BTUH: Proposed stove sizing guide for a 12 hour tending interval

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lsayre
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Post by lsayre » Thu. Oct. 26, 2017 5:55 am

A few new rule of thumb guidelines:

1.5 x Homes required (coldest day) BTUH Input/1000 = Minimum suggested firebox capacity in Lbs. of anthracite

2.1 x Homes required (coldest day) BTUH Output/1000 = Minimum suggested firebox capacity in Lbs. of anthracite

Or if rearranged a bit:

Firebox capacity in lbs. of anthracite x 1000 / 1.5 = A respectable actual maximum BTUH Input rating for your stove

Firebox capacity in lbs. of anthracite x 1000 / 2.1 = A respectable actual maximum BTUH Output rating for your stove

The goal here is to allow for sufficient coal remaining in the stove at the end of 12 hours to reliably be able to shake down, reload, and regain a new 12 hour fire. You can certainly push a stove harder than this to some degree (coal delimited), but you may not be able to reliably and consistently do it while tending at 12 hour intervals.

As I always say: All of the heat you are going to get out must be in the coal to begin with. No magic here. Some stoves may do a bit better, and some a bit worse, but in all likelihood, within about +/- 10%.

 
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Lightning
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Post by Lightning » Thu. Oct. 26, 2017 3:37 pm

lsayre wrote:
Thu. Oct. 26, 2017 5:55 am
Firebox capacity in lbs. of anthracite x 1000 / 2.1 = A respectable actual maximum BTUH Output rating for your stove

110 pounds (capacity) x 1000 / 2.1 = 53280 BTUs per hour?

 
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lsayre
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Post by lsayre » Thu. Oct. 26, 2017 3:45 pm

As an assistant here, a fireboxes capacity in lbs. of anthracite should be right close to:

0.033 x L" x W" x H" (for rectangular)

0.033 x 3.14156 x radius" x radius" x H" (for round)
Last edited by lsayre on Thu. Oct. 26, 2017 3:57 pm, edited 1 time in total.

 
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lsayre
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Post by lsayre » Thu. Oct. 26, 2017 3:50 pm

Lightning wrote:
Thu. Oct. 26, 2017 3:37 pm

110 pounds (capacity) x 1000 / 2.1 = 53280 BTUs per hour?
Yes! And that's actually quite a lot of output BTUH's. At -17 degrees F. outside I can keep my house warm on about 7,000 less than that. Actually it's 52,380 though....

Any need for more output than that might drop you into the range of tending every 8 to 10 hours rather than 12.

Manufacturer ratings generally seem to fall in the range of twice as many BTUH (or sometimes more), but I'm trying to be honest here.

 
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Post by Lightning » Thu. Oct. 26, 2017 4:04 pm

Cool, I agree with your formulas. Only on the coldest days do I break 100 pounds per day so would it be safe to say that my furnace is sized pretty close to fit heating needs?


 
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Post by Lightning » Thu. Oct. 26, 2017 4:06 pm

I usually break a 100 pounds per day maybe 2-3 days per year.

 
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lsayre
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Post by lsayre » Thu. Oct. 26, 2017 4:14 pm

I'd guess that to achieve most manufacturer ratings you would need to burn at a rate that requires tending every 6 hours. One exception is for European Union (EU) sourced stoves, which tend to be rated far more accurately, and just per chance generally come in at ratings which are about half of those found on this side of the pond.

 
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Post by lsayre » Thu. Oct. 26, 2017 4:21 pm

Lightning wrote:
Thu. Oct. 26, 2017 4:06 pm
I usually break a 100 pounds per day maybe 2-3 days per year.
I've only done it once. The day that I burned about 105 - 110 lbs. when it hit -17 deg. here 3 winters ago.

 
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Post by lsayre » Thu. Oct. 26, 2017 4:58 pm

This post I made awhile ago seems appropriate to link here.

Post by lsayre - The Worlds Tiniest/Easiest Residential Heat Loss Calculator?

My simplistic approach yields as the per sq-ft requirement:

5,000 HDD's x 0.004 = 20 Output BTU's per sq-ft
5,500 HDDS x 0.004 = 22 Output BTU's per sq-ft
6,000 HDD's x 0.004 = 24 Output BTU's per sq-ft
6,500 HDD's x 0.004 = 26 Output BTU's per sq-ft (I'm about here)
7,000 HDD's x 0.004 = 28 Output BTU's per sq-ft
7,500 HDD's x 0.004 = 30 Output BTU's per sq-ft
8,000 HDD's x 0.004 = 32 Output BTU's per sq-ft

 
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lsayre
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Post by lsayre » Mon. Oct. 30, 2017 6:12 am

Working this out for the 75,000 BTUH input (manufacturers rating) "ComfortMax 75" (my stove), I get:

Firebox L x W x H = 12" x 18" x 8" (where 8" is the level height of the coal bed) = 1,728 cubic inches of firebox capacity

1,728 x .033 = 57 lbs. of coal to fill the firebox

57 x 1,000 / 1.5 = 38,000 BTUH input

And lastly:

38,000 x 2 = 76,000 BTUH input (vs. the manufacturers rating of 75,000 BTUH input)

This correlates rather well with the assumption that the manufacturers BTU rating for my stove is about twice what is practical.


 
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Post by KingCoal » Mon. Oct. 30, 2017 11:03 am

yep, it worked out about dead on for my DS too.

 
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Post by ddahlgren » Wed. Nov. 01, 2017 1:00 am

The original post makes sense with an efficiency of 72%. Question how much coal is presumed left to shake and load reliably?

 
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Post by lsayre » Wed. Nov. 01, 2017 4:43 am

ddahlgren wrote:
Wed. Nov. 01, 2017 1:00 am
The original post makes sense with an efficiency of 72%. Question how much coal is presumed left to shake and load reliably?
If you are actually getting about 71% efficiency for 12 straight hours it works out to be pretty close to 2/3 of the coal burned and 1/3 remaining at the end of 12 hours. But many (to most?) coal stoves likely do not achieve long term 71% efficiency in the real world, so for them it could be as little as 1/4 of the initial fill remaining unburned after 12 hours. And with the inevitable ash and partially consumed coal (the coal that is actively burning) left behind with each shaking it could go a bit lower yet (as ash and partially burned coal effectively shrink the firebox capacity).

Is 1/4 of the initial coal remaining sufficient to reliably and repeatably restore a coal fire within a reasonable time-frame? Remember that we are talking about running the stove hard and hot for 12 hours in order to get to the point of only ~1/4 remaining.

 
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Post by ddahlgren » Wed. Nov. 01, 2017 10:56 am

I have noticed the diminishing firebox game. The first 4 or 5 tendings are text book and by the time you get to #20 there is a lot more fiddling around to clear the grates. Also with my house and plaster walls the first 2 days need to run pretty hot to get the plaster warm then back way off.

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