Impact of Efficiency vs Firebox Size on Output

[avsmusic1]

I wanted to get the expert's opinions on total output / heating impact based on these 2 criteria. Simplifying things and applying general reason (instead of deep experience) I would assume an equation sort of like (Firebox size) * (HHV) = (Adjusted Output capacity). When I play that out a little I'm not certain it holds though. If we treat the other variables as constants here is a comparison on the larger Jotul offerings currently

	Firebox	HHV	Adjusted Output Capacity
f55	2.95​	68%​	2.005115​
f500	2.54​	78%​	1.99136​

This would suggest that these 2 stove are somewhat interchangeable in their heating ability. Obviously there is the factor of convective design on the F55 vs radiant on the F500 but i'd assume that's more how the stove distributes the heat more than the total heat produced. I can't imagine the only difference between these is aesthetic and how they distribute the heat though, right?


Someone with more experience help me reconcile this please

 

[Highbeam]

The size of the stove determines both max heat output as well as total lbs of wood per load. The surface area of each stove at 800 degrees determines the output and how well it heats your room.

The only thing efficiency gives you is how often you need to load it.

 

[ABMax24]

I see what you are trying to do, but shouldn't the last column be called something like "useful heat output per load"?

I do think you are on to something, it's something I've noticed when looked at stove specs, and especially ZC fireplace specs. Some of these fireplaces have massive fireboxes, but in reality are only going to deliver similar output per load to a wood stove with 2/3 the capacity, because of the difference in efficiency.

Your table would be crucial for heat output in things like overnight burns, as your table shows the heat delivered during a load would be the same, one stove just consumes more wood to deliver the same total heat.

Kind of like how cars are rated in mpg or L/100km. Except for stoves it could be something like lb/10,000btu.

 

[avsmusic1]

The size of the stove determines both max heat output as well as total lbs of wood per load. The surface area of each stove at 800 degrees determines the output and how well it heats your room.

The only thing efficiency gives you is how often you need to load it.

Maybe I'm misunderstanding the efficiency ratings. I was under the impression that HHV is roughly the % of BTUs put into your home. The ability to hold more fuel could be offset but it's inability to distribute that into the home. To ABMax's point - maybe my naming is off and it's more "effective btus to the house". I was thinking of it sorta like Crank vs Wheel horsepower

In your experience though, the real world way the efficiency manifests is burn times? All my babbling is more theoretical lol... though i would like a way to convert some of these stats into one real world comparison measure of what actually matters to me - heat in the house

 

[Highbeam]

Efficiency tells you nothing about heat output unless you specify a burn rate. For example, 5# burned per hour, 10,000 btu per lb (false but easy math), 50% efficiency stove, means you are delivering 25000 btu per hour regardless of stove size.

Consider the available burn rates for a stove and how it matches your needs. Then, the bigger firebox just means a bigger fuel tank.

The epa list provides max and min btu outputs for each stove. That includes the efficiency variable.

 

[avsmusic1]

Efficiency tells you nothing about heat output unless you specify a burn rate. For example, 5# burned per hour, 10,000 btu per lb (false but easy math), 50% efficiency stove, means you are delivering 25000 btu per hour regardless of stove size.

Consider the available burn rates for a stove and how it matches your needs. Then, the bigger firebox just means a bigger fuel tank.

The epa list provides max and min btu outputs for each stove. That includes the efficiency variable.

Thank you Highbeam - this is more helpful than you know. First, I hadn't realized the btu output in those reports reflects the efficiency - that's basically exactly what I'm after. Also, despite having looked over them previously, I didn't recognize that the test burns were done at different air control levels so they do provide a useful range.

Thanks again!

 

[Highbeam]

Oh man, now I’m second guessing myself if efficiency is included in the btu output figures. Regardless, the efficiency spread is pretty small. If you need numbers to that level of precision then I suggest you research the test methods.

The btu output listing on the epa site may not be perfect but it’s probably the best tool we have for cutting the crap from marketing brochures. Those output numbers even reward/punish the different burn technologies by showing us how narrow the output range can be.

Efficiency numbers are important for wood savings. Emissions for those who like to limit pollution.

 

[bholler]

Thank you Highbeam - this is more helpful than you know. First, I hadn't realized the btu output in those reports reflects the efficiency - that's basically exactly what I'm after. Also, despite having looked over them previously, I didn't recognize that the test burns were done at different air control levels so they do provide a useful range.

Thanks again!

Btu output can reflect efficency. But not just btu output. You also have to factor in the ammout of fuel used during a given time to produce those btus. Old stoves like fishers Alaskas etc had massive btu outputs. But they used very large amounts of wood to make those btus for a short time. And dumped allot of the potential btus up the chimney in wasted heat and emissions that could have been burnt for more btus.

 

[EbS-P]

One would think to calculate the efficiency you need the total btus loaded in the firebox and the total btus given off by the stove. Using the efficiency to then calculate the max btu would be rather circular, unnecessary and rather unhelpful but it could be done. So I think your idea of efficiency times firebox size is a useful measure but really the difference between high and low efficiency is lamas 15% and the difference in fire box size could be 300%. So your metric “weights” size more than efficiency but the whole point is to find which one give more heat so I think it has merit.
Evan