The EC-10 has a proprietary cast iron eutectic heat exchanger that is resistant to thermal shock and corrosion. The cast iron is flexible and resilient which allows for lower temperature operation especially with outdoor reset controls. This yields significant energy savings. The three pass design of the Eutectic makes it very quiet too. The Eutectic boiler, like other Slant/Fin oil boilers, will operate cleanly under all the different operating conditions. We.ve done the testing and have matched burners and components to this boiler to make sure the boiler will stay running clean after you leave the job. AFUE is 87%.