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 = I x R

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 = L x W

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 x w

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 = l x w x h

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