Garn BTU calculations

[Jayf19]

1 cord of wood = 8x4x4 :. 128 cubic feet
128 cubic feet is 957.5 gallons

The Garn Jr has a combustion chamber of 110 gallons
Fill'er up and you get 8.7 loads per cord (957.5 G / 110 G)

1 cord of Tamarak is 19,500,000 BTU's

1 wood load should give 1,792,167 BTU @ 80 % efficiency (advertised efficicency) (19.5M / 8.7 loads * 80 %)
or 1,568,146 BTU's @ 70 % efficiency (19.5M / 8.7 loads * 70%)

------------------------------

The nominal storage that comes with the Garn Jr is 980 gallons, so with a delta T of 70 degrees for the storage, the heat load to bring the water from 120 to 190 is 548,800 BTUs (70 degrees * 980 gallons * 8 BTUs).

Conclusion : Something is off; way off. Can anyone enlighten me ?

 

[arbutus]

Check your firebox size. 36" long x 24" diameter = 9.4 cubic feet.

 

[Jayf19]

Check your firebox size. 36" long x 24" diameter = 9.4 cubic feet.

Thank you!!

 

[Jayf19]

Although it still seems a bit off :
1 cord of wood = 8x4x4 :. 128 cubic feet
Garn Jr has a firebox of 36"x24" (uses 16 to 24 inch logs - so 18 for the current calculations) = 9 cubic feet

Fill'er up and you get 14.22 loads per cord (128 cubic feet / 9 cubic feet)

1 cord of Tamarak is 19,500,000 BTU's

1 wood load should give 1,096,875 BTU @ 80 % efficiency (advertised efficicency) (19.5M / 14.22 loads * 80 %)
or 959,766 BTU's @ 70 % efficiency (19.5M / 14.22 loads * 70%)

The brochure mentions a maximum output of 180k BTU/hour with a 3-hour burn (540k per load) using white oak (which has 24.2M BTUs per cord)

 

[maple1]

White oak - at 24" long.

IOW, the entire combustion chamber volume does not get totally filled with wood.

I suspect?

 

[Jayf19]

White oak - at 24" long.

IOW, the entire combustion chamber volume does not get totally filled with wood.

I suspect?

Could be... but to get to 540,000 per load, that would equate to a 30 % capacity fill (per my calculations - which I think I'm still missing something) and I doubt a manufacturer would release data at 30 % capacity....

 

[velvetfoot]

Could they be using "cord" instead of "face cord"? A face cord is 30% of a cord.

 

[Jayf19]

Could they be using "cord" instead of "face cord"? A face cord is 30% of a cord.

I am the one using a cord (from a source using a standard 4x4x8 cord) as a variable to determine the output of my type of wood; Garn is simply stating the maximum BTU/hour output based on a single load (not a cord) of white oak.

 

[velvetfoot]

ok, sorry

 

[maple1]

Call Garn & ask them.

BTW as an aside, when I measured what I considered to be usable space inside my firebox, I got a number that was about 1/3 less than what was indicated in the specs. For whatever that's worth.

 

[mike van]

Jay, first thing, it would be impossible to fill every cubic inch of anyones firebox with wood. Even if you use sawmill cut 4x4s, you'll still have gaps. Only 14 loads per cord is ridiculous. I've been burning one since the end of December, in one of the coldest winters we've had in years [i'm 62, I remember a few] With 2 a day firings, a full cord will last me close to 3 weeks. Wood varies so from load to load or even between pieces in a load that true data [like from a slide rule] is impossible to get. Garn sells a good product, they have for years. But, by all means if you think they're trying to b.s. everyone, call them before calling them out.

 

[heaterman]

BTU's generated is actually easily figured by looking at the output side of the equation.

If you take 1000 gallons (8,330 pounds) of water and drop it from say....... 190* down to 140* you have a 50 degree drop.
That equates to about 416,000 btu's.
Now if you want to get a rough handle on input and efficiency, weigh the wood required to bring the water back up to 190*.

