Wood stove BTU numbers VS. The real world

[Alan Gage]

I've tried searching but haven't been able to come up with what I'm looking for. Namely, how accurate are the stated BTU numbers on wood stoves?

My reason for asking is that I'm building a small and very well insulated house this spring and will be heating it with wood, hopefully 100% (electric backup). It will be a single level, open floor plan, and probably a little over 900 sq. ft.

I've run it through Rescheck and done some online heat loss calculations and while I realize they're probably not entirely accurate I'm coming up with a heat loss for the structure of between 17,000-20,000 BTU/hour at -20 degrees.

I've already ordered an Englander 17-VL since it was on sale so I'm kind of locked in on that but I need to nail down placement. Englander gives the stove a peak BTU reading of 40,000 BTU/hour. I'm assuming this is ideal conditions and that I can't expect to get that much heat out of it for very long before it starts tapering off. But what can I expect to get out of it? What percentage of peak output can I expect for a stove with a nice load of oak after burning for 3 hours? 5 hours? 8 hours? A nice graph showing the BTU output of a wood stove stove over an 8 hour burn would sure be helpful.

If I can realistically expect to get 30-40K BTU out of the stove I'll be more inclined to put it where I want, which is pretty close to some windows. If real world numbers will be considerably lower I'll look at a more centrally located position along an interior wall.

Any input appreciated.

Alan

 

[wellbuilt home]

I think the heat out put is ok if you don't see -20 very often .
The problem will be in fire box size , 1.1 cf is small for a over night burn .
I think you will only see peak heat for 2/3 hours then another 2 hrs of warn temps .
My minimum fire box size would be around 2cf just to get a good 6 hr burn .
40,000 btus seems like a lot to get from 1.1 cf fire box .
If you want to sleep all night you will need to go bigger .
John

 

[joefrompa]

My 2.2 cubic foot firebox will likely give off 30-40k btu's at peak running conditions (not peak temps) and with the fan on high. I can probably squeeze that range out for 90 minutes max. Lopi says ~70k btu can be extracted, but my guess is that's with the blower all the way on high and the stove top at the very top of the possible safe operating range.

As John said, your problem is likely be to one of size. You'll want to have some perfectly sized LARGE pieces to put in there before bed to give you anywhere near acceptable output for 6-8 hour stretches.

That being said, at 900 square feet with outstanding sealing and insulation, you should have very little loss too. So if you can get the place cozy, it might get a little cold after a nights rest but you might find that perfectly acceptable.

 

[jimbom]

Your stove will be fine, it is the house to watch.

My little Atlanta AC5 steel stove is rated at 34,000 Btu/hour and it will do that just fine. However, I have to tend it a little more than if I just cruise at 20,000 Btu/hour. I am sure you will be warm regardless of stove location. Especially if you make sure you use house wrap, foam/tape all the penetrations in the wrap and the top and bottom plates, and seal all electrical and plumbing openings in the ceilings, floor, and walls. My experience is air leaks are very correlated to heat loss. I am not sure heat loss calculation methods have a very good way of accounting for leaks. There is the crack method and the air changes methods, but really, these are hard to apply before construction completion. So my bottom line for you is if you get a tight house, you will be warm with the stove you have ordered.

Another thing to consider is the really few hours per year when you will be running at or near the design temperature. My design temperature is 0 F. On a thirty year average, only 9 hours per year does it get colder than 0 F. Only 18 hours is it 0 to 4 F. Only 40 hours is it 5 to 9 F. Almost all of these low temperatures are recorded from midnight to 8am. So out of the 8760 hours in a year, I have a few where we might have to cuddle up or light off the AC5. And we do, because I have a tiny heat plant. No excess capacity at all. It is much more efficient that way.

For fun I looked up Duluth IAP, Minnesota. I show a design temperature of -20 F (I have old data, so this may have changed). Only 22 hours below -20 F. Only 40 hours from -20/-16. And only 90 from -15/-11. So a heat plant that was maxed out at -20 F would only have 22 hours per year when it would need a 5 F boost from standby heat or when the indoor temperature would be 65 F instead of 70 F. Sadly, very few American homes are sized close to requirements. Many designers add 20 to 50% extra capacity just to be safe.

