HVAC FORMULAS
CFM = ______BTU/Hr / ( 1.08 x Temperature Difference)
TON OF REFRIGERATION The amount of heat required to melt a ton (2000 lbs.) of ice at 32°F
288,000 BTU/24 hr.
12,000 BTU/hr.
APPROXIMATELY 2 inches in Hg. (mercury) = 1 psi
WORK =
Force (energy exerted) X Distance
Example: A 150 lb. man climbs a flight of stairs 100 ft. high
Work = 150 lb. X 100 ft.
Work = 15,000 ft.-lb.
ONE HORSEPOWER =
33,000 ft.-lb. of work in 1 minute
ONE HORSEPOWER =
746 Watts
CONVERTING KW to BTU:
1 KW = 3413 BTU’s
Example: A 20 KW heater (20 KW X 3413 BTU/KW = 68,260 BTU’s)
CONVERTING BTU to KW
3413 BTU’s = 1 KW
Example: A 100,000 BTU/hr. oil or gas furnace
(100,000 / 3413 = 29.3 KW)
COULOMB =
6.24 X 1018 (1 Coulomb = 1 Amp)
OHM'S LAW =
E=IxR
I=E/R
R=E/I
Where:
E = voltage (emf)
I = Amperage (current)
R = Resistance (load)
WATTS (POWER) =
volts x amps or P = E x I
P(in KW) = (E x I) / 1000
U FACTOR =
Reciprocal of R factor
1/R=U
Example:
If R = 19:
U = 1 / 19 = .05 (BTU’s transferred / 1 Sq.Ft. / 1ºF / 1 Hour)
VA
(how the secondary of a transformer is rated) = volts X amps
Example: 24V x .41A = 10 VA
ONE FARAD CAPACITY =
1 amp. stored under 1 volt of pressure
MFD (microfarad) =
1 MFD = 1 Farad / 1,000,000
LRA (Locked rotor amps) =
LRA = FLA x 5 (NOTE: This is a commonly used rule-of-thumb, not a direct conversion)
TEV (shown in equilibrium)
46.7# (Bulb Pressure) = 9.7# (Spring Pressure) + 37# (Evaporator Pressure)
Where:
Bulb Pressure = opening force
Spring and Evaporator Pressures = closing forces
RPM of motor = (60Hz x 120) / (No. of Poles)
1800 RPM Motor – slippage makes it about 1750
3600 RPM Motor – slippage makes it about 3450
DRY AIR =
78.0% Nitrogen
21.0% Oxygen
1.0% Other Gases
WET AIR =
Same as dry air plus water vapor
SPECIFIC DENSITY = 1 / Specific Volume
SPECIFIC DENSITY OF AIR = 1 / 13.33 = .075 lbs./cu.ft.
STANDARD AIR =
24 Specific Heat (BTU’s needed to raise 1 lb. 1 degree)
SENSIBLE HEAT FORMULA (Furnaces):
BTU/hr. – Specific Heat X Specific Density X 60 min./hr. =
X CFM X DT
.24 X .075 X 60 X CFM X DT = 1.08 X CFM X DT
ENTHALPHY = h =
Sensible heat + Latent heat
TOTAL HEAT FORMULA
(for cooling, humidifying or dehumidifying)
BTU/hr. = Specific Density X 60 min./hr. X CFM X Dh
= 0.75 x 60 x CFM x Dh
= 4.5 x CFM x Dh
Where Dh = Change in Enthalpy
RELATIVE HUMIDITY =
Moisture present / Moisture air can hold
SPECIFIC HUMIDITY =
Grains of moisture per dry air
7000 GRAINS in 1 lb. of water
DEW POINT =
When wet bulb equals dry bulb
TOTAL PRESSURE (Ductwork) =
Static Pressure + Velocity Pressure
CFM =
Area (sq. ft.) X Velocity (ft. min.)