It's easy yo mix units of volume (cords) and units of weight (tons) and get some weird numbers.

 

[jebatty]

... and the final steps of math: 50* x 8.33 lbs/gal x 1000 gal / 6040 btu/lb = 69 lbs of wood at 20% MC and 400F stack temperature, or put another way, 1.8 lbs of wood to raise a 1000 gal tank 1F. Start with that, and then apply an efficiency discount factor for the boiler based on specs or experience. This only works more or less exactly when all you are heating is a tank with plumbing and tank well insulated and a fully mixed tank to get a uniform temperature. You will have ongoing heat loss from the boiler, plumbing and tank, as well as wood variations and stack temp variations, which I wrap up into an efficiency factor discount.

Unless you are buying cords by weight (like the mills do where I live) and not volume, cords at best provide a very rough approximation of btu content, weight at known MC is much more accurate.

 

[JP11]

Funny story.. my uncle buys and sells logs all over the world. His first order with a certain foreign country had a whole bunch of logs and even a sawmill.

The loading at the port of their ship was not going well. After DAYS of trying to load the logs, they realized it wasn't even CLOSE to being big enough to fit the log load they had purchased, thus making the logs they did get very expensive. The company was flabbergasted.

They kept sputtering to my uncle about how many cubic meters the hold was in the ship. Finally fed up my uncle said..

WELL, as soon as you figure out how to POUR logs into that hold.. you let me know!



JP

 

[jebatty]

Batch load burning (burn a load, fire out and heat in storage, burn again when needed, clean out ash from boiler between burns as needed) is much different than continuous burning. The big Garn WHS 3200 in my tests was able to maintain about a 500,000 btuh output continuous burn for less than a day before it had to be allowed to burn out and then remove ash/coals before burning could be continued. But at that output rate, the big Garn was 80% efficient in transferring btu's from the burn to the system based on delta-T of supply/return and known gpm water flow.

IMO for non-lamda gasification boilers, real world operating efficiency in the 80-85% range is pretty realistic.

 

[maple1]

WELL, as soon as you figure out how to POUR logs into that hold.. you let me know!

First you get a big-azz chipper, then you....

 

[Jayf19]

Jay, first thing, it would be impossible to fill every cubic inch of anyones firebox with wood. Even if you use sawmill cut 4x4s, you'll still have gaps. Only 14 loads per cord is ridiculous. I've been burning one since the end of December, in one of the coldest winters we've had in years [i'm 62, I remember a few] With 2 a day firings, a full cord will last me close to 3 weeks. Wood varies so from load to load or even between pieces in a load that true data [like from a slide rule] is impossible to get. Garn sells a good product, they have for years. But, by all means if you think they're trying to b.s. everyone, call them before calling them out.

I completely agree with what you are saying, and I was not calling BS on the company, I was calling BS on MY calculations -lol. I knew something was off on my part and was requesting help to identify where I went wrong. Sorry if it seemed to be the other way around.


And you are right about this year being one of the coldest winter in years; I don’t remember the last time I’ve seen the weather go 60 below…

s actually easily figured by looking at the output side of the equation.

BTU's generated is actually easily figured by looking at the output side of the equation.


If you take 1000 gallons (8,330 pounds) of water and drop it from say....... 190* down to 140* you have a 50 degree drop.

That equates to about 416,000 btu's.

Now if you want to get a rough handle on input and efficiency, weigh the wood required to bring the water back up to 190*.


It's easy yo mix units of volume (cords) and units of weight (tons) and get some weird numbers.

Thanks for the input heaterman. I'm still trying to design the system I intend to install this summer; so right now all the info that I have is based on what I read and the calculations I'm able to make. I don’t have access to hard data and so I will need to look further into the weight of my wood supplies and determine how much I can fit in the firebox of whatever .