 

[joefrompa]

Also, consider installing an outside air kit so that your stove is pulling cold air from the outside of the house and sending it up the chimney, and your room gets all the radiant effects without having the stove sucking up some conditioned air. This is even more pertinent in a newly constructed, well-sealed house.

My guess is that you'll find yourself heating the house to the level of toasty quite easily.

 

[Battenkiller]

It's best to look at some burn cycle charts to get a little illumination on this. In the real world, peak output is almost never close to the rate a given device will output over an entire burn cycle. Very few makers send their stoves out to get tested in calorimeter rooms (the only real test), so they use stack loss data against fuel weight loss data (with the stove sitting on a scale) to get realtime estimates.

In essence, if a stove is placed on a scale and charged with a load of wood, and realtime stack losses show that the stove is burning at "X" combustion efficiency over a certain time, the loss in weight x the efficiency factor x low heating value of the fuel (usually about 7800 BTU/lb for wood containing 20% water by weight) x duration of the burn = heat output for that time period. It can be extremely accurate, but in the case of the EPA tests themselves, much of the stack loss is unaccounted for because the EPA doesn't really care about anything except for PM (at this point in time, at any rate).

Bottom line, expect an average heat output of about 1/3 or less of the "peak" output over the course of a cold-to-cold burn cycle.

 

[SolarAndWood]

If I run wide open with the blowers on high, I can do 85K using the 7800 BTU/lb method. The manufacturer states 47K as real world with 90K theoretical max.

 

[Battenkiller]

If I run wide open with the blowers on high, I can do 85K using the 7800 BTU/lb method. The manufacturer states 47K as real world with 90K theoretical max.

My stove is rated at only 50K, so I guess I'd need two of them side-by-side to get the output of a single BKK. Wouldn't want to be in the same room with them, though. %-P

 

[Alan Gage]

Thanks for the replies. I realize I'll have problems getting long burns from such a small firebox but I'm ok with that. If I have enough coals left over in the AM to light some small splits I'll be happy. I actually prefer a house cooler than most. 70 degrees and over I start getting uncomfortably warm.

BK - An average heat output of 1/3 or less from peak over a burn cycle would give me lots of room to play so that's good news. Even if it averaged 1/2 of peak output I should be sitting pretty.

Jimbo - Good point about the amount of time the outside temp actually stays at the design temp. It was colder than average here this winter but we never made it to -20. Lots of nights of 10-15 below and highs around 0. Last year we hit it a few times as well as setting a record of -32. But like you said, it doesn't stay there long. I've tried running the numbers with design temps of -10 and -15 and it certainly helps out.

You're right about calculating air loss, that will be the wild card. I'm going to be very careful about building a tight envelope. The plan is 2x6 walls full of cellulose insulation, OSB sheathing, housewrap, and at least 2" of rigid foam on the exterior (with taped seams). That will give me another R10 as well as creating a thermal break for the studs. Flat ceiling with at least R-60 blown in. I've never lived in a well insulated and tight house so I'm really interested (excited actually) to see how well it holds heat.

I'm also worried about the stove being too hot for the majority of the burning season. I'm used to burning 24/7 from December through the end of February. I'm afraid the house will hold heat so well that except in the coldest weather the coals will be burned down to nothing before it's time to reload. Which will mean starting a fire from scratch most of the time, which is a pain. That's one of the reasons for wanting to place it by some windows, to let some of that heat get away. But I don't want to lose too much of it and run out of BTUs when it gets really cold. I suppose I can always break down and turn on the backup heat if that's the case.

I guess there are worse problems to have....like trying to heat my current house with the wood stove when it's -10 and the wind is howling.

Thanks for all the great info everyone!

Alan

 

[Alan Gage]

BTW, Battenkiller, that's one of my favorite Bob Dylan songs.

Alan

 

[VCBurner]

I think you would have been better off with tha 13-nc for the longer burns with those outdoor temps you stated. Especially since you plan on doing most of the heating with your stove.
Good luck with the new stove.

