The list of abbreviations used in a set of engineering drawings varies from office to office.  Be sure to check the front section of the drawing set for the abbreviations used within.

AAir or Compressed Air
ACAir Conditioning
ACDAutomatic Control Damper
ADAccess Door
AFAir Foil
AHUAir Handling Unit
AMPAmpere
APAccess Panel
APDAir Pressure Drop
ASAir Stream
ATCAutomatic Temperature Control
ATMAtmosphere
BDDBack-Draft Damper
BHPBrake Horsepower
BIBackwards Inclined
BODBottom of Duct
BTUBritish Thermal Unit
BTUHBTU per Hour
CENTCenter or Centrifugal
CFCubic Feet
CFMCubic Feet per Minute
CHChilled or Chiller
CHWChilled Water
CHWRChilled Water Return
CHWSChilled Water Supply
COCarbon Monoxide
CONNConnection
CTCooling Tower
CTBDCooling Tower Blow Down
CUHCabinet Unit Heater
CWRCondenser Water Return
CWSCondenser Water Supply
DDrain
DBDry Bulb (Temperature)
DEGDegree
DDCDirect Digital Control
DIADiameter
DIMDimension
DPDifferential Pressure
EAEach or Exhaust Air
EAHUExhaust Air Handling Unit
EATEntering Air Temperature
EFExhaust Fan
EMEREmergency
EMSEnergy Management System
ESPExternal Static Pressure
ETExpansion Tank
EUHElectrical Unit Heater
EWTEntering Water Temperature
EXH or EXExhaust
EXTExternal
EXPExpansion
FFahrenheit
FAFree Area or Fire Alarm
FCFlexible Connection
FCUFan Coil Unit
FDFloor Drain, Fire Damper, or Fire Department
FLAFull Load Amps
FLEXFlexible
FLRDRFloor Drain
FPMFeet per Minute
FPSFeet per Second
FRPFiberglass Reinforced Plastic
FSFlow Switch
FTFeet
FTRFin Tube Radiation
GGas
GAGauge
GALGallons
GALVGalvanized
GFUGlycol Feed Unit
GPHGallons per Hour
GPMGallons per Minute
GRGrade
HBHose Bib (Connection)
HDHead
HPHorsepower or High Point
HRHour
HRUHeat Recovery Unit
HTGHeating
HWRHot Water Return
HWSHot Water Supply
HZHertz (Cycles per Second)
IDInside Diameter
INInches
KWKilowatt
LATLeaving Air Temperature
LBPound
LFLinear Feet
LDLinear Diffuser
LPLow Point
LPSLow Pressure Steam
LRALocked Rotor Amps
LUVR or LVRLouver
LVDRLouvered Door
LVGLeaving
LWTLeaving Water Temperature
MAXMaximum
MBH1000 BTUH
MCAMinimum Circuit Amps
MDMotorized Damper
MECHMechanical
MINMinimum
MUMake-Up Water
MUAMake-Up Air
NCNoise Criteria or Normally Closed
NONormally Open
NOMNominal
OAOutside Air
OAIOutside Air Intake
OCOn Center
ODOutside Diameter
ODPOpen Drip Proof
OVOutlet Velocity
PCFPounds per Cubic Foot
PDPressure Drop
PHPhase
PRVPressure Reducing Valve
PSIPounds per Square Inch
PSIAPounds per Square Inch - Absolute
PSIDPounds per Square Inch - Differential
PSIGPounds per Square Inch - Gauge
PVCPolyvinyl Chloride
RRadius
RAReturn Air
RETReturn
RHRelative Humidity
RLARunning Load Amps
RLFRelief
RPMRevolutions per Minute
RTURoof-Top Unit
SASupply Air
SCRScreen
SCTSaturated Condensing Temperature
SDSmoke Detector or Smoke Damper
SESmoke Exhaust
SENSensible
SFDCombination Smoke / Fire Damper
SHCSensible Heat Capacity
SPStatic Pressure
SFSquare Feet
SSStainless Steel
SUPSupply
TTemperature or Thermostat
TEFCTotally Enclosed Fan Cooled
TEMPTemperature
TON12,000 BTUH (Cooling Capacity)
TSPTotal Static Pressure
TSTATThermostat
TYPTypical
UCUndercut (Door)
VVolts
VAVVariable Air Volume
VDVolume Damper
VELVelocity
VFDVariable Frequency Drive
WBWet Bulb Temperature
WCWater Column
WGWater Gauge
WPDWater Pressure Drop
WTDWater Temperature Difference
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