All brands of HS Flamingo have the perfect double combustion system. It is a supporting supply of additional air to the combustion chamber. In the picture you can see the primary air that goes under the grate and which is necessary for any burning. Furthermore, additional air is supplied from the rear part of the chamber and is intended to support the combustion of gaseous particles which is called secondary / double combustion. This supply is referred to as tertiary.
ATTENTION! Many sellers and manufacturers often use the term "tertiary combustion" and confuse it with the term tertiary air supply. There is only primary (solid) and secondary (gaseous) combustion.