... and the final steps of math: 50* x 8.33 lbs/gal x 1000 gal / 6040 btu/lb = 69 lbs of wood at 20% MC and 400F stack temperature, or put another way, 1.8 lbs of wood to raise a 1000 gal tank 1F. Start with that, and then apply an efficiency discount factor for the boiler based on specs or experience. This only works more or less exactly when all you are heating is a tank with plumbing and tank well insulated and a fully mixed tank to get a uniform temperature. You will have ongoing heat loss from the boiler, plumbing and tank, as well as wood variations and stack temp variations, which I wrap up into an efficiency factor discount.


Unless you are buying cords by weight (like the mills do where I live) and not volume, cords at best provide a very rough approximation of btu content, weight at known MC is much more accurate.

Great information! I shall look further into this and add the info to my calculations.

 

[Boil&Toil]

As far as I recall, Garn specifically mentions leaving space in the firebox - my recollection is that it's supposed to be about half-empty when "filled" (not saying people don't overdo that, but that's my recollection from when ours was installed and we were getting informed about how to load it properly.)

Edit - another one of the lies [my workplace] told us, per actually digging up a Garn manual, which says you can basically pack it full. I'm not sure who's supposed to be benefitting from the BS they fed us here, but it's all too typical.

 

[flyingcow]

Jayf19, I may have missed it, is this a new build? Or are you looking to burn wood to replace oil/propane/etc?

If you are replacing oil/propane/etc is there a history of useage?

 

[Jayf19]

Jayf19, I may have missed it, is this a new build? Or are you looking to burn wood to replace oil/propane/etc?

If you are replacing oil/propane/etc is there a history of useage?

It's a new build; replacing a standalone wood stove which was poorly designed, combined with electric heating. I also only have 3 months history (Dec-Feb)...

 

[SmokeEater]

1 cord of wood = 8x4x4 :. 128 cubic feet
128 cubic feet is 957.5 gallons

The Garn Jr has a combustion chamber of 110 gallons
Fill'er up and you get 8.7 loads per cord (957.5 G / 110 G)

1 cord of Tamarak is 19,500,000 BTU's

1 wood load should give 1,792,167 BTU @ 80 % efficiency (advertised efficicency) (19.5M / 8.7 loads * 80 %)
or 1,568,146 BTU's @ 70 % efficiency (19.5M / 8.7 loads * 70%)

------------------------------

The nominal storage that comes with the Garn Jr is 980 gallons, so with a delta T of 70 degrees for the storage, the heat load to bring the water from 120 to 190 is 548,800 BTUs (70 degrees * 980 gallons * 8 BTUs).

Conclusion : Something is off; way off. Can anyone enlighten me ?

Jay, my "wood book" says that dry Tamarack has a heating value of 20.8 mmBtu/cord. If the wood was air dryed would it be about 20% mc? If so then a cord might have more like 16.69mmBtu/cord. Also a cord of wood is 128 cubic feet and your firebox has a volume of 9.4 cubic feet. If you put wood in the Garn at the same density as it was piled into the cord then it might take 128 cubic feet/cord divided by 9.4 cubic feet/load. The number of loads would be 13.6 loads or 7.3% of the cords 16.69mmBtu/load or at 80% eff, the Garn should receive about 981,000 Btus.

 

[Jayf19]

Jay, my "wood book" says that dry Tamarack has a heating value of 20.8 mmBtu/cord. If the wood was air dryed would it be about 20% mc? If so then a cord might have more like 16.69mmBtu/cord. Also a cord of wood is 128 cubic feet and your firebox has a volume of 9.4 cubic feet. If you put wood in the Garn at the same density as it was piled into the cord then it might take 128 cubic feet/cord divided by 9.4 cubic feet/load. The number of loads would be 13.6 loads or 7.3% of the cords 16.69mmBtu/load or at 80% eff, the Garn should receive about 981,000 Btus.

Pretty much the same conclusion I came to, except I didn't factor in impact of moisture. All in all, I guess it depends on how full the loads were for their calculations. I emailed a rep to get more info on how they came up with the advertised numbers.