EFFICIENCIES OF WOOD STOVES IN THE US AND IN EUROPE

This article explains in detail how the efficiencies of wood stoves are calculated using the higher heating value or lower heating value of the fuel. In case of condensing wood stoves, the higher heating value must be used.

 

The US uses higher heating value, HHV, as the basis for calculating efficiency, Europe uses lower heating value, LHV.

 

This leads to European stoves having calculated efficiencies greater than US stoves, which deceives lay people in the US, because they end up thinking the European stoves "are more efficient".

In the near future, the EPA will be requiring European stoves sold in the US be tested in the US, using HHV, so consumers can compare apples to apples.

Results of Analysis

 

In the US, efficiency is 554141/679400 = 81.56% at 20% excess air, 265F at stove exit

In Europe, efficiency is 553904/634370 = 87.32% at 20% excess air, 265F at stove exit

 

- The above efficiencies are for 100% combustion, as in a calorimeter test. See Appendix

 

- EPA efficiencies, based on steady firing conditions during laboratory testing; conducted by professionals; using carefully prepared, cribbed wood; using new, clean stoves, would be less, because 100% combustion would not be achieved.

 

- Real-world efficiencies achieved by Joe and Jane User, using various wood sources; using wood with various moisture content, MC; using not clean stoves; using aging stoves, and having daily start-ups and shut-downs, would be significantly less. Decreased efficiency means increased wood consumption and increased PM10 particulate and volatile organic carbon, VOC, emissions.

 

Summary	US	Europe
Wet wood, 20% MC, lb	100	100
Ash, lb	1	1
Dry wood, lb	79	79
Heat value, Btu/lb	8600	8030
Heat in, Btu	679400	634370
Heating and Evaporating water, Btu	89662	44869
Heating CO2, Btu	9032	9032
Heating air, Btu	26565	26565
Heat out, Btu	554141	553904
Efficiency, %	81.56	87.32

Analysis

Searching the internet for answers regarding US and European wood stove efficiency leads to endless confusion, because folks writing about the issue do not know the technical details, and basically make up their own versions! This analysis aims to achieve some clarity.

Heat Values

The available heat of fuel is measured in British thermal units (Btu), and each type of fuel has a specific Btu content per unit of fuel. By definition, one Btu is the heat required to raise the temperature of one pound of water by 1 degree F.

 

Natural gas has a heat value of about 100,000 Btu per therm; 100 cubic feet, CCF, is called a therm.

Propane has a heat value of about 92,000 Btu per gallon.

No. 2 heating oil has a heat value of about 140,000 Btu per gallon.

A kilowatt-hour (kwh) of electricity has a heat value of 3,412 Btu.

Oven-dry wood of any hardwood species has a heat value of about 8,600 Btu/lb.

Resinous softwood, such as shortleaf pine, has a heat value of about 9,050 Btu/lb.

 

Three Stages of Wood Combustion

It is difficult to evaluate available heat value of wood because of the complex process of obtaining heat from wood. Wood combustion occurs in three consecutive overlapping stages.

 

In the first stage, heat is actually absorbed and water in the wood is evaporated.

 

In the second stage, volatile matter is liberated and burned. The volatiles ignite, burn and give off heat at about 1,000 degrees F.

 

In the third stage, most of the volatile matter has been used up, and the surface of the remaining residue (charcoal) reaches a glowing temperature and burns when brought into contact with oxygen in the air. This combustion exposes additional surface area until the entire mass is consumed, leaving only ash.

 

Because heat is needed in the first stage of burning wood to remove water, the more water in the wood at the time you place it in your wood-burning unit, the less heat for space or hot water heating.

 

Chemical Composition and Growth of Wood

 

A tree, to grow itself, combines CO₂ and H₂O molecules to form C₆H₁₂O₆, glucose, a sugar.

The glucose is mixed with water taken up by the roots.

That mixture is drawn from maple trees to make maple syrup in Spring.

Glucose is the only molecule that photosynthesis can create!

Plants modify glucose to make all the other organic molecules on which lifeforms depend.

Plants often remove one H2O molecule from glucose to chain together long and strong molecules of C₆H₁₀O₅, cellulose, which is what nearly all wood is made of.

 

If we multiply the components (C, H, and O) by the known atomic weights, we calculate the weight composition of the cellulose.

Carbon 44.5%, Hydrogen, 6.2%, Oxygen, 49.3%.

