Numbers play a large part in the HVAC industry. The numbers involved are more complex than, the difference between 35 and 95 degrees. A good quality HVAC engineer needs to be familiar with complex numbers and calculations. As with any mathematical solution, it is easy when you know how.

**A Complete Guide To HVAC Formulas**

There are many tools a HVAC engineer can use to get the number right. Engineers can use reference books. Or as most engineers do, use a phone app. Yes it is easy when you know how. However, it is vitally important that engineers understand the numbers and calculations. As a failure to do so can and will result in mistakes being made.

Understanding the basic formulas involved in the HVAC industry will help you work quicker and more efficiently. There is nothing wrong with relying on a phone app or book. But engineers should be able to make calculations. Not only during the HVAC exams, but also in the field.

**Electrical Formulas**

- E= voltage (emf)
- I = amperage (current)
- R = resistance (load)
- Ohm’s law states:
- E = I x R which also means:
- I = R/E
- R= E/I

**Watts**

- P = E x I
- or if your P is in kilowatts:
- P = (E x I)/1000
- U Factor
- U = 1/R That is, if R = 4, U = ¼, or 0.25
- 3 Phase Voltage Unbalance
- = (100 x maximum deg. from average volts) / Average Volts

**Farads**

- One Farad capacity: 1 amp stored under 1 volt of pressure
- MFD (microfarad) = 1 MFD = 1 Farad / 1,000,000

**Coulomb**

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

**Work and Horsepower**

**Work**

Work = force x distance

So if you weigh 180 lb, and climb stairs 100 ft high, the work is 180 lb x 100 ft = 18,000 ft-lbs (foot pounds)

**Horsepower (HP)**

- 1 Horsepower = 33,000 ft-lb of work in 1 minute
- HP to KW to BTU
- 1 HP = 746 Watts
- 1 KW =3413 BTU

**Ton of Refrigeration**

The amount of heat to melt 1 ton of ice at 32 degrees F. This amount is 12,000 BTU per hour, which is 288,000 BTU in 24 hours

**RPM**

- 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

**What’s in the Air**

- 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)

**Heat and Humidity**

- Enthalphy = h = Sensible heat + Latent heat
- 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
- 28 Inches Of Wc (water column) = 1 psi
- Furnace Efficiency: % Efficiency = energy output / energy input

**Useful HVAC Formulas**

For conditions other than standard air:

Total Heat (BTU/hr) = 4.5 x cfm x Δh (std. air)

Sensible Heat (BTU/hr) = 1.1 x cfm x Δt (std. air)

Latent Heat (BTU/hr) = 0.69 x cfm x Δgr. (std. air)

**Other Formulas**

- Total Heat (BTU/hr) = 500 x gpm x Δt (water)
- GPM cooler = (24 x TONS) / Δt (water)
- Fluid Mixture Tm = (Xt1 + Yt2) / X + Y (this works for air or water)
- BTU/hr = 3.413 x watts = HP x 2546 = Kg Cal x 3.97
- Lb. = 453.6 grams = 7000 grains
- psi = ft. water/2.31 = in. hg/2.03 = in. water/27.7 = 0.145 x kPa
- Ton = 12,000 BTU/hr = 0.2843 x KW
- HP (air) = cfm x Δp (in.H2O)/6350 x Eff.
- HP (water) = gpm x Δp (ft.)/3960 x Eff.
- gpm = 15.85 x L/S
- cfm = 2.119 x L/S
- CFM = ______BTU/Hr / ( 1.08 x Temperature Difference)
- Therm = 100,000 BTU = MJ/105.5
- Watt/sq. ft. = 0.0926 x W/M²
- PPM (by mass) = mg/kg

**Conclusion**

These formals should be studied as part of the HVAC exam. They will be part of the HVAC engineer certification process. It is important that you understand these formulas and calculations. You will need to understand the numbers and calculations when you are in the field. You can use books, software and smart phone apps to help you understand these calculations further.