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