Recently I’ve seen pellet boiler marketers advertising remarkably high “efficiencies” for their equipment.  Often the equipment they’re touting is imported from Europe so they can be forgiven for hyperbole if they’re confused about the difference in common ways of measuring and reporting thermal efficiency.  However, customers must learn to recognize unrealistic claims made by those anxious to promote their heating products.

It is customary in the United States to use the higher heat value (HHV) of fuel as the basis for computing thermal efficiency when talking about solid fuels.  Without becoming too technical, this means that the energy resulting from combustion is compared against the energy transferred to the water in computing thermal efficiency.  When fuels are burned, water vapor is a common by-product that can often be seen condensing above chimneys on cold days.  Turning water to vapor requires energy and represents a factor in limiting the thermal efficiencies of fuel burning appliances that allow the latent heat of vaporization to escape out the chimney in the water vapor.

The maximum thermal efficiencies for systems burning fuel and allowing the escape of water vapor are in the 85% to 87% range.  To achieve efficiencies higher than 85%-87% boilers must capture the water vapor from the flue gas and condense it capturing the latent heat of vaporization rather than allowing it to escape with the flue gas. This is done in some modern fossil fuels boilers and the OkoFEN SMART, a low output pellet boiler for highly efficient buildings.

If you see reported efficiency values for non-condensing boilers that are greater than about 85-87%, the value reported is most likely the European value using the lower heat value (LHV) of the fuel, which disregards the heat lost to water vapor.  That value is roughly 7 to 10% greater than the American measure.  Subtract 7 to 10% from the reported value to compare it with equipment measured to American standards.

Dutch Dresser is the Managing Director of Maine Energy Systems, manufacturers and distributors of MESys/OkoFEN boilers.