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

Interesting saskwoodburner, I do same thing except during negative 30 weather when try to keep it above 600. Yes then I burn more wood. Yes stack temps go up, usually less than 100 degree increase. But those temperatures for me, are measured in weeks not months.

 

"If" I could burn at 600 or better and get decent burn times, I sure would. You can run it like a rocket, but then all you're left with is a ton of coals. But 400-500 is the sweet spot and it keeps the place to a decent temp. I also have a much smaller building to heat than most. It's not the end of the world to blip the furnace every now and then, and I'm okay burning 500 liters/100-125 gallons a year of furnace oil. Just need it slightly more controllable in the cold.

 

I know somebody that did this with a wood furnace, he has a very tall chimney and really high draft. A barometric damper was not wanted, and a manual damper is not a good idea on a furnace with a thermostat controlled damper. He was able to find out from a factory rep (off the record) that the fire could be controlled by restricting both the primary and secondary air beyond factory settings. It took some experimenting to get it right, but he has been running this way for years now, with only excellent results.

 

I think on the Englander 17, it restricts the primary quite well so I won't tinker with it. Hard for me to explain, but I'll try. The cut out where air goes in (inside, beneath the stove) is a square,and rotated at a 45. The plate that slides and regulates air is a square. So its a square sliding on a diamond. Throttling back the air isn't linear, but once you get down to the last little bit, it's cut back hard.

 

Quite a few stoves back just prior to the epa mandates but late enough to incorporate secondary burn had adjustable secondary air. I as far as I can see due to EPA regs that all vanished which is a shame as every install and use is different and in no way is like their mandatory testing/ emmisons desk jockey rules. So we are left with re-inventing the wheel as it were. I think we are all aware of the coal build up when pushing the current venue of stoves hard.

 

Was there ever any guidance for how to adjust secondary air? Like, maybe, turn it down until flames roll slowly? Or maybe the minimum air to allow secondary combustion?

 

There should always be enough secondary air to burn off all the flammable gasses being produced by the wood in the firebox. Not too much in excess though as that just cools down the secondaries and pushes more air through the stove, carrying heat outside. But as the goal we have to reach is low emissions, most stoves seem to push too much air to the secondaries because they run cleaner that way than with too little air. And giving the owner / operator control over the secondaries would result in some number of stove being used without enough secondary air and 'dirty', so we do not get that knob.

My only complaint with the I.S. and secondary air is that there is so much of it, it actually burns the wood from the top- down, until the firebox is about half- full, then the fire slows down because the fuel is not getting that secondary air anymore, just the primary air. So the stove runs harder than should for the first 1/3 of the burn, and then far too slow for the last 1/3 of the burn because secondary air does not help in the least in burning down the coals. It is just the nature of the beast for modern stoves I think.

Brian

 

Basic instruction from late 90's stove was to close off the primary and open secondary till you got the rolling flames at top of fire box. I don't remember much more than that other than you could control how intense the secondaries were. I still have the stove - will be re-installing it later this year - just installing a nc 30 at present + a 25 pdv pellet unit (basement) in home- Propane eating me alive at present. After all the installs about 3 years to break even, but I will be a lot warmer and the leak in my wallet will be almost sealed. hw on propane(needs replacing also). note outdoor units banned.

 

Brian, are you sure the linkages in your IS are working? On my IS I can dial the secondary air WAY back. It is linked to the primary air obviously, but I can control that pretty well in the first third of the burn with the IS’s manual air control.

What I cannot control on the IS directly and what I think causes it to run hot that first 1/3 is the cat air supply. I believe the cat is getting its air from a third source on the other side of the stove that is controlled by the automatic flap. Once the stove heats up, that flap opens and flows as much air as the draft can pull. I have had pretty good success controlling this with a flue damper. Shutting down the draft can bring down or hold down the cat temp and the stove top temps, but you need to open it back up later in the burn.

 

Matt Fine, my stove does same thing. A 2015, engage that cat dial air back, it looks like mini blow torches coming down from top.

 

Yeah, the linkage is working, and I too can dial the secondary air back with the draft control. The problem is that the ratio of primary to secondary air is not where I think it should be or, I guess, want it to be. There is too much secondary air, especially around the 1/4 to 0 draft range. And the secondaries cannot be shut down, even with the draft closed all the way. You can verify all of this with a simple light and mirror and just looking under the left / bottom (left viewed from the front, the door latch side) and you can easily see both drafts, their unique shapes and the relative openings of each a different openings.

As to the cat, yes, there is a third air supply to that too but it is in the back of the stove and cannot 'back- feed' the airbox through the secondary air holes in the top plate. If that air circuit did feed the airbox, it would be at the front because it would have to spill out of the combustor well (my words- whatever Woodstock calls it) and actually become part of the primary draft.

And a flue damper definitely does help but again, it does not adjust the ratio between the primary and secondary air feeds. That is controlled by a steel linkage along with the shapes cut into the draft throttle plates.

Brian

 

Can't see what you would gain here as the secondaries burn smoke and particulates.... turn them down and I would assume that fuel is just lost through the chimney. I would say you are simply running up against the limitations of your stove firebox design.

 

I'm not sure if you're addressing BDF or the general idea of cutting back secondary air, but I think there's something to be gained by fine tuning. I think slowing the air down would help to get a better burn, with more dwell time in the hot stove, rather than blow it out the chimney. My goal would be to make the stove burn as if it's -20 C out, even when it's -40 C.

