Explain the physics behind shutting the air control in increments

I'm not sure why it works but it does. I find that if I jut slam it shut from wide open, there are secondaries but they are not sustainable, while if done incrementally, the secondaries can last 1 to 3 hours depending on the wood, the way it's loaded etc.

so many variables, so little time.

any given stove will operate differently based on variables, chimney location, construction, height, climate, barometric pressure, temperature, wood type, the list goes on.

do what works for you.

i can burn some stuff and run my oslo up to 600 degrees, then slam the air totally shut and she'll cruise like the love boat.

other times, different variables, i'll try the same and BOOM BOOM....out go the lights.

just all depends.....

Warm in RI said:

I'm not sure why it works but it does. I find that if I jut slam it shut from wide open, there are secondaries but they are not sustainable, while if done incrementally, the secondaries can last 1 to 3 hours depending on the wood, the way it's loaded etc.

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I imagine for a "fresh" fire from a cold start it has a lot to do with sustaining high enough firebox temps, but may also have to do with the rate of outgassing relative to the proportion of primary air entering. The need for primary air varies greatly with the size and type of wood in my downdraft stove.

If you have dry wood, well established fire so that the outer surface is beginning to coal, and hot, hot, temps by the burn tubes, then it is fairly stable to sustain the secondaries and you can shut down the primary pretty far. It sounds like you had these conditions when you get to 500 F.

I do the incremental method so I can start turning down the primary air earlier, I can go to 50% many times when my stove top temp is 350 to 400 F, and then to 75% after another 5 minutes, and then all the way shut (or 90% shut) in another 5 minutes or so. Just went through a reload and an incremental shut down, and I just shut off the primary air and the corner of the stove top is about 425 F, secondaries are steady and will be increasing as the wood becomes enveloped.

So I think the incremental method allows you to start to remove primary air earlier, but if your too earlier, you lose the secondaries.

Is there a benefit to switching over to secondary air ASAP? I think so - burn more efficient, less heat lost up the chimney, wood lasts longer, imho.

Right now, there is no flame coming off the wood (half load), just glowing coal surface, all flames are on the top coming off the burn tubes, secondaries rolling strong, corner stove top is still around 425 F. This is probably somewhat lower heat output than when I have a full load and cranking at 650. But I am burning less wood, extending the burn time, and I don't need max heat output.

One of the nice things about having a wood stove hobby is you get to experiment and learn how to load and control your burn for a variety of conditions - it is like learning to drive. ;-)

When you put a fresh load of splits on a bed of coals and fully open the primary air control, most of the air supply is going to hit the coals and get them blazing, which will in turn heat the new wood and get it to start offgassing. When the primary air is open, the pressure inside the stove is nearly equal to the air pressure in the room, since the large primary inlet does not offer much resistance to the passage of air. In addition, the flue temperature is typically low, often starting below 300f or lower (since that is usually why you reload - insufficient heat output). Due to the low flue temperature, the draft is low (draft is the engine that powers the flow of air through the stove). Due to the fact that the air pressure in the room and in the stove is nearly the same, there is little pressure differential to induce air to flow through the secondary air system.

So, as the coals are glowing red hot and the wood starts to ignite, one will get a lot of flame action. But the roof of the stove is not yet up to full temperature, so in addition to the flames, there will be unburnt hydrocarbons and since the secondary air is neither flowing nor sufficiently hot, if you were to look outside you would see smoke coming out the chimney. How long this smoke continues to be emitted is something you have a little control over and is one of the reasons for a progressive closing of the primary air. Basically, you have to wait for the flue temperature to rise beyond 400F (measured with a probe type thermometer that penetrates substantially into the interior of the pipe). Once you pass that point, you should start reducing the primary air. What has changed ? 1) the draft is much stronger at 400f than potentially 200f or below 2) the secondary tubes and or baffle is now hot enough that if secondary air were to flow, it should immediately begin burning smoke and contributing additional heat to the firebox and reducing pollution 3) by reducing the primary air several things are happening: a) The pressure drop through the primary air control will increase. This means that the pressure inside the stove will decrease. b) decreasing pressure inside the stove will induce air from inside the room to flow into the secondary air system and emerge from the burn tubes/baffle. c) the "total" airflow into the stove is reduced, which means that the temperature of the flue gasses will rise faster. d) the amount of primary air going to the base of the fire is reduced. There is a careful balance to be maintained, since if the airflow to the base of the fire is cut too much too soon, the splits may stop offgassing and the entire burn process may stall.

There is another reason for progressively closing the primary air and that is with a full load of fuel, if the firebox temperature is allowed to climb too high, shutting off the primary air may be unable to prevent the continued rapid offgassing of the load. This typically leads to a sustained secondary burn and flue temperatures over 1000f which in turn has the draft at maximum force to the extent that the air may whistle as it enters the secondary air system as well as the airwash system, which frequently has its own (fixed) entrance.

The reasons above are why one should always be able to achieve the longest burn with the least wasted heat up the flue and the least smoke on startup by progressively closing the primary air after reloading. It also carries the least risk of runaway when one has the stove loaded to the gills and conditions that favor strong draft (high inside / outside temperature differential).

Great explanation, Keith.

I sure wish I wouldn't have forgotten about your 7110. I think that would've been a great option for my upstairs. Having now owned stoves from 4 manufacturers, I am extremely impressed with the quality of Morso stoves!

The 7110 is still for sale. Had some interested parties, but no takers yet.