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