 

I get that too, but dial back further (all the way down) and they go from blow torches to mini bic lighter flames. I can’t see why I would want less secondary air than that, or with the exception of coaling at the end, why I would want more primary air.

In my IS (March 2016 build) once the stove is up to temp and the cat takes off, I can shut the air intake all the way down and it will keep getting hotter. With a full load, the cat probe temp will often go to 1500+. This is not from too much secondary air IMHO, but due to the wood smoldering off volitile compounds and uncontrolled CAT air burning them. Open the air control and the cat temp may drop a bit, but the extra air burns more in the box and the stove top temps that were (possibly) already warmer than I want get even hotter.

I think this is what Brian is experiencing when he says the first third of the burn is hotter than he would like. I don’t think less secondary air or more primary air are the answers. Heck, you could dial the air to the box off nearly 100% like making charcoal and it will still gas off and burn is the Cat and leave you with a hot stove and a firebox full of charcoal. The ways I have found to control it are 1) put in less fuel or 2) cut back the draft to slow total airflow, including Cat air.

As another benefit, dialing back the draft and also opening up the air control to let more primary air across the glass can help keep the glass cleaner, but so far, in my experience, if you load the stove to the gills and then want a slow burn with lower ST temps, the glass is going to get dirty.

This is all talking about the first 1/3 of a burn with a good sized load. The last 1/3 of the burn is a different story. That is when independent controls would be nice. Shut off the secondary air and turn up the primary air. Even better if some of that primary air comes from below the grate. That is a whole different topic though.

 

What happens on the Woodstock Ideal Steel is that the secondaries start what amounts to a secondary fire at the top of the firebox and actually consume firewood splits directly by pushing aid down onto them. It is easily seen as the air jets bore little holes into the firewood. Once the firewood supply in the firebox is burned down by, about, 1/3, then the secondaries can no longer reach the firewood and so the bottom or primary draft becomes what drives the stove and the secondary air just gets pushed into the chimney. Because there is too much secondary air, it takes over the speed and intensity of the burn from the primaries. So that leaves us with two choices: 1) set the stove for the 'long burn' with the secondaries accounted for, which results in the last 1/2 or so of the burn being far too slow to supply enough heat to the house, as well as intensifies the generation of coals or 2) set the stove for the primary burn and let it burn too hot and fast for the first 1/3 or so of the burn when the secondaries are not just burning wood products but the actual wood fuel itself.

This season is pretty much toast for me but with any luck at all, I will be in better shape to again tinker with the stove next season. I plan on both addressing the secondary air input as well as overall control of the stove's burn automatically- something I started on long ago but unfortunately had to abandon due to other time constraints.

And none of this should imply the I.S. does not run efficiently because it does do so. I am merely trying to squeeze the last 10% out of the stove as I would with any stove; it is my firewood, I either paid for it or procured it and always have to haul it into the stove, discard the ashes and I have a high priority for burning as efficiently as is reasonably possible. And I am willing to spend some time and money in chasing this efficiency because once it is achieved, it goes on year after year. The identical thing can be said for inefficiently: unattended, it goes on year after year with the consumption of more wood than is needed.

Brian

 

Well, not quite what I meant. If the primary and secondary air were dialed back to zero, even the pyrolysis or 'smoldering' would stop and the charred firewood would be left in the stove. So I believe the primary air should control the burn, the secondary air should burn off the combustible volatiles when and if produced, and the cat. should catch all the rest and consume them. If a step can be skipped entirely, it is the secondary burn because the cat. will take care of all of the particulate, volatile material and gasses regardless of any secondary burn, just like a pure cat. stove.

So what I find is the secondaries on the I.S. are actually making a primary fire but on top of the fuel supply, which I am not a fan of. It is cool to watch, and the stove burns cleanly and very well in that mode but it does not give the longest, most efficient burn possible, at least IMO.

Brian

 

Brian, I think the easiest answer to what you want to do is to close down the existing air control hard at the beginning of the burn, and then have a servo open it back up in the later phases to keep the heat up and coals down.

A basic setup with just two air settings would be easy to program triggered off the Cat temp. Cat temp 800* or warmer, close the air supply to set point 1 (all the way closed or at 1-3 smaller ticks depending on conditions). If the fire was well lit before clamping down, it will not go out and will feed the cat for hours. After the volitiles are cooked off, and the cat temp falls below 800, open the air to set point 2 between 1/4 and 1/2 open. That will keep the STT from dropping and the coals from building too much.

The HeaterMeter I use to control my Kamado could be adapted to do this task fairly easily except its thermocouple circuit doesn’t measure hot enough. I think a component modification would fix that.

 

The IS is a catalytic stove also, a hybrid, there by that which secondaries don't get the cat does. I think BDF is trying to minimize the fuel consumption of the secondaries in beginning third of burn.
I still think a mechanism to open air supply after x number of hours would be ideal

 

Great idea Matt, I think we are on same page.

 

Been pondering away to kick open the air control on the 30 say after 6 or more hours to get one up on the coal build up so that when I get home I have a lesser amount of coals and can have a larger reload. Also been playing with some ideas about forced primary air at the later stage of coaling. I do not always have fuel that will burn leaving little coal to reduce the original coal load.