 

If all wood were completely cellulose, this would be the exact composition of every piece of wood. But that is not the case. All plants and trees also create other organic molecules they need, not just cellulose, for their structures. So, even though every piece of wood is primarily cellulose, the actual percentage composition varies a little. Actual wood generally varies within the following ranges:

C, 49.5% - 53.1%; H2, 5.8% - 6.7%; O2, 39.8% - 43.8%

 

Our Wood, for analysis purposes

 

We will use 100 lb of wood, 20% MC, i.e., 80 lb of dry wood.

We assume our wood at C, 51%; H2, 6%; O2, 43%.

 

Freshly harvested wood contains about 50% water after it is cut, split and delivered

Firewood should be stacked for about 1 to 2 years, so winds can blow through the wood pile to achieve about 20% MC.

 

The more correct, and more useful way, to describe the MC of wood is to use the weight of the wet wood as the basis.

If 50 lb of wood initially had 40% moisture, there would be 30 lb of dry wood.

MC = (50, wet - 30, dry)/50, wet = 0.4. There are hand-held meters to measure MC.

 

All pieces of wood contain small amounts of materials that cannot burn, which create ash after burning. That unburnable material is rarely much over 1%. We assume our wood has 1% ash. Our 79 lb of dry wood has:

 

79 x 0.51 = 40.29 lb of C

79 x 0.06 = 4.74 lb of H2

79 x 0.43 = 33.97 lb of O2.

 

Theoretical Chemical Energy of Our Wood

 

Each lb of C will generate 14,100 Btu of heat energy, if completely oxidized into CO2; 40.29 x 14100 = 568,089 Btu.

Each lb of H2 will generate 61,000 Btu of heat energy, if completely oxidized into H2O; 4.74 x 61000 = 289,140 Btu.

O2 cannot oxidize and has no chemical energy content in it.

The total theoretical chemical energy of our wood is 857,229 Btu.

 

Table 1 summarizes the data.

 

Table 1/Energy	General	Our wood, 20% MC	Ash	Our wood, dry	Energy	Our wood energy
Element	Range, %	%	%	lb	Btu/lb, dry	Btu/79 lb
C	49.5 - 53.1	51		40.29	14100	568089
H2	5.8 - 6.7	6		4.74	61000	289140
O2	39.8 - 43.8	43		33.97	0	0
Total		100	1	79.00		857229

 

Real-World Energy of Our Wood

 

The theoretical heat value of our wood is 857229/79 = 10,851 Btu/lb, dry

The real-world heat value falls well short of this theoretical value.

The less-than-theoretical values likely are due to the energy required to break down the various organic compounds of the cellulose.

The generally accepted heat value of dry wood, based on calorimeter testing in laboratories, is 8,600 Btu/lb. It is called the higher heat value, HHV.

 

In the US, the heat value for our 79 lb of dry wood would be 79 x 8600 = 679,400 Btu, because the US uses HHV.

In Europe the heat value would be 79 x 8030 = 634,370 Btu, because Europe uses the lower heating value, LHV. See Appendix.

 

NOTE:

The EPA mandates 8600 Btu/lb be used for rating of wood/pellet stoves and wood chip boilers.

European authorities mandate 8030 Btu/lb be used for rating.

As a result, the efficiency percent of European heating units calculates to a higher value than US units, which tends to confuse/deceive US lay people who might want to buy a European unit, “because it is more efficient”.

 

Heating Water

 

We have to heat and evaporate water from three sources:

 

1) The water in 100 lb of seasoned wood; about 20 lb of water.

 

2) The water created by combustion of H2. The chemical reaction is 2 H₂ + O₂ --> 2 H₂O. Because we know the atomic weights of each, we have 4 + 32 = 36 lb of water, or 1 + 8 = 9 lb of water. Our wood has 4.7 lb of H2, thus 4.7 x 9 = 42.3 lb of water.

 

3) The humidity in the combustion air. We assume 648 lb of combustion air (calculated below) containing 10 lb of water vapor is used by the fire.

 

The total water is 20 + 42.66 + 10 = 72.66 lb.

 

The water is heated from the wood/air initial winter temperature, say 35F, to 212F, which takes 177 Btu/lb,

The water is evaporated, which takes 1050 Btu/lb

The water vapor is heated from 212F to the stove exit temperature of 265F, which takes about 7 Btu/lb. See note.

 

In the US, total heat required is 72.66 x (177 + 1050 + 7) = 89662 Btu. That heat disappears up the chimney.

 

In Europe, total heat required is 30 x (177 + 1050 + 7) = 37020 Btu, plus 42.66 x (177+ 0 + 7) = 7849 Btu, for a total of 44869 Btu. That heat disappears up the chimney. Heat of evaporation is not counted.

