Efficiency question

[GrumpyDad]

I was always told, fireplaces are at best 15% efficient w/ many actually being in the negative.
Meaning 15% of what you burn goes toward heating a space and the rest is waste.

So if stoves are saying they are upwards of 70something % efficient, how does that occur when a STT of say, 500 degrees and a cat probe reads over 1000 degrees...wouldnt that mean that 50% is translated to heat and the other 50% just goes flying out of the chimney?

 

[begreen]

So if stoves are saying they are upwards of 70something % efficient, how does that occur when a STT of say, 500 degrees and a cat probe reads over 1000 degrees...wouldnt that mean that 50% is translated to heat and the other 50% just goes flying out of the chimney?

This is comparing heating efficiency vs combustion efficiency. The catalyst is within the firebox envelope. It may be 1000º, yet the flue gas temperature may be only 400º. The fireplace is pulling a large volume of room air up the chimney, while the wood stove is sending a comparatively small volume up the 6" stack.

 

[ABMax24]

Your on the right path, kinda.

Theoretical efficiency is calculated by the temperature and oxygen content of the flue gases when using a known fuel of known moisture content. The higher the flue temperature and the more oxygen in the flue gas the lower the efficiency.

Cat temperature doesn't really play into it.

 

[stoveliker]

You're talking about energy flow - where does it go. That is what the efficiency numbers mean: x percent gets pushed up the chimney and 1-x percent gets deposited into the room.

Energy flow equations contain the surface area thru which that energy flows, as well as the temperature (difference).
The stove top is much larger than the cat, so for the amount of energy that flows from a small cat to the stove top and then into the room, you can have much lower temps on the stove top reaching the same energy flow, because the surface area is much larger.

The fact that the stove top is metal (fairly good heat conductor) helps there as it spreads the heat to a larger area to radiate into the room.

So simply comparing temperature values does not work.

 

[Ashful]

Already three good answers, so I can't add anything useful, other than to point out that 70% is a piss-poor stove by today's standards. Think more like 80 - 83%.

 

[stoveliker]

Numerically true. But in practice, I think 10% difference in efficiency is the start of where one could notice differences in wood usage per season. Any efficiency difference below 10% will be tough to measure using the shed method imo (i.e. the volume emptied from a wood shed after a season).

So piss-poor. But only just noticeable.

 

[Ashful]

Good point, stoveliker.

WRT fireplaces and heating, it's been quite a subject this week, so I'll repeat what I've already said in other threads. Fireplaces can actually heat the room they're in pretty well, but due to the volume of make-up air they require, they cause a net-negative heating effect in the many rooms of your house not immediately adjacent to the (even secondary) radiant heating of the fireplace.

So, while the efficiency measured directly in front (line of sight) might be 15%, I assume that you'll never see positive efficiency as measured from any space not in the line of sight of said fireplace.

 

[begreen]

Already three good answers, so I can't add anything useful, other than to point out that 70% is a piss-poor stove by today's standards. Think more like 80 - 83%.

70% is not terrible. Note that the Ashford 30.2 squeaks in for the tax credit at 76% and the Princess Insert by a hair at 75%.

 

[Ctwoodtick]

I was always told, fireplaces are at best 15% efficient w/ many actually being in the negative.
Meaning 15% of what you burn goes toward heating a space and the rest is waste.

So if stoves are saying they are upwards of 70something % efficient, how does that occur when a STT of say, 500 degrees and a cat probe reads over 1000 degrees...wouldnt that mean that 50% is translated to heat and the other 50% just goes flying out of the chimney?

If you ever want to see a good test of open fireplace inefficiency, hang up a sheet in a doorway. We used to do this to hold heat back in the day. The bottom of that sheet gets pulled constantly a good 6 inches toward the direction of the fireplace.

 

[EbS-P]

Already three good answers, so I can't add anything useful, other than to point out that 70% is a piss-poor stove by today's standards. Think more like 80 - 83%.

Anyone want to do the math and figure out what efficiency hit you take by burning 23%MC vs 17%. I’m beat. Tough week.

 

[begreen]

Anyone want to do the math and figure out what efficiency hit you take by burning 23%MC vs 17%. I’m beat. Tough week.

Here's a more extreme example

The importance of seasoned wood - Dry vs Wet

This is a somewhat extreme case comparing dry wood @ 16% moisture and wet wood @38%. 38% is just under dripping wet or like green wood that was split recently. Adding a second video. Short and to the point. Further thoughts: https://hearth.com/articles/chimneysweep/howetwd.htm

www.hearth.com

 

[Ashful]

70% is not terrible. Note that the Ashford 30.2 squeaks in for the tax credit at 76% and the Princess Insert by a hair at 75%.

No, that is misleading. You are posting the HHV efficiency, and very few people will ever run a Princess at HHV for most of the burn.

What’s the stat? 90% if the people run at or near LHV 90% of the time? LHV on Princess was once posted at 86% efficiency. I think the more recent test method dropped it a few points, but BK doesn’t seem to publish LHV on their site anymore (or at least not that I saw in a few seconds browsing on a phone screen).