 

[Jimbob]

Here's another heat-loss calculator to try, just for kicks:
http://www.h-mac.com/heat-load-calculator/heat-load-calculator.php

It would be interesting to see if it gives you the same #'s you have.
At any rate, 20,000 btu/hr is good. If your stove is slightly too small, it would be VERY cheap to supplement it a bit with the electric on the coldest days.

At 7800 btu/lb. you'd be burning about 3 to 3-1/2 lbs of wood per hour, assuming 75% efficiency (most stoves nowadays are at least that).
And that would be only on the very coldest of days.

Sounds like an ideal setup.

 

[Jimbob]

A nice graph showing the BTU output of a wood stove stove over an 8 hour burn would sure be helpful.

Any input appreciated.

Alan

The only input I can give you on this is a link to a thread where I track temps and time on a 13 lb. load of wood.
Wood is said to be 7500-8000 btu/lb.

Link:
https://www.hearth.com/econtent/index.php/forums/viewthread/70922/P22/#820508

The thread isn't scientific, but it gives you an idea. BTW, 13 lbs of wood will give you roughly the same amount of heat, whether you burn it in a 1.1 ft³ stove, or a 3 ft³ stove.

 

[madison]

Like men talking about their "members" or bank accounts, divide by at least half to get the real number

 

[wkpoor]

Like men talking about their "members" or bank accounts, divide by at least half to get the real number

I was thinking the same thing. Take any # divided in half and then you won't be disappointed.

 

[thechimneysweep]

I've gotta weigh in here about the numbers being used in this thread for the available btu content in a pound of wood.

In a laboratory, using an oxygen bomb calorimeter to burn bone-dry wood ( 0% moisture content ) in an atmosphere of pure oxygen, it is possible to extract 8660 btu's from a pound of wood.

That number, known as the Absolute Heat Value, is sometimes used in scientific calculations, but, while useful in analyzing theoretical problems, the absolute fuel value of wood does not reflect real-world available heat. For one thing, the wood you burn in your stove is not dried to 0% moisture content: even a well-seasoned woodpile will contain about 20% water, which doesn't produce any heat in the fire. A 1-lb. stick of wood at 20% moisture content is only 80% wood, so it only contains about 6928 btu's of available heat energy (8660 x 80%). This is known as the High, or Theoretical Heat Value of fuel wood.

Even the Theoretical Heat Value must be adjusted, because 100% of that Heat Value is not available to be extracted to the room. The explanation is twofold:

1) A portion of the 6928 btu/lb must be used to boil away that 20% moisture content, as well as whatever humidity is present in the combustion air and the water that is created as a product of combustion.

2) Another portion must be "lost" up the chimney to establish the average 300° - 350° F flue temperature necessary to maintain an adequate updraft so the fire will burn properly.

I'll leave the math to Battenkiller here, but the bottom line is, once we subtract the heat value needed to boil off the water and the heat value needed to keep the chimney at 300° - 350° F, a pound of fuel wood at 20% moisture content actually only contains about 6200 btus that are available to be delivered to the room. This is known as the Low, or Available Heat Value of fuel wood, and is the number most commonly used in firewood btu/cord charts.

 

[BrowningBAR]

If I run wide open with the blowers on high, I can do 85K using the 7800 BTU/lb method. The manufacturer states 47K as real world with 90K theoretical max.

My stove is rated at only 50K, so I guess I'd need two of them side-by-side to get the output of a single BKK. Wouldn't want to be in the same room with them, though. %-P

The difference is, I think the Vig is 50k real world. I don't think that is Max BTU out put.

 

[Alan Gage]

JimBob - I ran the calculations from your link and came up with similar numbers, about 16.5K BTU/hour. I did it quick at work so I didn't have all the exact numbers so it's not as accurate as it could be. I also wasn't quite sure how to do insulation. I plan on at least R60 in the attic but they largest choice they had was R40; so I chose R40 and R20. Had to do the same for the walls.

This is the other online calculator I've been using: http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm

ResCheck is a bit more detail oriented than anything I've found online, wanting to know how the house is framed and it differentiates between cavity insulation and full coverage insulation, like exterior rigid foam. It's a fun bit of software that's easy to use and free to download from the Department of Energy. http://www.energycodes.gov/rescheck/download.stm

VC Burner - I've actually been using the Englander 13 in my double wide the past few years. It's certainly not a well insulated or well sealed house but the stove does a good job until the wind picks up or it drops to -5 or lower. My new house will actually be less square footage and a world apart when it comes to holding heat. I'm pretty sure the 13 would heat me right out of the place.

