A question about BTU's

[jb1951]

According to their brochure information, the Vermont Castings Defiant has a 3.4 cu ft fire box and puts out 60,000 BTU's and the Jotul Oslo 500 has a smaller firebox (2+ cu ft), but puts out 70,000 BTU's. How is this possible? I would think the larger the firebox, the more BTU capability. Could someone please explain?

Thanks,

Jan

 

[EddyKilowatt]

That's BTUs *per hour*, isn't it?

Kind of more of a matter of how fast they can blow air thru the firebox (well, suck through, actually) -- how fast they can burn the fuel -- than the actual size of the firebox. But at the same heat output rate, the smaller firebox will burn for less time, or will have to be reloaded more often.

Since we're burning solid fuel, the surface area of the fuel matters, as well as the air supply rate. But the EPA test fuel is 2x4's with carefully controlled air spaces between them... so there's no shortage of fuel surfaces to burn to get those high BTU numbers for the brochure. Burning real logs, the story could be different... folks who burn those two stoves can tell you how they do in real-world conditions.

It's interesting to consider that the "firebox" in your car engine is only a couple hundred cubic inches -- a fraction of a cubic foot -- but it can burn a couple gallons of gas an hour, easily half a million BTUs of energy content. "An engine is an air pump" the engine tuners say, and the same applies to a wood stove (when connected to its chimney system).

Eddy

 

[webbie]

Welcome! You might want to read the article about choosing a stove.
https://www.hearth.com/econtent/index.php/articles/choosing_a_wood_stove
The short answer is that it is possible they both PEAK at 50,000+, but you will probably never use them that way. The larger firebox will burn longer at a given heat output....that is really the key. Burn time is a function of size and output combined. See this handy little calculator.
https://www.hearth.com/econtent/index.php/articles/burn_time_calculator
Of course, one thing remains unsaid. Don't buy a 3.4 CF firebox unless you have a big wood pile!

 

[Gooserider]

Lots of discussion on this in other threads. Essentially stove BTU numbers are marketing gimmicks, they put down whatever they think will sell best. A given volume of the same sort of wood has a certain number of BTU's - you can within limits burn it quickly, get a high BTU/hour output number, and not mention that you are having to reload the stove every hour or two. Or you can burn it slowly, get a lower BTU/hour output number, but go much longer between loadings - either way if you multiply BTU's/hr * burn time, you get the same total number of BTU's out, but it looks more impressive in the brochure if the Jotul can beat the VC with the bigger firebox - (we won't mention that the VC will give a much longer burn if you ran both stoves at their rated outputs...)

While stove design does have some impact on burn times / heat outputs, it seems that the best overall predictor of "real" heating ability, or the ability to put out the desired amount of heat for the desired time between reloadings, is firebox size.

Craig and I have a minor disagreement here - he says don't buy a bigger firebox than you can get the wood for; I say buy the heating capacity you want, and adjust your wood supply plans accordingly...

Gooserider

 

[downeast]

You're all correct about heat output: EPA tests are heat INPUT using over dry pallet-type wood, testing for emissions particulates.

The one measure that is not tested is efficiency of the stove. That it may be a "marketing gimmick" is possible; yet used as designed, each stove has it's own efficiency measure. EPA and manufacturer's test are done in lab or ideal conditions.
From the above posts: firebox size, wood species and moisture content, draft, surface area, and flame path in designed "re-burning" technologies are all part of the picture.

More important is your home structure and climate as in the computed charts for sizing the stove. Heating requirements of the Pacific Northwest ( warm rarely below 0 C and wet high temperate), Virginia, Connecticut or even the central plateau of Spain, are nowhere near most of Canada and our northern U.S. average winter temperatures. Older northern homes have little insulation efficiency, as with most older homes in northern Europe; the technology and knowledge of insulating is recent in the past 20 years. New construction here uses a minimum of R-19 walls ( 6") and R-40 ( 12") roofs. Many older homes here in northern New England have no insulation or roof venting ( for a "cold roof"),leaky storm windows, and open granite foundations. Most older homes in southern Europe, including Spain, and the British Isles have plumbing pipes on the outside walls. That would be impossible here.