I almost fell for it, until I thought about it for a time.
There offer to do duct cleaning to save my core from damage. Over 2000.00 dollars duct cleaning. Think about it, an air return is placed in most homes above the core were there is a filter. Air goes over the filter then through the core at no time do abrasive particles come in contact with the. While I do know ducts do need to be cleaned, there is very little way large particulates can get into the inside core.