 

Europe decided the hydrogen-related water vapor is an inherent part of wood burning and beyond the control of heating unit designers/engineers.

 

Table 2 summarizes the data

Table 2/Heating water		35 F to 212 F	Evaporation	212F to 275 F	Heat
	lb	Btu/lb	Btu/lb	Btu/lb	Btu
Wood	20
H2 burning	42.66
Humidity	10
US	72.66	177	1050	7	89662
.
	30	177	1050	7	37020
	42.66	177	0	7	7849
Europe					44869

Heating CO2

 

We have to heat 187 lb of CO2 (calculated below) produced by burning our wood.

It is heated from initial winter temperature 35F to 265F at the stove exit.

Heat required = m x Cp x delta T = 187 x 0.21 x (265 - 35) = 9032 Btu. That heat disappears up the chimney.

 

Heating Air

 

The complete combustion of our wood, requires 0.79 x 184* (calculated below) = 145.36 lb of O2. See note.

Our wood contains 34 lb O2 (see above), so the added O2 would be 145.36 - 34 = 111.36 lb.

This means 111.36/0.2315 = 481 lb of combustion air is required to burn our wood, because air is about 23.15% O2.

 

* The 184 is the theoretical air for 100 lb of dry wood. We have 79 lb of dry wood.

 

This would require very smart oxygen molecules that go to exactly where needed for combustion. In the real world, additional air is provided, called excess air.

 

We assume 20% excess air, which is applicable for efficient wood stoves. See note.

This means 1.2 x 481 = 577 lb of air is supplied to the stove.

 

Heating Remainder of Combustion Air: The rest of the combustion air = 481, air - 111.36, O2 used up = 369.64 lb, is about 79% nitrogen, N2, which is heated from initial winter temperature 35F to 265F at the stove exit.

Heat required = m x Cp x delta T = (481 - 111.36) x 0.25 x (265 - 35) = 21,255 Btu. That heat disappears up the chimney.

 

Heating Excess Air: The 20% excess air does not participate in the reaction, but is heated from initial winter temperature 35F to 265F at the stove exit.

Heat required = m x Cp x delta T = (577 - 481) x 0.24 x (265 - 35) = 5310 Btu. That heat disappears up the chimney.

 

Total heat required is 21255 + 5310 = 26565 Btu. That heat disappears up the chimney.

 

Table 3 summarizes the data

Table 3/Heating air
Our dry wood, lb	79
Theoretical O2/lb of dry wood	1.84
Theoretical O2 for our dry wood	145.36
O2 in our dry wood, lb	34
Supplied O2, lb	111.36
O2 fraction in air	0.2315
Theoretical air for our dry wood, lb	481
Excess air factor	1.2
Supplied air, lb	577
	Heat excess air	Heat combustion air
Theoretical air for our dry wood, lb	481	481
Excess air, lb, 577 - 481	96
O2 used up		111.35
Remainder is mostly N2		369.64
Cp	0.24	0.25
Stove exhaust temp, F	265	265
Fuel temp, F	35	35
Heat air, Btu	5310	21254
5310 + 21254		26565

 

APPENDIX 1

C + O₂ —> CO₂. By atomic weights, this is 12 + 32 = 44.

If we divide by 12, we have: 1 + 2.667 = 3.667

Therefore, one lb of C in the wood (or any other fuel) requires 2.667 lb of O2 (whether from inside the fuel, or from added air, or from another source), to produce 3.667 lb of CO2.

 

2 H₂ + O₂ —> 2 H₂O. By atomic weights, this is 4 + 32 = 36.

If we divide by 4, we have: 1 + 8 = 9

Therefore, one lb of H2 in the wood (or any other fuel) requires 8 lb of O2 (whether from inside the fuel, or from added air, or from another source), to produce 9.0 lb of water vapor.

 

Theoretical Combustion Air for One lb of Dry Wood

 

1 lb of H2 + 8 lb of O2 —> 9 lb of H20

O2 required is 0.06 lb H2 x 8 = 0.48 lb for burning H2

O2 required is 0.51 lb C x 2.667 (above calculated) = 1.36 lb for burning C

Theoretical O2 required is 0.48 + 1.36 = 1.84 lb for 1 lb of dry wood

 

Dry wood contains 0.43 lb O2, so the added O2 would be 1.84 – 0.43 = 1.41 lb.

Theoretical combustion air for 1 lb of dry wood = 1.41/0.2315 = 6.1 lb.

