Efficiency – What does it really mean?

[TMonter]

I see a lot of people here argue about efficiency and it comes up a lot in fan discussions and such so I figured I’d cast a little light from a combustion engineer’s standpoint.

There are two types of efficiency that you should really care about, heating efficiency and combustion efficiency. These while linked are separate efficiencies.

Combustion efficiency is how completely you are burning the piece of wood in your stove. This means if you were to do a stack test on the stove while burning, the stove with the least amount of un-combusted material (except for ash) leaving the flue would be the most combustion efficient. However, this is NOT the same as heating efficiency.

Heat efficiency is how much of the wood’s energy actually stays in your house versus how much goes out of the stack. More efficient stoves have lower stack outlet temperatures. This is because the sensible heat loss out of the flue is the biggest cause of stove inefficiency.

Be aware, good seasoned cordwood cannot really be burnt above an efficiency of 77-78%. There are limitations on condensing flue gas (creosote), complete combustion/good mixing and moisture losses that prevent better heating efficiencies.

Total Heating Efficiency = Heat Input – Dry Flue Gas loss – Moisture Loss – Unburnt Carbon Loss – Moisture in air loss.

The only real way of increasing heating efficiency is to lower your stack temperature assuming your stove has good combustion efficiency.

 

[Roospike]

I asked the question as to how a stove is sold and i assume its combustion efficiency as per the % the stove company gives you ( not the EPA % # )........I wasn't given a answer.

 

[TMonter]

That would be my guess Roo. When we get our combustion analyzer at work calibrated in a couple weeks I plan on climbing on my roof and seeing what the emissions from my stove look like. I'll report back on what I find.

 

[Roospike]

Awesome .......... We love stuff like that.

I'm sure there would be just so many varables to figure "Heat efficiency" but with the same test load to figure "Combustion efficiency" you would / should get a Heat efficiency answer.

I think Heat efficiency % is the numbers we would ALL LOVE TO SEE as we dont see them now.

 

[begreen]

I agree about combustion efficiency. However, if one were to stick to strictly the stove and not the home environment, I would expect heating efficiency to be interpreted as how well the stove radiated or convected the btus released by burning into the room instead of up the stack. The subject of how well the house retains the heat injected into the interior envelope is a separate issue IMHO.

 

[Roospike]

I agree about combustion efficiency. However, if one were to stick to strictly the stove and not the home environment, I would expect heating efficiency to be interpreted as how well the stove radiated or convected the btus released by burning into the room instead of up the stack. The subject of how well the house retains the heat injected into the interior envelope is a separate issue IMHO.

I agree , and there would be no real world way to figure interior envelope efficiency .

 

[TMonter]

The subject of how well the house retains the heat injected into the interior envelope is a separate issue IMHO.

Agreed. The house efficiency is a separate issue. If you add 50k Btu/hr into a room and the room is poorly insulated it's not the stove's fault it can't keep up.

Stove heating efficiency is heat delivered - heat lost to other factors such as flue gas sensible loss.

 

[pistonslap]

I agree completely. My house is brick over terracotta block and plastered walls. My only insulated walls are my kitchen and my son's room. When it is above mid to high 20's my wood furnace keeps the house well heated. Below that, the stove and chimney temps are the same, but the only warm rooms are the insulated ones.

 

[elkimmeg]

When i did the Vc tour I was allowed into the testing lab there a sequoia pre fab was set up for expermentation it seems the engineers were trying to solve the air wash issues VC went out and purchaces common firewood to test it. It was also connected to the combustion analyzer. they use filters to capture the smoke particulates for EPA emissions testing. there are thermo coupler attached everywhere to anylize stove and flue gasses temps. they even showed filter of past emessions stoves and one that did not pass. The most impressive test is the glass breakage test
where they hold a 3 lb steel ball attached toa rope pick it up waste high and let it go It the glass breakes or cracks it fails the UL testing. Asz you can imagine that's susposed to allow for loading a log and smacking the glass. UL test are taken very serious and very costly to test stoves

 

[Andre B.]

Question: Efficiency – What does it really mean?
Answer: Efficiency is the ratio of the effective or useful output to the total input in any system, inputs and outputs must be measured in the same units.

It can be applied to any input to any system, just happens that energy efficiency is the most common.

Total Heating Efficiency = Heat Input – Dry Flue Gas loss – Moisture Loss – Unburnt Carbon Loss – Moisture in air loss.

This equation should be.
Total Heating Efficiency = (Heat Input – Dry Flue Gas loss – Moisture Loss – Unburnt Carbon Loss – Moisture in air loss) / (Heat Input)
or
Total Heating Efficiency = (Useful Heat Output) / (Heat Input)

 

[Martin Strand III]

At the risk of opening old wounds, please consider:

The amount of the heat that is actually transferred into the room depends on a number of factors:

1. Combustion efficiency. How completely an appliance burns the wood

2. Heat transfer efficiency (thermal efficiency). How much of the heat generated in the firebox escapes through the chimney into the atmosphere (stack loss) depends on the firing strategy and the thermal mass of the heater. The combustion efficiency multiplied by the heat transfer efficiency, is the
overall efficiency, but this term still does not fully characterize the true efficiency of a heater.

3. Linearity of output. For clean combustion, wood requires a brisk, hot fire which results in short burn times. If the heater has no storage capacity the fire must be damped, otherwise all the heat is transferred into the room during the burn time. As a result, the room becomes overheated while the fire is burning and under heated after the fire has died down. An overheated room loses more heat to the environment because of the higher temperature differential between the room air and the outdoor air.

4. Partial charge efficiency. Many stoves do not handle small fuel charges efficiently because high temperatures are needed to obtain non-smoldering combustion.

And how efficient your wood heater operates depends on 2 more factors:

(1) Installation - location on outside v inside wall. Heater too big for house? Flue draw?
(2) Operation - Is wood green? Firebox load? Adequate air?

Your operating technique accounts for the largest variations in your woodstove’s heating efficiency.

Aye,
Marty

Grandma used to say, "Repetition is a valid learning aid."

 

[Jags]

When we get our combustion analyzer at work calibrated in a couple weeks

. oooohhhhh, toys. I WANNNA PLAY! I would love to see how my stoves performance changes with different burn techniques (i.e. air open, closed, 500 deg stove top v.s. 650 degree stove top,etc.). I can see graphs and charts dancing through my head now.

 

[Rhone]

I think Heating efficiency is too random. It's one thing if the fuel is of a consistent manner and dealt with mechanically but it isn't. The fuel is random, controlled by people who's experience and knowledge are technique are vastly different, and with external random influences (weather, wind, even gravity as the pile may fall on itself extinguishing flames). I've mentioned this previously, take my wife and I for example. Same unit, same install even. She uses about 20 pages of newspaper and a single piece of kindling to get the fire started, she keeps the air at max until she's warm and then shuts it down to as low as possible when she's warm enough at which point the fire smolders, then turns it back on high when she wants heat again. She also throws in the wood without care as to placement and air flow dynamics. Me, I use no paper to start my fire. I load the unit up half way with wood, put kindling on it, rip off a thin piece of oak fiber, light the oak fiber, and hold it under the kindling until it gets going. I then put my wood with plenty of air space, and my secondary burn gets going within usually 5 minutes. I then slowly turn down the air, and I know I can't turn it down below about 30-40% depending on what I see. Case in point, when I pull onto my street and see the neighborhood covered in smoke I know my wife is home early. Her fires raise the house temp about 4-5F my fires are 6-10F. Pretty big discrepancy in efficiency between her and I and that's just between two people using the same unit. Imagine the difference between two people in different installs using different units.