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