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