The issue of efficiency can be somewhat confusing if you're not sure
which numbers are being used. If one stove manufacturer publishes only
HHV numbers, and another manufacturer publishes only LHV, a direct
comparison of efficiency cannot be made. Most manufacturers only
publish LHV efficiencies since they are higher and that looks better
than the HHV numbers. We are now starting to publish both on all of our stoves LHV efficiencies to help you make a
direct comparison to stoves that simply list just one and HHV efficiencies because
HHV is used by the US EPA as the "actual" efficiency and because virtually all other types of heating equipment in North America is
rated using HHV.
Both HHV and LHV are references to the Heating Value of a particular fuel, specifically firewood in the case of wood stoves. They are abbreviations for "Higher Heating Value" and "Lower Heating Value".
HHV value represents a calculation where the water product in the combustion gas is considered in the liquid form (all of the BTU's in the wood as usable input into the stove)
LHV is calculated with the water product considered in the vapor form. (most of the BTU's in the wood as usable input into the stove)
Simply put, HHV assumes all the water component is in a liquid state (condensed) at the end of combustion and that heat recovered from that condensation can be put to use. With that considered the efficiency value for HHV will be lower due to a higher quantity of energy to be considered relative to the appliances measured temperature release. The maximum heat output value is derived from the certification test data and represents a refueling cycle more representative of homeowner usage than that prescribed by the test protocol