, push the primary air control close to its lowest setting; the fire will settle into a lower burn rate with a small flame and more secondary gas ignition. The fire will now burn slowly and steadily. The longevity and intensity of the fire depends on the type, size and quality of the wood, the strength of the chimney draft, and other variables that vary from installation to installation. High draft installations or situations may require using a low burn rate setting. In the morning, you should find a bed of hot coals buried within the ashes. The insert should be warm to the touch but not hot. To restart the fire without relighting, simply stir and rake the ashes with a poker until the hot coals have come to the surface. Place a handful of kindling and a few small logs on the coals, close the door, and fully open the primary air control. The fire should re-ignite within 5 to 10 minutes. Once the fire is burning well, add a few larger logs, close the door, leave the primary air control fully open, and allow the fresh logs to ignite. Running a high burn rate at least once a day burns off accumulated creosote from within the insert and Hearthstone Clydesdale Fireplace Insert Model 8491 Page 25 of 33 venting system. Now, in the morning after an extended low burn rate, is a good time to create your daily high burn rate; run it for about 20 minutes or so. Not only does this hot fire promote a clean stove and chimney, it also helps keep the glass cleaner for easy viewing of the fire within. Once the fire is burning briskly, regulate the primary air control to a medium setting for a medium, to medium low burn rate.
 
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I don't think one should try this without instrumentation on the flue. If we did a wide open air fire in our stove with a full load of wood, our flue system would be in meltdown mode.
 
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flame from the tubes is the hottest flame. Flame coming of the wood is not as hot. The more dry your wood is the more you can shut down the primary and burn on secondaries. Less dry wood requires more air because you have to use more of that energy to dry the wood rather than use the energy to fully consume the wood and convert it to heat in the box.



So if you close off the primary air to, say, only 1/3rd open you will achieve a longer burn time on a given load of wood. You will also be engaging the secondary burn fully too. This style of burning seems to promote maximum efficiency as well.

But... if you want to dump heat into your home quickly do you get the same efficiency (total BTUs given off), albeit shorter burn duration, from the same load of wood if you run primary air 100% open?

Either way the secondaries are engaged (giving you that added efficiency). So in my mind it seems the total BTUs released would be the same regardless of where the primary air lever is left for the duration of the burn. Am I missing anything here?
 
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flame from the tubes is the hottest flame. Flame coming of the wood is not as hot. The more dry your wood is the more you can shut down the primary and burn on secondaries. Less dry wood requires more air because you have to use more of that energy to dry the wood rather than use the energy to fully consume the wood and convert it to heat in the box.

This year we have only one year seasoned oak mixed with 1yr sweet gum. We're wasting a lot to boil off the water as it burns. Next year and all forward should be more typical. But with the fire ripping for extended durations I can go upstairs and put my bare hands on the stainless pipe and leave them there.

When I disassemble the stove pipe to clean it I'll give it a good inspection. If something failed what would it be? Pop rivets? Braized seam?
 
flame from the tubes is the hottest flame. Flame coming of the wood is not as hot.
It seems a bit different with a cat stove. When I have some flame in the stove, the sides get a bit hotter. Even though the cat may not burn quite as hot due to the flame eating more smoke, I think there's a bit more total heat coming off. I would have to study it closer to make a definitive statement...usually don't burn with a lot of flame in the box.
 
As far as I can tell, with my set up there is no trade off. It's just a loss if I were to leave the air intake wide open. I basically fully shut down every load. Even if the wood load is less seasoned than I had hoped or intended, I just wait longer before shutting it all the way down. Shutting the intake down always increases the stove top temps for me. So it's a a total win for me to shut down. Higher stove top temps, longer burn times and less wood burned.
 
So for me, once the fire is established I close down the primary air control and leave the secondary air control fully open for an efficient hot burn. This seems to be inline with the guidance from Jotul for burning wood:

AIR!
The wood is utilised in the best way when the draught control is fully open and the flames are intense. That will also reduce pollution, because gas particles are combusted and produce heat instead. Once your home is warm, the temperature is regulated by the amount of wood, not the air control.
 
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Blowndiffuser, what Jotul stove do you have and how large is the area being heated? The Jotul stoves I have owned do not have a secondary air control, just one primary air control.
 
So for me, once the fire is established I close down the primary air control and leave the secondary air control fully open for an efficient hot burn. This seems to be inline with the guidance from Jotul for burning wood:

AIR!
The wood is utilised in the best way when the draught control is fully open and the flames are intense. That will also reduce pollution, because gas particles are combusted and produce heat instead. Once your home is warm, the temperature is regulated by the amount of wood, not the air control.
Are you calling a flue damper as your "secondary" maybe?? And your stove air control as your "primary" ??:confused:
You mentioned "draught control". ????
 
I notice when I have a good fire going and have been slowly closing the primary air, that as soon as I reach a point where the flames switch over fully to secondary burn type flames ( burning at the tubes) the stove top temps WILL indeed climb hotter. Briefly.

The temp will spike but for a short time, as soon, the temps will return back to a more settled/controllable level. ( I can opt to turn on the fan at this point if needed too). Then it will cruise there until it eventually needs more air to sustain a higher stove top temp . (then I can open air some more if I choose)
 
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