How much secondary air is enough or too much? What if it was adjustable?

Just a fun way to visualize air restrictions.

 

All of the air in my 13NC enters the stove thru the OAK manifold at the rear bottom of the stove. I've looked inside the OAK while operating the air control on the front of the stove. The air control only slightly restricts primary air.................thus directing more air or a greater percentage of the air to the secondaries and tertiaries given stove temps and outside temp etc. I still think the total volume of air stay the same..................it's just feeding the fire from different points.

 

I have heard rumor to this effect, but have never seen any evidence of it. On my 30-NC, the hottest STT and flue temp are both with primary wide open, and lowest with the primary closed. Likewise, in all other positions, more closed = cooler, and more open = hotter. Though it is far from linear, the adjustments are more "sensitive" once the spring is past the lip.

Do other 30-NC users see the same?

 

It would be possible to design a stove that behaves like this, but it wouldn’t be typical.

Excess airflow will definitely kill efficiency of the stove, especially secondary and cat air that will not fan the fire. There is only so much fuel to be burned by the secondaries and cat and any extra air will just cool things down while also speed the flow of BTU’s out the chimney. To some extent extra primary air will fan the flames and increase the heat output, but that has its limits as well. If the flow is too fast for the oxygen to be consumed, it will cool the flame and in the extreme blow it out. For example a candle with a very small combustion zone.

 

I also see this with my wood furnace where I can see the flue temp and the "STT" at the same time via digital thermometer...once the wood has been charred good and the firebox temp is up, the intake damper closes, the flue temp drops and the "STT" goes up...if the airflow volume was still the same going up the stack, the flue temp would continue to rise as the internal firebox temps skyrocket (those are also monitored via an internal thermocouple...and it just starts to run good at 120o*F by the way...)

 

I do not know on the NC13 but if it follows the Nc30 there are 2 additional intake ports for the air wash of the glass, they are not regulated at all. a long time ago I asked Englander about hooking the secondary air intake to the outside air intake - never got a definitive answer other than something along the line of it might be OK. In theory the secondary air should be preheated before being introduced to the fire box.

 

I can't speak for any other stove, but that is categorically not correct for the Ideal Steel. Closing the air control directly reduces the amount of air entering the firebox and drafting up the chimney. The draft meter responds very cleanly to the air control, and you can see the response on the flames as well.

If it were otherwise, it would not be possible to control the burn times as well we do.

 

Others and myself have mentioned this already as well. I can't see it being any other way.

 

The theory is that primary air hits the fuel and increases burn rate like a bellows. Secondary air is fed up above the fire and increasing it beyond what is needed to complete secondary combustion will just blow heat up the stack.

 

I believe the little ports in front only feed the doghouse hole. All airwash air (primary)comes through the controlled oak hole.

 

Same. The stove and flue always cool as the air control is pushed in. There is no heating up of the stove when you reduce air.

Th nc30 is very controllable. It can be snuffed at will. A bit surprising really since the user could really pollute the air.

 

nc 30 dog house air reduces as you move control in

 

Not mine. That air comes in under the front of the stove from two small dime sized holes behind each front leg. There is no regulation of that air.
That little bugger chews holes through anything in it's path.

 

This for mine as well.

 

Yep, I can go at will from clean and hot burn to flameless and smoldering in a few minutes.

 

We have established that pushing in the air control on an nc30 does a great job of reducing the burn rate. At some point it looks like everything is shut off when you push that control rod in but we know that like the secondary air tubes, the doghouse hole is unregulated.

 

I have a Woodstock Ideal Steel and it has two air controls that are coupled; adjusting the air control adjusts both. I find that the stove is a little too generous with the secondaries because when fully loaded and the air cut back, a fire starts on the top of the load of splits and burns pretty aggressively until the fuel load burns down a bit, then the stove runs a lot cooler. I think I will try trimming the secondary air back a bit and see if that results in a move even burn. As long as I do not make a huge change, I do not even think it will cause the stove to put any more smoke because there is a cat. above the secondary burn area, and it has its own air supply that seems to be more than ample.

