Higher and Lower Heating Values for Wood Fuel 

By: Rick Curkeet, PE – Chief Engineer 

Higher Heating Value (HHV) – This is the total energy produced by complete 

combustion of fuel. It is measured in a “Bomb Calorimeter” (ASTM E711) by 

burning a dry sample in pure oxygen in a container immersed in a known quantity 

of water. The temperature increase of the water once the container has 

completely cooled is precisely measured to determine the heat produced. 

For nearly all wood species the HHV is between 8500 and 9000 Btu/lb. The 

value of 8600 currently specified for Oak in ASTM E2618 and EPA M28 WHH is 

therefore reasonable. 

Lower Heating Value (LHV) – This is a measure of the heat energy available 

from combustion without recovery of latent heat from water vapor. In NA we 

base LHV on a reduction from HHV based on the amount of water generated 

from combustion of hydrogen contain in the fuel molecule. We do not deduct for 

latent heat of water that comes along with the fuel as “moisture content”. In the 

EU procedures, latent heat of water from moisture content is also deducted when 

LHV is calculated. 

LHV = HHV – 10.3 (%H x 9) 

Latent Heat of water is the energy required to convert water from a liquid to 

gaseous state. It is approximately 1030 Btu/pound of water. When water vapor 

condenses, this same amount of heat is recovered. 

EXAMPLE 

One Pound of dry wood – HHV = 8600 

Water from combustion – most wood contains about 6.5% hydrogen by weight. 1 

pound of hydrogen in the fuel produces 9 pounds of water. 

Therefore burning on pound of dry wood produces 0.065 x 9 = 0.585 lb H2O 

Latent heat of 0.585 lb/H2O = 0.585 x 1030 = 602.5 Btu. 

LHV = 8600 -602.5 = 7997 Btu/lb(dry) 

At 20% MC(dry), 0.20 lbs of H2O is added for each pound of dry fuel. This 

requires another 0.20 x 1030 = 206 Btu to covert from liquid to gas form. 

Thus LHVeu = 7997-206 = 7791 Btu/lb(dry) 
Water from MC should not be deducted when calculating LHV. 

Suppose we burn 1 lb of dry wood. We would liberate 8600 Btu of energy. If we 

then cool the products of combustion to their initial temperature, we will recover 

the 8600 Btu’s produced and the most of the water produced will condense. But 

if we only cool the combustion products to a temperature that is just high enough 

to prevent condensation of the water (about 160 F) we will lose both the latent 

heat (602 Btu) and the sensible heat it takes to maintain the combustion products 

at 160 F – about 250 Btus. Thus, the most we can recover in a non-condensing 

process is 8600-602-250 = 7748 Btu. It should be noted that the sensible heat 

loss depends on the amount of excess air as well which must be heated to the 

exhaust temperature and represents additional heat loss. So the theoretical 

maximum efficiencies are never actually achieved in practice. 

Since free water in the form of moisture content is also involved there is also a 

loss due to the latent heat of this water. At 20% MC it’s equivalent to burning a 

pound dry wood and placing 0.20 lbs of water in the combustion chamber. It will 

take another 206 Btu’s to convert this water to vapor and 8 more to maintain it at 

160 F. So, the most heat we can recover from a non-condensing system burning 

20% MC wood is about 7534 Btu/lb dry wood (1.2 lb wet wood). 

Thus, the theoretical maximum Higher Heating Value efficiency of a noncondensing wood burning appliance is (7534/8600) x 100 = 87.6% 

Based on LHV as defined in NA the maximum theoretical efficiency for a noncondensing system is (7534/7997) x 100 = 94.2% 

The ratio of Higher Heating Value to Lower Heating Value is 8600/7997 = 1.075. 

Therefore, appliance heating efficiencies based on Higher Heating Value can be 

converted to Lower Heating Value efficiencies as follows: 

hLHV = hHHV x 1.075