This site still seems to have the old numbers:

wood BLAZE KING Princess Insert | Hearth Products | Great American Fireplace in Menomonie, WI

www.greatamericanfireplace.com

Edit: just noticed you said “insert”. Never looked at them, so maybe they are that bad? The comment to which I was responding, and indeed my own post said “stove”, not “insert”.

 

[ABMax24]

No, that is misleading. You are posting the HHV efficiency, and very few people will ever run a Princess at HHV for most of the burn.

What’s the stat? 90% if the people run at or near LHV 90% of the time? LHV on Princess was once posted at 86% efficiency. I think the more recent test method dropped it a few points, but BK doesn’t seem to publish LHV on their site anymore (or at least not that I saw in a few seconds browsing on a phone screen).

This site still seems to have the old numbers:

wood BLAZE KING Princess Insert | Hearth Products | Great American Fireplace in Menomonie, WI

www.greatamericanfireplace.com

Edit: just noticed you said “insert”. Never looked at them, so maybe they are that bad? The comment to which I was responding, and indeed my own post said “stove”, not “insert”.

LHV and HHV refers to the way efficiency is calculated with regard to the water present in the fuel, it has nothing to do with the burn rate of the appliance.

lhv vs hhv - Google Search

 

[Ashful]

“Nothing”? No correlation, whatsoever?

And do you agree 70% advertised efficiency is a good stove by today’s standards, too?

 

[stoveliker]

I thought It is strictly the state of water present in the combustion products, not water present in the fuel. And that does correlate with burning high or low.

 

[begreen]

The EPA ratings are done over a series of runs testing emissions over a range of burns from the lowest to the highest. Different homes are going to need different amounts of heat depending on outside temps and the heat loss of the house. One needing good heat in the winter may be running the stove at or close to maximum output. We see this frequently with people reporting their burn times dropping down to 8 hrs. for a cat stove that on very low output can achieve 24+ hrs burn time.

The EPA testing of the Ashford 30.2 at the high burn rate listed the HHV at 68.51% and LHV at 74.05%. At the medium burn rate, the same stove tested at 73.41% HHV and 79.34% LLV. At the lowest burn rate the numbers climbed to a respectable 81.44% HHV and 88.02% LLV. Trouble is, at that rate the stove was only measured to be putting out 11,553 BTU/hr. or about what 2 electric space heaters would put out. At 45º that would not be enough to heat our house, never mind when it's 20º outside. My guess is that in our house a cat stove would be running at fairly high output for most of our burning season and our numbers would be in the 70% range.

Another thing that favors the BK test results is that high output is limited by the thermostat. When comparing the high-heating run, the Ashford 30.2 is putting out only about half the heat that a PE Summit is. So yes, the PE stove then is testing at 70%, but with twice the heat output. At low output, (14,303 BTU/hr) the Summit is right under the 75% threshold. This is in no way putting down the performance of either stove, but it does illustrate the limitations of testing, tax credits, and marketing based on these numbers.

Test Reports - Blaze King

www.blazeking.com

 

[stoveliker]

Well said imo.

One nitpicking: This (high heating run limited by the thermostat), I would say "is limited by the air intake design". Because the thermostat is designed firstly to provide an even heat output. It is not there primarily to limit the output per se?

Though the thermostat is indeed used to enforce that.

But what I really want to say is that this would be a good reason for a hybrid stove? I presume the limit is because the cat doesn't do well enough at higher flow rates to meet EPA rules. Tube stoves scale their "cleaning up the exhaust"-activity better with flow rate, I think, due to the draft dependent supply of secondary air.

I also do wonder about your "I'd run not low" in winter because I thought you had fairly mild winters there? (Unless a less than ideal home?) Because other research has shown (and bkvp has posted the paper) that most folks run medium to low most of the time. Then being In a fairly mild winter season area, is be surprised you'd run so high.

I truly wonder what the influence is of the necessary cycling of stove output for a no thermostatically controlled stove does to the settings. A persistent even output allows to run a but lower on the dial than having lower output times needing a higher outputs catch up time as well.

Anyway, back to doing leaves.

 

[ABMax24]

“Nothing”? No correlation, whatsoever?

And do you agree 70% advertised efficiency is a good stove by today’s standards, too?

70% isn't great, but it's not terrible. Reality is even most natural gas appliances are limited to about 82% efficiency before condensation in the flue becomes an issue, at least until you get into the PVC pipe vented varieties.

My stove is in the low 70's % efficient. I'd love to try an 80%+ efficient BK in my house, but I'm not convinced I wouldn't plug the cap with creosote icicles at -20c when operating on low-med burn. Or have condensation or even rain in my chimney. I form icicles on my cap as is. Granted I'm a special case with 39ft total flue length (max 40ft by code here) with 36 vertical feet from stove collar to cap.

I daily drive a 1 ton diesel truck that on a good day converts 30% of the fuel to mechanical energy, I'm not loosing sleep about a possible 10-15% wood savings with my current wood stove. I'd switch to a BK because of the thermostat and reduced emissions more than anything.