Presently, {HEAD ~ HVAC systems} operate in {TAIL ~ four modes}: heating, cooling, off and fan on 
operate in
Presently , { HEAD ~ HVAC systems } operate in { TAIL ~ four modes } : heating , cooling , off and fan on
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH