Stove efficiency vs air flow

[bbqribman]

This is my first year with our new non-cat stove. I've been reading up here, and that has been really helpful, but I can't seem to find out one thing. If I'm looking for max heat output of a stove, where do I run the air control? My engineer mind is struggling with this one. I know it seems like the most efficient use of fuel is to run it with the air flow as low as possible while still burning all the smoke with secondaries. It seems that way you are minimizing the heat you are dumping up the flue and getting the most time to transfer heat.

But if the goal is max heat, is there something different you should do? Do you run it more wide open? There is only a certain amount of BTUs per piece of firewood. But when you consume them faster with a wider air flow are you dumping more heat up the chimney and losing the gain? Or does burning faster simply allow you keep the initial hotter burn stage more frequent? Thanks for the help.

 

[begreen]

Good question. The answer is, it depends. There are a lot of variables in this equation including the flue system draft, wood species and dryness, outside temps, and the stove itself. The amount of air is going to vary with the time of year and the state of the burn. But in general opening up the air a little will increase the intensity of the burn. Opening the air up a lot however may reduce secondary burning which will allow more unburnt gases to head up the flue. This can result in smoke and/or a hotter flue and a cooler firebox, neither of which are desirable. Short answer, open it up a little for more heat, but not a lot. Watch the stove top and flue temps to be sure that you are accomplishing the desired result.

 

[jophysx]

Just get an idea of the target temperature you want to hit. Then open the draft to the point where you are getting the temperature you want, but don't open it any further. You will be sacrificing a little bit of efficiency, but if you need the extra heat, that's the price you pay.

-Jim

 

[Grisu]

But when you consume them faster with a wider air flow are you dumping more heat up the chimney and losing the gain?

That's essentially it. Take this as an example. Let's say you load wood with effective 400,000 BTU in your stove. You close the air fully and burn it down in 8 hours you get 50,000 BTU per hour. Or you don't close the air fully, thereby sacrificing 25% of the BTU up the flue for effective 300,000 BTU. Burn that down in 4 hours with a hotter stove and you get 75,000 BTU per hour. You go through more wood but the house will be warmer. How warm depends on the stove. You can adjust the air that the stove gets maximally hot without overfiring it. At that point you will get the most BTU from your stove. If it is not enough you need a bigger or a second stove.

 

[Sprinter]

My engineer mind is struggling with this one.

So did mine. Here's what I do now. Start out by adjusting for best efficiency, then adjust upward as needed for output. Wood stoves are for comfort, so adjust accordingly.

 

[DougA]

So did mine. Here's what I do now. Start out by adjusting for best efficiency, then adjust upward as needed for output. Wood stoves are for comfort, so adjust accordingly.

And when it's super cold outside, you lose efficiency to gain heat. That is why it's better to buy a slightly larger capacity stove and you are in the efficiency zone more often. I learned that the hard way.

 

[branchburner]

If I'm looking for max heat output of a stove, where do I run the air control? I know it seems like the most efficient use of fuel is to run it with the air flow as low as possible while still burning all the smoke with secondaries.

It is fine to sacrifice some efficiency for extra output, but you want to be sure you don't overfire the stove. Variations in stoves and flue systems mean some chimneys really suck... literally. Downdraft stoves like mine are known to easily overheat under excessive draft conditions if given too much air. Some people have tried to heat too much house with too little stove, and run their stoves too hard in the process. Excessive temps can shorten the lifespan of some bricks and/or refractory components in certain stoves (like mine).

As begreen says, the many variables involved mean there is no one single air setting that will yield predictable results under all conditions. And keep in mind, you can't control the secondary air, which flows more strongly under certain conditions (weather, stage of burn cycle, etc.)... so under very strong drafting conditions, my stove can sometimes burn at less than optimal efficiency, a bit hot, even with the primary air fully shut. (Those are usually windy, low temp conditions, so I want the extra heat anyway.)

I often use my flue temps to decide where to set the air... too low, and I'm probably not getting good secondaries, but too high and I'm sending too many BTUs up the stack.

