Wood Stove vs. Coal Boiler efficiency

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lsayre
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Post By: lsayre » Fri. Dec. 29, 2017 8:42 am

I've been running a wood stove along with my boiler for several days, and from what I can tell based upon coal projected to have been burned (for the case of not burning wood) vs. coal actually burned, and combined with compressed sawdust wood blocks being burned, the wood stove is only around 55% overall efficient, if I assume that my coal boiler is overall 70% efficient. The amount of wood blocks I've burned so far should have saved me 152 pounds of coal, but in actuality have only saved me 119 lbs. of coal. A difference of 32 lbs.

Nearly all of this discrepancy (27 of the 32 lbs.) I can chalk up on paper directly to the wood stove keeping my living room zone at 75 degrees on average vs. 70 degrees for the coal boiler. If the wood stove could somehow maintain the zone it is heating at 70 degrees, the use of wood alongside of coal would in the end be much closer to a wash on a BTU's for BTU's basis.

The wood stove is clearly greater than 55% efficient, but it consistently overheats (75 degrees vs. 70 degrees), and that measures out as a large efficiency strike against it from a cost perspective (albeit that from a comfort perspective 75 degrees does feel much nicer than 70 degrees).

The bottom line is that I would expend roughly 27% more BTU's if burning wood to heat my home. So even if (as is the case for me) the BTU to BTU cost of wood blocks vs. coal is nearly identical, wood loses out to coal. Plus it loses out in the expended effort department big time.
Last edited by lsayre on Fri. Dec. 29, 2017 12:14 pm, edited 1 time in total.


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hotblast1357
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Post By: hotblast1357 » Fri. Dec. 29, 2017 8:46 am

And how often are you tending the stove? Vs the boiler..

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Post By: Rob R. » Fri. Dec. 29, 2017 8:49 am

That is the difference between space heat and central heat.

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Post By: Pacowy » Fri. Dec. 29, 2017 9:10 am

I would expect a coal boiler to have efficiency advantages over a wood stove due to use of water as a heat exchange medium and (generally) greater heat exchange surface area.

Mike

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Post By: Rob R. » Fri. Dec. 29, 2017 9:15 am

Me too. Wood is tricky - so much of the btu content is on the volatiles, that you have to fire it very hard to have good combustion efficiency.

The efficiency of a wood burning appliance that is able to do this and harness the heat from the flue gasses is actually quite good - e.g. The AHS Wood Gun.

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Post By: Dave 1234 » Fri. Dec. 29, 2017 9:15 am

Tread very lightly Larry !

When your faithful coal boiler finds out you are putt'in the wood to a stove............. things could get ugly!!

Dave :lol: :clap:

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lsayre
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Post By: lsayre » Fri. Dec. 29, 2017 9:16 am

hotblast1357 wrote:
Fri. Dec. 29, 2017 8:46 am
And how often are you tending the stove? Vs the boiler..
Wood stove, roughly every 5.5 hours on average. Boiler every 2 - 3 days when it is this cold.

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lsayre
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Post By: lsayre » Fri. Dec. 29, 2017 9:25 am

When I throttled both of the the air inlets and the exhaust flow diverter way down on a relatively fresh load of wood in an attempt to keep the stove temperature from spiking and maintain at a lower temperature, and at achieving longer burn times, I actually experienced a rather quite large puff-back from the wood. That was the end of the experiment in starving the wood of air in order to force it to burn cooler. I called DS Machine and explained it to them, and they told me that for wood I can't cut way back on the over the fire air (they called it the air-wash) like I would do for coal, and in fact they recommend running the over the fire air wash control wide open with wood, and especially with compressed sawdust blocks that have only 5% moisture. I've left it mainly open since that conversation, and no more puff-backs, but at the expense of a much hotter stove. It tends to run at 550 - 560 degrees at its hottest spot when set to avoid puff-backs.

DS Machines alternative to leaving the air wash control wide open was to place a paper clip on my bimetallic thermostat flapper to keep it from being able to fully close. They emphasized that this is was not as good a choice, but would likely also work to prevent wood puff-backs.

On the positive side, the window in the door is staying clean since I stopped throttling down on the air wash control.


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Post By: Lightning » Fri. Dec. 29, 2017 9:32 am

Is there no middle ground with the trying to control the burn rate?

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Post By: lsayre » Fri. Dec. 29, 2017 9:32 am

Dave 1234 wrote:
Fri. Dec. 29, 2017 9:15 am
Tread very lightly Larry !

When your faithful coal boiler finds out you are putt'in the wood to a stove............. things could get ugly!!

Dave :lol: :clap:
Yes, I can see things starting to fail already in retribution. :lol:

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Post By: lsayre » Fri. Dec. 29, 2017 9:34 am

Lightning wrote:
Fri. Dec. 29, 2017 9:32 am
Is there no middle ground with the trying to control the burn rate?
Not that I can see for compressed sawdust wood blocks, unless you don't mind an occasional boom. I'm certain that I could throttle it down once I was convinced that the volatiles had burned off. I'm also certain that I could open it up further and see 700 degrees, so 550 degrees is OK by comparison.

Don't ever let anyone tell you that you can't get puff-backs from wood.

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Post By: Dave 1234 » Fri. Dec. 29, 2017 9:44 am

lsayre wrote:
Fri. Dec. 29, 2017 9:34 am
Not that I can see for compressed sawdust wood blocks, unless you don't mind an occasional boom. I'm certain that I could throttle it down once I was convinced that the volatiles had burned off. I'm also certain that I could open it up further and see 700 degrees, so 550 degrees is OK by comparison.

Don't ever let anyone tell you that you can't get puff-backs from wood.
Larry, IDK how much wood burning you have done, but I always marveled at the glowing stove pipe we would get on a really cold night here. I never looked to see what the temp of glowing tin is but I know a sh............t load of heat was going up the stack.

Dave

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lsayre
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Location: N/E Ohio, between Medina and Wadsworth

Post By: lsayre » Fri. Dec. 29, 2017 9:58 am

I should clarify that after the volatiles are gone the stove settles down to 400 - 450 degrees for a couple hours, and when it falls from there it is time to feed it again and drive it back up to ~550 - 560 degrees again.

Another plus for the compressed sawdust wood blocks is their unbelievably low ash content. I'm initially guessing at ash removal once a week, and even at that it won't be much ash to dispose of from a volume or weight perspective. Perhaps half a hods worth.
Last edited by lsayre on Fri. Dec. 29, 2017 10:39 am, edited 2 times in total.

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Post By: Lightning » Fri. Dec. 29, 2017 10:01 am

lsayre wrote:
Fri. Dec. 29, 2017 9:34 am
Don't ever let anyone tell you that you can't get puff-backs from wood.
Oh I know lol, I've had coworkers tell me about frantic wives calling them because the wood stove was violently huffing at them :lol: of course that's when I introduce the coal card haha

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Post By: franco b » Fri. Dec. 29, 2017 10:15 am

Half the heat in wood is in the gas. Not burning it means smoke, but burning it overwhelms the heat exchange.

The problem was solved hundreds of years ago with massive masonry heat exchange, or in modern stoves with pellet feed a little at a time steadily.


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