 

APPENDIX 2

The higher heating value, HHV, is determined with a calorimeter in a laboratory. Fuel is fed in at 59F and combusted with pure oxygen, i.e., no excess air, until combustion is complete. Heat is extracted from the products of combustion to reduce their temperature to 59F. Any water vapor due to combustion of hydrogen in the fuel, 2 H2 + O2 -> 2 H2O, is condensed and cooled to 59F. The laboratory tests are performed with pure oxygen to achieve near 100% completion of reactions. Such measurements often use a standard fuel temperature of 15C or 59F. See URLs.

 

https://www.ddscalorimeters.com/why-high-pressure-oxygen-is-used-in...

https://en.wikipedia.org/wiki/Calorimeter

 

The lower heating value, LHV, is determined by subtracting the heat released due to condensing the water vapor, i.e., LLV = HHV – condensation heat released by the water vapor from H2 burning.

 

Evaporation loss due to burning hydrogen

In a heating stove or a boiler, the water vapor goes up the chimney.

 

2 lb H2 + 16 lb O2 --> 18 lb H2O

Carbon in 100 lb of dry wood is 48 lb

Hydrogen in 100 lb of dry wood is 6 lb

0.06 lb H2 + 0.48 lb O2 --> 0.54 lb H2O

Heat of vaporization is 1058.2 Btu/lb water

Evaporation loss = 0.54 x 1058.2 = 570 Btu/lb*

LHV = 8600, HHV – 570, evaporation loss = 8030 Btu/lb, dry

* Excludes heat loss from fuel inlet temperature to chimney outlet temperature.

 

The evaporation loss disappears via the chimney. Thus, the maximum possible boiler efficiency, with 100% complete combustion, could be 8030/8600 = 93.4%, if all other losses were ignored. See Appendix of URL

http://www.windtaskforce.org/profiles/blogs/the-case-against-intens...

APPENDIX 3

The temperature must be at least 250F at the chimney exit (higher, say 265F, at the stove exit) to prevent creosote and acids from condensing on the flue interior surfaces.

Creosote will not condense on surfaces above 250F. Modern flues are insulated to ensure the temperature stays above 250F during steady operation conditions.

Creosote is flammable and burns hot. If a flue is coated with creosote and ignited, perhaps by a spark going up the flue, it can cause a serious chimney fire that can lead to a structure fire.

Fires can be avoided by using recent-model stoves and flue standards, burning dry wood (20% MC), keeping fires hot enough to maintain flue temperatures of at least 265F at the stove outlet.

https://en.wikipedia.org/wiki/Wood-burning_stove

 

APPENDIX 4

In case of gas and propane fired, hot water heaters, the evaporation energy (Item 2) is partially recovered by cooling the flue gases at the heater exhaust to about 130F and HHV must be used, even in Europe.

 

Such cooling of exhaust gases has been used by the gas/propane, hot water heaters of Viessmann (in my basement) and Buderus for at least 35 years. Those heaters have corrosion-resistant PVC exhaust systems, and the fuels are extremely clean compared to wood, and emit near-zero particulates.

 

If a house were highly sealed and highly insulated, and the radiators were adequately seized for 120F hot water supply, the gas/propane heaters likely would operate in condensing mode (95% efficiency) for the entire heating season. See URLs.

 

http://www.windtaskforce.org/profiles/blogs/co2-emissions-from-logg...

http://www.windtaskforce.org/profiles/blogs/burning-wood-produces-e...

APPENDIX 5

Older Wood Stoves

 

Older wood stoves, pre 1990, such as Franklins and Potbellies, use about 40% excess air, and have very incomplete combustion, and have high flue gas temperatures, and have leaks, which yield efficiencies of about 50% during steady firing conditions, much less during start-ups and burn-downs, which can happen once a day.

Their PM10 emissions are about 30 to 60 g/h during steady firing conditions, much higher during start-ups and burn-downs, which can happen once a day. They are toxic smokers!

 

Modern, EPA-Certified Wood Stoves

 

The emissions from modern appliances for combustion of log wood are dominated by inorganic ash during ideal steady firing conditions.

Inorganic ash would be most of the emitted particulate.

Organic carbon and soot may be less than 10% of the emitted particulate.

 

However, during the start-up, and low burn-rates, and fuel addition, the combustion conditions will be much less than ideal, causing increased emissions of organic carbon and soot, i.e., much greater than 10% of the emitted particulate.

 

The emissions from modern log wood appliances may increase 5 to 10 times, if they are not appropriately operated. Then the emissions are most likely dominated by soot and organic carbon, instead of inorganic ash.