The secondary air is there to provide a much more complete burn and is generally a very good idea I believe. But again, on the I.S. it tends to take over for the first 1/3 of the burn and cause the stove to run hot because it is not just burning off smoke that rises up but actually providing direct air to burn the top of the fuel load. When the stove is cranked up much beyond first gear, the little air holes will actually drill holes into the splits at the top of the stove. Cool to see but I think maybe not as efficient as it could be.

I suspect though that for passage of EPA regulations the secondary air has to be controlled with the primary. In fact, the primary air can actually be shut down on the I.S. but the secondary air cannot be even with the draft control fully 'closed'.

Brian

 

I loved reading this thread. Wanted to add my experience.

I just got through dialling in my "cabin stove," a Scan A10. It's Danish made, with a ~1.4cf box, convection design and huge glass (stock photo below). When first produced, they were made as mine was, with 2 secondary air injections--a manifold at the top back of the box, and a tube in the top front. Upon first using it on my long stack with mucho drafto, no matter where the primary air control was set, the secondaries went violent and made quick work of the wood and consequently was a short burn time. Flue temps usually matched STT when cruising. Also, I think the tube produced turbulence at the top front and it smoked out the door quite readily in spite of my strong draft. Hmm, frustrating. "I guess those European emissions standards are strict and there's just tons of air in here" was my thinking.

A bit of research taught me that at some point they simply dropped the front tube from the design. (My guess is in ~2010 when Jotul took over the line.) Youtube videos of newer Scan A10 burns confirmed the change (no tube) and showed lovely lazy secondaries and hot flames in the box. Looked great. So I tried it--removed the front tube. All secondary air from the top rear manifold now. Wow, big change, she burns like a top, long and hot. No runaways or blast furnace effect. Secondary flames are evident throughout the burn, at least when you'd expect to see them. Virtually no smoke out the door. STTs are regularly above flue temps now when cruising. Still clean glass and clean exhaust, no blackening anywhere etc. etc. This stove is a true joy to use now, with enough coals for startup in the morning now, and boy that flame show is sweet.

I do agree that it's all a fine balance largely dependent on unique factors of each installation. With unregulated secondary air, draft plays a big role I would think. Just wanted to share my experience, which leads me to think that with secondary air, "just enough" is indeed the best.

Further to my anecdote, while researching Scan stoves (they're a rare breed in NA it seems), many newer models have primary and secondary air controls. Those europeans... often a step ahead when it comes to efficiency...

Not my pic, but here's the Scan A10:

 

I wonder how much of these differences are related to draft and how hot STT are "normal" BDF, a great guy engineer type and I have same stove. From chatting he runs his generally speaking with STT closer 300 while mine closer to 450 on average. Assuming saskwoodburner would be higher temps than myself on average, definitely longer based on his colder temps he reports.
Moreover, I generally assume east coast guys are burning higher BTU wood. All of which could be better controlled with more/better air control.

 

I appreciate all the thoughts and opinions you guys dish out. A few are on the same page as me, a few not, but that's okay, it's good to hash out stuff in the open even if opinions differ. I am going to pursue a way of throttling back the secondary air . A bit late in the year to matter, but for a summer project. Honestly, it probably won't matter in most normal weather, but those times of extended deep freeze or extra windy days, I'm sure it'll be worth the head scratching!

Canadian border VT , I only pursue high temps once in awhile off the line on a cold start, or in mornings when I need a bump in heat. For the most part, I try to stay between 400-500 F on side or face of stove. This seems to be best in my situation, for a bit less heat for a longer burn time. I can bump it up higher, but then the efficiency goes down (I believe) as stack temps rise a fair bit blowing fire between the baffles, while stove temps rise slightly. Of course, all this goes to heck in a handbasket when the outside temps mess with me.