I'm really interested to see how things go next winter and you can be sure I'll be posting my results....hopefully it will be bragging.

Alan

 

[SolarAndWood]

If I run wide open with the blowers on high, I can do 85K using the 7800 BTU/lb method. The manufacturer states 47K as real world with 90K theoretical max.

My stove is rated at only 50K, so I guess I'd need two of them side-by-side to get the output of a single BKK. Wouldn't want to be in the same room with them, though. %-P

I'd buy it puts out twice the 2 cu ft stove it replaced. It also reliably burns much lower. If you are in front of the stove when its running wide open, you definitely feel the heat. If you aren't directly in front of the stove, it is a non-issue.

 

[Alan Gage]

Just for fun I punched up the numbers on my current house, the double wide. Had to make some educated guesses (have no idea of the R value of the old windows) and I think the house probably got the benefit of doubt overall since it's hard to quantify leakiness. Anyway, it's more than double (about 40K btu/hour) the numbers I'm coming up with for the new house.

Chimney Sweep - Thanks for the clarification.

Alan

 

[webbie]

How about this simple calculator which uses the firebox size and type of stove to give a pretty good estimate of what to expect?:
https://www.hearth.com/econtent/index.php/articles/burn_time_calculator


choose the 1.1 cubic feet and the stove type and you will get some estimates.

 

[thechimneysweep]

Or, how about a formula that takes into account the efficiency rating of your specific stove?

(( firebox size in cu. in.) x ( 0.015 ) x ( 6200 ) x ( stove efficiency )) / ( burn time )

To get the firebox size in cubic inches, we multipy the manufacturer's stated cubic foot measurement by 1728.

The 0.015 is the weight of the load per cubic inch. To get this number, we used an average of the top 60 species from our firewood comparison chart, and adjusted to compensate for airspace between pieces, using the ratio that a 128 cubic foot woodpile only contains about 85 cubic feet of wood. This creates that we're not using the weight of X cu.ft. of wood, we're using the weight of the wood that will fit in a X cubic foot firebox.

The 6200 is the available BTU (heat) content per pound of fuelwood at 20% moisture content.

For stove efficiency, we use the manufacturer's tested Low Heat Value rating.

For burn time we use 8 hours, although smaller numbers can be used for smaller fireboxes.

More at http://www.chimneysweeponline.com/wscomp8.htm

 

[webbie]

Most of those numbers are in that calculator......roughly, of course, but so is anything related to this stuff.

Heck, I failed Algebra.....

 

[webbie]

Here are the rough assumptions in that calc:
function CalculateBTUs(fields)
var Soft = 25;
var Medium = 30;
var Hard = 38;
var ExtraHard = 44;
var FireboxCorrectionFactor = 0.5; // correct for empty/wasted space in the firebox after loading
var FireboxVolume = parseFloat(fields.FireboxVolume.value);
var StoveEfficiency;
switch(fields.Efficiency.value)
{
case 'Gasification Wood Boiler':
StoveEfficiency = 0.75;
break;
case 'Newer EPA freestanding catalytic stove':
StoveEfficiency = 0.7;
break;
case 'Newer EPA freestanding non-catalytic stove':
StoveEfficiency = 0.63;
break;
case 'Newer EPA fireplace insert':
StoveEfficiency = 0.55;
break;
case 'Older freestanding stove - airtight':
StoveEfficiency = 0.48;
break;
default:
StoveEfficiency = 0.4;
var BTUsPerPound = 7000;
-------------------------------------------

the numbers above, all put together, are close enough for this type of thing.

 

[RustyShackleford]

... I'm coming up with a heat loss for the structure of between 17,000-20,000 BTU/hour at -20 degrees.

You mean outside temp of -20 and inside something reasonable (not a 20-degree temperature differential) ? Wow, that's incredibly efficient, on the order of 200 BTU per hour per degree of temperature differential. When I did the calc for my fairly-well insulated 1400 sq-ft place, I came up with something like 800. Well done !