BASIC "HVAC" FORMULAS

BASIC HVAC FORMULAS

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1. CFM

CFM= ______BTU/Hr / ( 1.08 x Temperature   Difference)

2. HORSEPOWER

ONE HORSEPOWER =33,000 ft.-lb. of work in 1 minute

ONE HORSEPOWER =746 Watts

3. CONVERTING KW to BTU:

1 KW = 3413 BTU’s
Example: A 20 KW heater (20 KW X 3413 BTU/KW = 68,260 BTU’s)

4. 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)

5. COULOMB

COULOMB =6.24 X 1018 (1 Coulomb = 1 Amp)

6. OHM'S LAW

E = I x R
I = E / R
R = E / I

Where:
E = voltage (emf)
I = Amperage (current)

R = Resistance (load)

7. WATTS (POWER)

WATTS (POWER) =
volts x amps or P = E x I

P(in KW) = (E x I) / 1000

8. U FACTOR

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)

9. ? VA Rating in Transformer

Copper losses ( I²R)depends on Current which passing through transformer winding while Iron Losses or Core Losses or Insulation Losses depends on Voltage. So the Cu Losses depend on the rating current of the load so the load type will determine the power factor P.F , that's why the rating of Transformer in kVA,Not in kW.

VA = volts X amps
Example: 24V x .41A = 10 VA

10. ONE FARAD CAPACITY =1 amp. stored under 1 volt of pressure

11. MFD (microfarad) =1 MFD = 1 Farad / 1,000,000

12. LRA (Locked rotor amps)

LRA = FLA x 5 (NOTE: This is a commonly used rule-of-thumb, not a direct conversion)

13. RPM of motor

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

14. DRY AIR

DRY AIR =
78.0% Nitrogen
21.0% Oxygen

1.0% Other Gases

15.WET AIR 


Same as dry air plus water vapor

16. SPECIFIC DENSITY = 1 / Specific Volume

17. SPECIFIC DENSITY OF AIR = 1 / 13.33 = .075 lbs./cu.ft.

18.RELATIVE HUMIDITY

RELATIVE HUMIDITY =
Moisture present / Moisture air can hold

19. SPECIFIC HUMIDITY =
Grains of moisture per dry air

7000 GRAINS in 1 lb. of water

20. 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

21.TOTAL PRESSURE (Ductwork)

TOTAL PRESSURE (Ductwork) = Static Pressure + Velocity Pressure

22. 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%

23. 3 PHASE VOLTAGE UNBALANCE =

(100 x maximum deg. from average volts) / Average Volts

24. NET OIL PRESSURE =
Gross Oil Pressure – Suction Pressure
NOTE: The suction pressure must be measured at the crankcase, not the service valve

25. COMPRESSION RATIO =

Discharge Pressure Absolute / Suction Pressure Absolute

26. HEAT PUMP AUXILIARY HEAT –
Sized at 100% of load

27. ARI HEAT PUMP RATING POINTS =
47°F and 17°F

28. 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

29. 1 psi    =  28 INCHES OF WC


30. 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:1

80,000 Input BTU’s
1000 (Heating Value per CF of Natural Gas)
= 80 CF/hr.

Example:2

80,000 Input BTU’s
2550 (Heating Value per CF of Propane)
= 31 CF/hr.

31. FLAMMABILITY LIMITS

Propane - 2.4-9.5 
Butane - 1.9-8.5
Natural Gas - 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%

32. CALCULATING OIL NOZZLE SIZE (GPH):

BTU Input = Nozzle Size (GPH)
140,000 BTU’s
OR
BTU Output
140,000 X Efficiency of Furnace


33. FURNACE EFFICIENCY:

% Efficiency = energy output / energy input

34. OIL BURNER STACK TEMPERATURE (Net) =
Highest Stack
Temperature minus
Room Temperature
Example: 520° Stack Temp. – 70° Room Temp. = Net Stack

Temperature of 450°

35. KELVIN TO CELSIUS:

C = K – 273

36. CELSIUS TO KELVIN:

K = C + 273

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