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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". The Higher Heating Value regards all of the BTU's in the wood as usable input into the stove, while the Lower Heating Value only regards most of the BTU's in the wood as usable input into the stove. All is higher than most, thus the higher value, and most is lower than all, thus the lower value. Why does the LHV only regard most of the BTU's as input? It has everything to do with moisture in the wood, and the evaporation of that moisture during combustion. Evaporating the moisture requires some of the BTU's from the wood. Here's a simple equation that illustrates the relationship between HHV and LHV...
All of the BTU's (HHV) - Some of the BTU's (used for moisture evaporation) = Most of the BTU's (LHV)
So, how does this affect wood stove efficiencies? It's simple mathematics, BTU's out divided by BTU's in. The dividend (BTU's out) is the same in both HHV and LHV calculations. It's the divisor that changes, a lower number for LHV, or the higher number for HHV. A lower divisor (LHV) will always produce a higher quotient (in this case, efficiency %).
If you have additional questions about HHV and LHV, and their differences, call us at 888-714-5294. We'll be glad to help explain it further.
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 being lower. We publish our LHV efficiencies to help you make a direct comparison to other stoves, and because the IRS allows the use of LHV for the purpose of qualifying for the 25C tax credit (expired 12-31-14, but up for renewal). We publish our HHV efficiencies because HHV is used by the US EPA as the "actual" efficieny on its certified stove list, and because virtually all other heating equipment in North America is rated using HHV. In order to compare the efficiency of a Kuma stove to that of your furnace, for example, you need the HHV efficiency number.
The EPA's New Source Performance Standards (NSPS) is the program that regulates emissions for wood burning appliances. The original NSPS was initiated in 1988, and the more common "Phase II" took effect on July 1st, 1990. A Phase II certified stove with a catalytic could produce no more than 4.5 grams/hr of pollution, and a Phase II certified stove without a catalytic could produce no more than 7.5 grams/hr of pollution. Now, 25 years later, the EPA has finally updated their NSPS to reflect advancements in technology. The new EPA regulations on wood burning appliances took effect on May 15, 2015, and is being implemented in two phases over the next 5 years. As of May 15, 2015, all regulated wood burning appliances manufactured in, or imported to, the United States, must produce no more than 4.5 grams/hr of pollution. In the new standards, there is no difference between a stove with a catalytic and one without. In May of 2020, all regulated wood burning appliances will be required to produce no more than 2.5 grams/hr of pollution. It's worth noting that as of June, 2015, the cost effectiveness of the 2020 phase has been challenged in court and is undergoing judicial review.
The new EPA standard mandates that we post these reports on our website. It's pretty boring stuff, but if you want to read through the EPA test data for one of our stoves, click the links below.