Just about to buy my first wood stove...

[Woody Stover]

How would you handle stoves with a convective jacket? The outside surfaces of my stoves remain pretty cool, while air is being forced behind that panel and over the surface of a screaming-hot steel firebox hiding within.

I guess you'd have to either try to shoot around the jacket, or remove it. Hard for me to say how I'd handle it, without seeing it.

 

[Woody Stover]

Or shoot the front of the firebox, or back if you have access to it.

 

[Woody Stover]

Hey, how do you know it's "screaming-hot" if it's covered up? 😏

 

[Ashful]

Hey, how do you know it's "screaming-hot" if it's covered up? 😏

Good point! I'm only guessing by the action visible in the firebox, and the ~1000F glowing cat, as well as the hot air coming thru the convection slots.

My point was that I think the problem is more complicated than you were implying. It's not as simple as surface temperature x area, when temperatures vary more than 2:1 over a given surface. I'd bet the FLIR or Fluke IR camera software has a BTU output estimation built into it at this point, integrating the surface temperature over a given area. If not, they should, but it's been awhile since I've checked out their latest. I used to just export the raw pixel data into Excel or MATLAB, and run a calculation there. Repeat for all surfaces in a radiation-only model, but convection will confound that.

 

[mdocod]

....
Be that as it may, it seems to me that the easiest way to assess a stove's output, without trying to sort through the varying BTU numbers posted online by the stove makers, EPA etc, would be to shoot the top and sides of the stoves in question with an IR thermometer, at different air settings from high to low.
Now, that might not account for all factors involved in the heat output of a stove, but it might work pretty well. I'm guessing that the sides of BK stoves don't get all that hot, due to the heat/radiation reflecting shields they have inside the box, and that more heat is flushed up the flue at higher burn rates, vs. what makes it into the room, compared to regular stoves. But the BKs are rated very efficient so I don't know how that figures into it..maybe because of their ability to run at very low output?

I agree with the IR gun assessment methodology. It's not perfect but it can help paint a picture of what is going on that may not have been obvious from other types of measurement or specification.

When a BK stove is idling along like a wood gasifier, I would expect that the top plate just above and behind the cat is probably the hottest part of the box, and being highly conductive welded steel, that energy can traverse out to the sides of the box as well, so despite it being a slow fire, the stove is acting like a giant heatsink for a tiny fire. Indeed I would expect that stove thermal efficiency improves as the burn rate is slowed down, so long as there's a combustion strategy that can maintain a slow fire that doesn't waste fuel.

Imagine that instead of loading the firebox with wood, setting it all ablaze, then choking it down, we took the exact same load of firewood, ground it up into sawdust or pellets, and fed it with a screw-feed/hopper mechanism to burn in a little optimized burner. Then imagine we take all the airflow that would have been going through the original fire and injected into the cat, and directed it all into this tiny little burner, something like a sawdust powered rocket-stove. Then feed that fuel to the little rocket stove over the course of say, 20 hours. Now imagine that we design that little burner assembly in such a way that the heat from the tiny but very hot clean burning fire is being absorbed into a heatsink the size of a traditional wood stove... Yep, that's going to be pretty thermally efficient. By the time the exhaust leaves the system it's going to be pretty cool.

Now, imagine if we shrunk the size of the stove-pipe so that we didn't have to waste as much energy up the pipe to prevent condensing out creosote... Pellet Stove!

HAHA.... Small fires in big stoves are indeed more thermally efficient. The EPA test strategy may not always standardize the fuel load for a given stove design in a manner that is optimal for efficiency. Some stoves would probably measure better on the efficiency side while still burning very clean if they were tested with less fuel in the box.