8431 Murphy Drive 

Middleton, WI 53562 

Telephone: 608 836 4400 

Facsimile: 608 831 9279 

www.intertek-etlsemko.com 

22 October 2008 

Thomas Stroud 

Senior Manager, Codes & Standards 

Hearth, Patio and Barbecue Association 

1901 North Moore Street, Suite 600 

Arlington, VA 22209-1728 

RE: Efficiency and Lower vs. Higher Heating Value of Fuel. 

Dear Tom: 

I’m am writing to provide information that I hope will clarify the issue of efficiency as 

it relates to solid fuel appliances and eligibility for tax rebates based on heating 

efficiency. As I understand the recently enacted legislation, there is a requirement 

that eligible appliances be at least 75% efficient. I also understand that this 

efficiency was selected based on rated efficiencies of European appliances. Since 

Europe uses the “Lower” or “Net” heating value of the fuel in calculation of 

efficiencies, and in North America we use the “Higher” or “Gross” heating value, the 

limit specified needs to be fully understood in determining what specific appliances 

should be eligible for the incentive. 

First, efficiency is defined as Heat Output divided by Heat Input. 

100

Input

Output ε •=

Output is generally determined by a stack loss process or direct measurement which 

yields identical results regardless of whether the Higher or Lower Heating Value of 

the fuel is used to determine the input. 

The difference between the Higher and Lower Heating value of a fuel is essentially 

the heat involved in converting water that is created in combustion from liquid to 

gaseous state. This “heat of vaporization” is about 1030 Btu/lb1. Lower heating value 

does not include this energy while the higher heating value does. In determining 

efficiency the Input is derived by multiplying the dry weight of the fuel burned by its 

heating value. Obviously, if the higher heating value is used, the fuel weight is 

 

1

 Literature sources for “heat of vaporization of water” report varying values in a range of 970 to 1060 

Btu/lb. 1030 Btu/lb is equivalent to the value specified in ASTM E711. 

Intertek Testing Services NA, Inc. 8431 Murphy Drive, Middleton, WI 53652 
multiplied by a larger number than when lower heating value is used and thus the 

denominator in the efficiency equation is larger and the efficiency is therefore lower. 

The standard method for determining solid fuel heating value is ASTM E711. This test 

directly measures the fuels Higher Heating Value. Per this test method, the Lower 

Heating Value is determined by calculation based on the hydrogen content of the 

fuel. Since the hydrogen is converted to water in combustion, this is a straight 

forward process. The formula (in inch-pound units) is: 

LHV ( ) •−= 9H%3.10HHV

Where %H is the hydrogen content of the fuel by weight. 

In wood fuels, hydrogen content typically averages about 6.5% with a range of 6 to 

7%. Higher heating values are around 8500 to 8700 Btu/lb of dry wood for commonly 

used fuel woods. Thus, for a typical efficiency test, the Higher and Lower Heating 

Values are: HHV = 8600 Btu/lb and LHV = 8600 – 10.3 (6.5 x 9) = 7997 Btu/lb. 

Thus the efficiency calculated using the Higher Heating Value is always about 93% of 

the efficiency calculated from the Lower Heating Value. That is: 

Efficiency HHV = Efficiency LHV X 0. 93 

Example: An appliance produces a total heat output of 75,000 Btu’s through 

consumption of 12.5 dry pounds of wood. The efficiency based on Lower Heating 

Value is: 

%75100

79975.12

000,75 =• • ε =

The efficiency based on Higher Heating Value is: 

%8.69100

86005.12

000,75 =• • ε =

This means that since the intent of the wood stove incentive is that appliances be at 

least 75% efficient based on the Lower Heating Value, this is mathematically identical 

as requiring at least 69.8% efficiency based on the Higher Heating Value. 

Why is this discussion important? In North America virtually all fuel burning heating 

appliances have their efficiencies determined based on Higher Heating Values. The 

Intertek Testing Services NA, Inc. 8431 Murphy Drive, Middleton, WI 53562
use of Lower Heating Values carries an implicit assumption that the latent heat of 

water from combustion is unrecoverable. This effectively defines 100% efficiency as 

recovery of all sensible heat (that is, the energy required to heat products of 

combustion from the ambient temperature to the temperature at which they exit the 

heated space) without recovering any latent heat through condensation of water. 

Since it is technically feasible – and, in fact, common in gas fired appliances – to 

recover a significant portion of this latent heat, the use of Lower Heating Value in 

efficiency calculations inevitably leads to the potential to claim efficiencies of greater 

than 100%. This is an obvious violation of one of the fundamental laws of thermal 

dynamics. No energy conversion process can produce more heat as an output than is 

available as an input. 

Practically speaking, it is generally undesirable to recover latent heat from wood 

burning appliances since this would require capture and drainage of liquid water. 

Further, naturally drafted appliances (those that depend on draft created by relatively 

hot flue gases to draw combustion air and exhaust products of combustion) cannot 

be operated with flue gas temperatures low enough to condense water and recover 

latent heat. This results in an effective upper limit of approximately 82%2 on heating 

efficiency based on the Higher Heating Value of the fuel. 

Conclusion: 

 

Since it seems clear that the intent of the solid fuel appliance incentive program 

recently enacted by congress is to apply to appliances with efficiencies of 75% or 

higher based on Lower Heating Values, the program eligibility requirement should 

be restated based on the Higher Heating Value of the fuel to require a minimum of 

69.8% efficiency. This does not in any way constitute a reduction of the intended 

minimum efficiency requirement. It will allow for efficiency ratings to be consistent 

with existing North American practice and consistent with the laws of physics and 

accepted definitions of ‘heating efficiency’. It will also eliminate an “apples-tooranges” comparison issue when efficiencies of appliances are compared across fuel 

types such as wood-to-gas, wood-to-oil or wood-to-coal. 

Sincerely, 

Rick Curkeet, PE 

Chief Engineer – Hearth Products 

 

2

 This value includes additional latent heat losses from fuel moisture content and sensible losses required 

to maintain flue gas temperatures above the dew point. 

Intertek Testing Services NA, Inc. 8431 Murphy Drive, Middleton, WI 53562