HOW TO CALCULATE AREA
Rectangular Duct
A=LxW
Round Duct
A = (Pi)r² ...or...(Pi)D²/4
RETURN AIR GRILLES –
Net free area = about 75%
3 PHASE VOLTAGE UNBALANCE =
(100 x maximum deg. from average volts) / Average Volts
NET OIL PRESSURE =
Gross Oil Pressure – Suction Pressure
NOTE: The suction pressure must be measured at the crankcase, not the service valve
COMPRESSION RATIO =
Discharge Pressure Absolute / Suction Pressure Absolute
HEAT PUMP AUXILIARY HEAT –
Sized at 100% of load
ARI HEAT PUMP RATING POINTS =
47°F and 17°F
NON-BLEND REFRIGERANTS:
Constant Pressure = Constant Temperature during
Saturated Condition in an evaporator or condenser
BLENDS –
Changing Temperature during Saturated Condition in an evaporator or condenser (See Glide)
28 INCHES OF WC =
1 psi
NATURAL GAS COMBUSTION:
Excess Air = 50%
15 ft.3 of air to burn 1 ft.3 of methane produces:
16 ft.3 of flue gases:
1 ft.3 of oxygen
12 ft.3 of nitrogen
1 ft.3 of carbon dioxide
2 ft.3 of water vapor
Another 15 ft.3 of air is added at the draft hood
GAS PIPING (Sizing – CF/hr.) =
Input BTU’s
Heating Value
Example:
80,000 Input BTU’s
1000 (Heating Value per CF of Natural Gas)
= 80 CF/hr.
Example:
80,000 Input BTU’s
2550 (Heating Value per CF of Propane)
= 31 CF/hr.
FLAMMABILITY LIMITS
Propane
Butane
Natural Gas
2.4-9.5
1.9-8.5
4-14
COMBUSTION AIR NEEDED
(PC=Perfect Combustion)
(RC=Real Combustion)
Propane
23.5 ft.3 (PC)
36 ft.3 (RC)
Natural Gas
10 ft.3 (PC)
15 ft.3 (RC)
ULTIMATE CO2 13.7% 11.8%
CALCULATING OIL NOZZLE SIZE (GPH):
BTU Input = Nozzle Size (GPH)
140,000 BTU’s
OR
BTU Output
140,000 X Efficiency of Furnace
FURNACE EFFICIENCY:
% Efficiency = energy output / energy input
OIL BURNER STACK TEMPERATURE (Net) =
Highest Stack
Temperature minus
Room Temperature
Example: 520° Stack Temp. – 70° Room Temp. = Net Stack
Temperature of 450°
KELVIN TO CELSIUS:
C = K – 273
CELSIUS TO KELVIN:
K = C + 273
ABSOLUTE TEMPERATURE MEASURED IN KELVINS
SINE = side opposite
COSINE = side adjacent
Sin = hypotenuse
Cos = hypotenuse
TANGENT =
side opposite / side adjacent
PERIMETER OF SQUARE:
P = 4s
Where: P = Perimeter and s = side
PERIMETER OF RECTANGLE:
P = 2l + 2w P – Perimeter
l = length
w = width
PERIMETER OF SQUARE
P = a + b + c + d (P = Perimeter)
a = 1st side
b = 2nd side
c = 3rd side
d= 4th side
PERIMETER OF CIRCLE:
C = (Pi)D = 2(Pi)r
Where:
C = Circumference
(Pi) = 3.1416
D = Diameter
r = radius
AREA OF SQUARE:
a = s² = s x s
A = Area
s = side
AREA OF RECTANGLE:
A=l xw
Where:
A = Area
l = length
w = width
AREA OF TRIANGLE:
A = 1/2bh
Where:
A = Area
b = base
h = height
AREA OF CIRCLE:
A = (Pi)r² = (Pi) D²/4
Where:
A = Area
(Pi) = 3.1416
r = radius
D = Diameter
VOLUME OF RECTANGULAR SOLID:
V=lxwxh
Where:
V = Volume
l = length
w = width
h = height
VOLUME OF CYLINDRICAL SOLID:
V = (Pi)r²h = (Pi) D²/4 x h
Where:
V = Volume
p = 3.1416
r = Radius
D = Diameter
h = height
CAPACITANCE IN SERIES:
C = 1 / (C1 + C2)
CAPACITANCE IN PARALLEL:
C = C1 + C2 + . . . . .