 

Recent EPA-certified wood stoves use about 20% excess air, and have nearly complete combustion, and have low flue gas temperatures, about 265F at stove exit, and have near-zero leaks, and yield efficiencies of about 80% during ideal steady firing conditions in a laboratory, much less during start-ups and burn-downs.

 

Their PM10 emissions are EPA-mandated at 4.5 g/h, as achieved during laboratory testing.

Their PM10 emissions will be EPA-mandated at 2 g/h in 2020, as achieved during laboratory testing.

 

The 2 g/h values likely would be 5 to 10 times greater, i.e., emitting at 10 to 20 g/h, when operated by Joe and Jane User, i.e., the real world. See section 4.6 of URL.

 

http://www.verenum.ch/Publikationen/Biomass-Conf9.2.pdf

http://www.windtaskforce.org/profiles/blogs/dartmouth-biomass-boile...

 

NOTE: About 65% (6.5 million) of all US wood stoves in use are older models, pre 1990, which use about 40% excess air.

Open fireplaces use at least 100% excess air, and have efficiencies of -10 to + 10%, and have PM10 emissions of about 30 to 60 g/h during steady firing conditions, much higher during start-ups and burn-downs, which can happen once a day; they should be outlawed.

 

https://www.epa.gov/sites/production/files/2016-02/documents/wood_s...

http://burningissues.org/comp-emmis-part-sources.htm

http://www.yolocleanair.org/woodsmoke.htm

APPENDIX 7

A few modern wood boilers are computer-automated to control the amount of air needed to burn wood at a very high temperature, about 2000F. This ensures almost all the fuel, including carbon remaining in ashes, and carbon monoxide, CO, and volatile organic carbon gases, VOCs, such as creosote and tar, are almost completely combusted.

 

Such stoves likely require acid resistant flue liners, such as stainless steel, because their stove outlet temperatures at about 265F to achieve about 80% efficiency, is very close to acid and creosote condensation temperatures. A regular-steel flue liner likely would corrode within a year, if acid condensation occurred.

 

https://erc.cals.wisc.edu/woodlandinfo/files/2017/09/mt198405hr.pdf

https://en.wikipedia.org/wiki/Heat_of_combustion

APPENDIX 8

The emissions for an EPA-certified wood stove, 2 g/h in 2020, with 80% efficiency and 50,000 Btu/h output (enough for a recent, well-sealed and well-insulated, 2000 sq ft house in New England) would be 1000000/50000 x 2.0 = 40 g/million Btu, or 0.0881 lb/million Btu, based on heat output, or 0.0881 x 50000/62500 = 0.0705, based on heat input. See table 4.

 

Table 4/Conversion of g/h to lb/million Btu
Btu/h	1000000
Stove output, Btu/h	50000
Stove input, Btu/h	62500
g/h	2.0
g/lb	454
g/million Btu	40
lb/million Btu (heat output)	0.0881
lb/million Btu (heat input)	0.0705

 

Heat delivered could be added to:

 

1) A hot water circulating loop

2) A warm air circulating loop

3) An open-floor-plan space

 

APPENDIX 9

The possible confusion for a buyer when looking at European wood stoves vs. US wood stoves may be reduced if the buyer refers to EPA’s database for EPA-certified wood stoves.  The database currently has information on 584 units (both log wood and wood pellets) and it lists the overall efficiency for 272 of stoves. See Note.

 

The EPA requires the efficiency testing be based on Canadian test method CSA B415.1-10, regardless of the manufacturing origin of the stove. The information shown in the EPA’s database should be unbiased regarding domestic or foreign, when comparing the efficiency of the stoves listed within the database.

 

Any new wood stoves sold in Vermont must be certified by the EPA as prescribed in 40 CFR Part 60 Subpart AAA. 

https://www.ecfr.gov/cgi-bin/text-idx?SID=b5ce57bd07e93896de9ad0125...

 

In theory, the EPA will continue to update the database as new information is received from manufacturers, such as test reports for new stoves seeking to be certified by the EPA.

 

About the database:  https://cfpub.epa.gov/oarweb/woodstove/index.cfm?fuseaction=app.about

Link to EPA’s database:  https://cfpub.epa.gov/oarweb/woodstove/index.cfm?fuseaction=app.search

 

NOTE: For the average buyer, it would be difficult to match the make and model of a particular stove listed in the database with any stove on the showroom floor at the stove shop, i.e., the database may be intelligible to insiders, but is nearly useless to the lay public.