HVAC Formulas
Download Our Free HVAC Formulas
Computrols believes that our customers should have the ability to operate their own HVAC system and building automation controls. That is why we do our best to provide them with as many free resources as possible. Here, you will find our HVAC formulas. On larger screens you can either scroll down to view each individually or download the PDF. On smaller screens such as phones you must download the PDF to view the formulas. The PDF includes:
 Dewpoint and Wetbulb Temperature
 Air Handling Unit Tonnage Output
 Chiller Tonnage Output
 Chiller Coefficient of Performance
 VAV Box Air Flow Rate (CFM)
 Heat Index Calculation
 Wind Chill Temperature Calculation
 Pressure Measurement
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Dewpoint and Wetbulb Temperature
The following equations are used to calculate the wetbulb temperature of air given the drybulb temperature and relative humidity %. The equation assumes that the ambient barometric pressure is constant at a value of 29.15 “Hg since the change in wetbulb temperature is very insignificant with changes in the ambient barometric pressure.
Input Variables  System Variables  Output Variables  

RH  Relative Humidity %  e  Ambient vapor pressure in kPa  Td  Dewpoint temperature in degrees C 
T  Drybulb temperature in degrees C  GAMMA  Constant based upon ambient barometric pressure  Tw  Wetbulb temperature 
DELTA  Constant  
Equations  
e  (RH / 100) * 0.611*EXP(17.27*T/(T+237.3))  
Td  [116.9 + 237.3 ln(e)] / [16.78 – ln(e)]  
GAMMA  0.00066*P (Use P = 98.642 kPa. This is equal to 29.15 “Hg… about the pressure we normally experience.)  
DELTA  4098*(e / Td + 237.3)^2  
Wetbulb Temperature in Degrees F Equals:  
Tw  1.8 * [[(GAMMA*T) + (DELTA*Td)] / (GAMMA + DELTA)] + 32  
Dewpoint Temperature in Degrees F Equals:  
Td  1.8 * [[116.9 + 237.3 ln(e)] / [16.78 – ln(e)]] + 32 
Air Handling Unit Tonnage Output
The following equation calculates the refrigeration output in Tonns of a coil.
Input Variables  Output Variables  

T1  Entering air temperature of the coil in degrees F  TONNS  Dewpoint temperature in degrees F 
T2  Leaving air temperature of the coil in degrees F  
CFM  Volume of air passing through the coil  
Equation  
TONNS  1.08*(T1 – T2)*CFM 
Chiller Tonnage Output
The following equation calculates the refrigeration output in Tonns of a chiller.
Input Variables  Output Variables  

T1  Chilled water return temperature in degrees F  TONNS  Energy output of the chiller 
T2  Chilled water supply temperature in degrees F  
GPM  Volume of water passing through the chiller  
Equation  
TONNS  GPM*(T1 – T2) / 24 
Chiller Coefficient of Performance
The following equation calculates the ratio of energy used to the energy output of a chiller.
Input Variables  

T1  Chilled water return temperature in degrees F 
T2  Chilled water supply temperature in degrees F 
GPM  Volume of water passing through the chiller 
KW  Kilowatts 
Output Variables  

COP  Energy output of the chiller 
Equation  

COP  (T1 – T2) * GPM * 0.0417 / (0.28433 * KW) 
VAV Box Air Flow Rate (CFM)
Input Variables  

A  Duct area in sq. ft 
Pv  Pressure in inches of H_{2}O from PV3 
Output Variables  

V  Velocity of the air 
CFM  Cubic feet of air per minute 
Equation  

Q  AV 
0.0763 is the density of dry air at 60^{o} F The duct diameter units are in ft. 

CFM  1096Π(Duct Diameter/2)^{2}(√(Pv/.0763)) 
Heat Index Calculation
The following equation calculates the heat index of the outside air.
Input Variables  

T_{f}  Outside air temperature in degrees F 
RH  Outside air relative humidity % (enter 50 for 50%, etc.) 
Output Variables  

HI  Heat index 
Equation  

HI  42.379+2.04901523T_{f}+10.14333127RH 0.22475541T_{f}RH6.83783×10^{3}T^{2}_{f} 5.481717×10^{2}RH^{2}+1.22874×10^{3}T^{2}_{f}RH +8.5282×10^{4}T_{f}RH^{2}1.99×10^{6}T^{2}_{f}RH^{2} 
Wind Chill Temperature Calculation
The following equation calculates the wind chill temperature of the outside air.
Input Variables  

V  Outside air velocity in Miles per Hour 
T  Outside air temperature in degrees F 
Output Variables  

WC  Wind chill temperature 
Equation  

WC  0.0817(3.71(V)^0.5 + 5.81 – 0.25V)(T – 91.4) + 91.4 
Pressure Measurement
Velocity Pressure  

Pv = (V/4005)^{2} or V = 4005 √Pv Where V = Air Velocity (FPM) 
Equivalent Measures of Pressure  

1lb. per square inch  = 144lbs. per sq. ft. = 2.036in. Mercury at 32°F = 2.311ft. Water at 70°F = 27.74in. Water at 70°F 
1 inch Water at 70°F  = .03609lb. per sq. in. = .5774oz. per sq. in. = 5774oz. per sq. in. = 5.196lbs. per sq. ft. 
1 ounce per sq. in.  = 1272in. Mercury at 32°F = 1.733in. Water at 70°F 
1ft. Water at 70°F  = .433lbs. per sq. in. = 62.31lbs. sq. ft. 
1 Atmosphere  = 14.696lbs. per sq. in. = 2116.3lbs. per sq. ft. = 33.96ft. Water at 70°F = 29.92in. Mercury at 32°F 
1in. Mercury at 32°F  = .491lbs. per sq. in. = 7.86oz. per sq. in. = 1.136ft. Water at 70°F = 13.63in. Water at 70°F 
Compression Ratio  

Compression Ratio  = Absolute Discharge Pressure / Absolute Suction Pressure 
Absolute Discharge Pressure  = gauge reading + 15psi 
Absolute Suction Pressure  = gauge reading + 15psi 
Refrigerant Mass Flow Rate  

Mass Flow Rate (Pounds/Minute) 
= Piston Displacement X Refrigerant Density = (Cubic Feet/Minute) X (Pounds/Cubic Feet) 