 

[bbqribman]

Thanks for all the replies. I appreciate the tips and advice. Currently, I don't have anything to measure flue temps. Sounds like something I'll have to look at getting. And since the stove is fairly shielded, I'm still trying to figure out a good spot to measure stove temps with my IR gun. I'm not sure my stove top would be useful, since its another layer of metal over the real stove box.

I live in a three story home with older construction. So not super great insulation or windows. Also the layout isn't conducive to a single stove heating the place. I know the stove can't heat the whole place. I'm just curious what the stove can do. Since I can run it with the air fully closed, it seems like I have a large amount of adjustment to use. How do you determine if it is overheating or overworking? The manual only refers to overfiring as the stove glowing or the flue glowing. I'm assuming they don't mean the burn tubes, as those can get a reddish tinge with the air fully closed. I was assuming I'd have to use electric to supplement in the rooms furthest away, so maybe its best to just stick in the efficient mode. Firewood is pretty cheap here though, so I was just trying to find out how to get more heat without being super wasteful.

 

[branchburner]

. How do you determine if it is overheating or overworking? The manual only refers to overfiring as the stove glowing or the flue glowing. I'm assuming they don't mean the burn tubes

Right, glowing is referring to the stove exterior, 900-1000f.

Generally, a stove will "cruise" with stove top temp at 550-650f, and you don't want them to be over 750f for extended periods. This is measured towards the center of the stove top in general, though it of course depends on the stove model (BTW, you should put yours in your sig, so we know what you are running).

My stove runs on "downdraft" technology, with an afterburner (=smoke-burner) in the rear of the stove. So while my stove top may be at 650f, my IR gun can read as high as 800f when pointed at that rear AB area. If I was to open the primary air further, or if I had excessive draft, that temp could run higher (some folks with these types of stoves have seen them glow a dull red back there).

Here's how many people find out they overfired a stove: they file a warranty claim and are told, "gee, the only way that component could fail is if you overfired your stove... therefore warranty void, claim denied." But the odds of that happening by opening your primary air from, say, 1/4 open to 1/3 open, is pretty slim. Usually folks are running a stove full open, way too hot, to try and make the stove do what it wasn't made to do. You can only run certain stoves so hard/hot before you hurt them, but a lot of stoves can actually be run hotter and harder than people give them credit for... it's not like they melt when they push 800f.

 

[bbqribman]

I added the stove into my signature. I'll have to see how I can get a stove top reading. I think it is shielded though, so if I IR guned the top, it doesn't get close to those temps. The door, or upper front face is I think the closest thing I can get to the actual stovetop.

Kind of a different question. Another thing I haven't figured out. After reloading the stove, I try to let it burn at full open for a bit to heat back up. When I throttle it down, sometimes I'll see smoke coming out of the chimney. This is with secondaries going full tilt, and almost propane like jets off the front burn tubes. What am I doing wrong. Is it not hot enough before I turned it down? Too much air so I'm pushing out unburnt smoke? Since secondaries are going, it seems like I got it hot enough. If I see smoke after startup, I'm assuming I'm doing something wrong. No one I know has a new EPA stove, and all the old stove burners look at me like I'm nuts when I ask them about stuff like this. This forum is great, but I feel like I'm going down a rabbit hole.

 

[fossil]

Fresh load of wood, burning vigorously on the outside and beginning to heat up is outgassing so vigorously that the secondaries just can't burn it all...plus some of what you're seeing may well be water vapor condensing as it reaches daylight. Even "dry" firewood contains moisture. Does what you're seeing from the stack dissipate quickly, or persist as it blows away?

 

[bbqribman]

It tends to dissipate pretty quickly.

This site led to get a moisture meter just to make sure "seasoned" meant dry. Prior to stacking it under tarps near my back door, I couldn't get measurements higher than 18%. Those were the worst numbers and I freshly split something to get that.

 

[Highbeam]

My engineer mind is struggling with this one.

Mine too.

So ideally, if you could strip all excess heat from the flue and stove, running the thing wide freaking open is the most efficient and highest output way to burn the wood. This is how masonry heaters or even wood furnaces work. Your stove isn't set up to strip heat like that though.

I run the NC30 hard and hot and my goal is to get the most heat out of it regardless of efficiency.