GAS LAWS:
Boyle’s Law: P1 V1 = P2 V2
Where:
P = Pressure (absolute)
V = Volume
Charles’ Law:
P1 / T1 = P2 / T2
Where:
P = Pressure (absolute)
T = Temperature (absolute)
General Gas Law:
(P1 x V1) / T1 = (P2 x V2) / T2
Where:
P = Pressure (absolute)
V = Volume
T = Temperature (absolute)
PYTHAGOREAN THEOREM:
c2 = a2 + b2
Where:
c = hypotenuse
a & b = sides
Capacity of Schedule 80 steel pipe in foot per length in US gallons:
1”
= .0374
1-1/4” = .0666
1-1/2” = .0918
2”
= .1535
2-1/2” = .22
3”
= .344
4”
= .5970
5”
= .947
Example 2-1/2" pipe = .22 x 10 feet = 2.2 gallons capacity
Infrared Thermometer Adjustment Values:
Material/Emissivity
Aluminum
Bright/0.09
Anodized/0.55
Oxidized/0.2 to 0.3
Brass
Bright/0.03
Oxidized/0.61
Chromium
Polished/0.08
Copper
Bright/0.05
Oxidized/0.78
Iron and Steel
Polished/0.55
Oxidized/0.85
Nickel
Polished/0.05
Oxidized/0.95
Zinc
Bright/0.23
Oxidized/0.23
Brick
Building/0.45
Paints
White/0.9
Black/0.86
Oil Paints (all)/0.92
Roofing Paper /0.91
Rubber /0.94
Silica /0.42 to 0.62
Water /0.92
Weights and Specific Heats of Substances
Material/Weight (Ib./ft³)/Specific Heat (Btu/lb)
Gases
Air (normal temp)/0.075/0.24
Metals
Aluminum/166.5/0.214
Copper/552/0.094
Iron/480/0.118
Lead/710/0.030
Mercury/847/0.033
Steel/492/0.117
Zinc/446/0.096
Liquids
Alcohol/49.6/0.60
Glycerin/83.6/0.576
Oil/57.5/0.400
Water/62.4/1.000
Others
Concrete/147/0.19
Cork/15/0.48
Glass/164/0.199
Ice/57.5/0.504
Masonry/112/0.200
Paper/58/0.324
Rubber/59/0.48
Sand/100/0.195
Stone/138-200/0.20
Tar/75/0.35
Wood, Oak/48/0.57
Wood, Pine/38/0.47
Linear Measurement Equivalents (U.S. Conventional - SI Metric)
1 inch =
2.54 cm
25.4 mm
25400 µm
1 foot =
12"
0.304 m
30.48 cm
1 yard =
3'
0.914 m
91.44 cm
1 micron =
0.0000394"
1 millimeter =
1000 µm
0.0394"
1 centimeter =
10 mm
0.3937"
1 meter =
100 cm
39.37"
1 kilometer =
1000m
0.62137 miles
Area Equivalents ( U.S. to Metric )
1 in²=
0.0065m²
1 ft²=
144 in²
0.093 m²
1 yd²=
1296 in²=
9 ft²=
0.836 m²
Volume Equivalents ( U.S. and Metric )
1 in³=
0.016 L=
16.39 cm³
1 ft³=
1728 in³=
7.481 gal=
28.317 L=
0.0283 m³=
28317 cm³
1 yd³=
27 ft³=
46656 in³
1 gal=
0.1337 ft³=
3.79 L=
231 in³=
3785 cm³
1 cm³=
0.0610237 in³
1 L=
61.03 in³
1000 cm³
0.2642 gal
Velocity Equivalents
1 mi/hr=
1.47 ft/sec
0.87 knot
1.61 km/hr
0.45m/sec
1 ft/sec=
0.68 mi/hr
60 ft/min
0.59 knot
o
1.1 km/hr
0.305 m/sec
1 m/sec=
3.28 ft/sec
2.24 mi/hr
1.94 knot
3.6 km/hr
1 km/hr=
0.91 ft/sec
0.62 mi/hr
0.54 knot
0.28 m/sec
Heat Equivalents
1 Btu=
252 calories
1054.4 J (Joules)
1 kcal (kilocalorie)=
1000 cal
4.1840 kJ
1 Btu/lb=
0.5556 kcal/kg
2.3244 kJ/kg
1kcal/kg=
1.8 Btu/lb
Btu/hr=
0.2931W