A low quality pellet, other than terrible combustion, will also have a low heat performance and will leave a lot of residue. Light pellet, dark pellet or mixed?
There is no absolute rule, generally the light pellet (deal) has a lower heat power and leaves less residue than the dark one (beech).
The dark one has greater heating power but leaves more residue. It is about finding the right compromise for one's needs. It is always advised to test the pellets before proceeding with the purchase of large quantities.
The certified pellet: It is surely appropriate to purchase certified pellets, from June 2011 a unique regulation on a European level, EN 14961-2, was introduced, which will divide the types of pellets into 3 categories with the "EN Plus" marking: A1 for the highest quality, A2 and B which identifies the lesser quality pellet only appropriate for industrial use.