Mastering Essential HVAC Formulas for Technicians

 Understanding and applying HVAC formulas is crucial for any technician aiming to excel in the field. This blog post will cover several important formulas related to gas consumption, airflow measurement (CFM), total heat, sensible heat, and latent heat. Each formula plays a key role in ensuring systems run efficiently, safely, and at optimal performance levels.

1. Clocking a Gas Meter to Measure Gas Consumption

Purpose: Clocking a gas meter helps HVAC technicians determine how much gas an appliance consumes, ensuring that equipment is correctly sized and systems run efficiently.

Steps:

• Turn on the appliance: Make sure it’s the only gas appliance running.
• Time the meter: Using a stopwatch, time one full revolution of the gas meter’s service dial.
• Record the time: Note how many seconds it takes for one full revolution.

Formula for Calculating Gas Consumption:

• If using a half-cubic-foot dial: Divide 1800 by the number of seconds.
• If using a one-cubic-foot dial: Divide 3600 by the number of seconds.

Once you have the cubic feet per hour, multiply it by the BTU value per cubic foot (typically around 1000 BTUs) to convert gas consumption into BTUs per hour.

For example, if a half-cubic-foot dial takes 24 seconds for one full revolution: $\frac{1800}{24} = 75 \, \text{cubic feet per hour}$ Multiply by 1000 BTUs: $75 \, \text{cfh} \times 1000 \, \text{BTUs} = 75,000 \, \text{BTUs per hour}$

2. Calculating CFM Using the Temperature Rise Method (Gas Furnace)

Purpose: Determining the airflow in cubic feet per minute (CFM) is essential for ensuring the furnace operates efficiently. This method works by using the temperature rise and the furnace's BTU output.

Formula: $\text{CFM} = \frac{\text{BTUs} \times \text{Efficiency}}{1.08 \times \Delta T}$ Where:

• BTUs: Heat output of the furnace.
• Efficiency: Given as a decimal (e.g., 85% efficiency = 0.85).
• 1.08: A constant for standard air.
• ΔT: The temperature difference between the supply and return air.

Example:

• Furnace BTU: 75,000 BTUs.
• Efficiency: 85% (0.85).
• Temperature difference: 50°F. $\text{CFM} = \frac{75,000 \times 0.85}{1.08 \times 50} = \frac{63,750}{54} = 1181 \, \text{CFM}$

This calculation provides the airflow required for optimal heating and system performance.

3. Total Heat Calculation Using a Psychrometric Chart

Purpose: Total heat (in BTUs) combines sensible heat and latent heat. Using a psychrometric chart helps technicians visualize the air's moisture and temperature properties, allowing for more accurate HVAC designs.

Steps:

• Plot the dry bulb and wet bulb temperatures on the psychrometric chart to find the enthalpy (total energy in the air).
• Find the enthalpy difference (ΔH) by subtracting the final enthalpy from the initial enthalpy.

Formula: $\text{Total Heat (BTUs)} = \text{CFM} \times 4.5 \times \Delta H$

Example:

• CFM: 1200.
• ΔH (from psychrometric chart): 11.7 BTUs. $1200 \times 4.5 \times 11.7 = 63,180 \, \text{BTUs}$

This calculation helps determine how much heat is added or removed from the air in a system.

4. Sensible Heat Calculation

Purpose: Sensible heat refers to the heat that causes a change in temperature without altering the state of the substance. Understanding sensible heat is critical for designing systems that maintain comfortable temperatures.

Formula: $\text{Sensible Heat (BTUs)} = \text{CFM} \times 1.08 \times \Delta T$ Where:

• CFM: Airflow in cubic feet per minute.
• 1.08: Constant for standard air.
• ΔT: Temperature difference between supply and return air.

Example:

• CFM: 1200.
• Temperature difference: 50°F. $1200 \times 1.08 \times 50 = 64,800 \, \text{BTUs}$

This calculation is used to determine how much heat the air absorbs as it moves through the system.

5. Latent Heat Calculation

Purpose: Latent heat refers to the heat required to change the moisture content in the air without changing the temperature. This is crucial for dehumidification processes.

Method 1: Subtracting Sensible Heat from Total Heat

• Total heat: 25,000 BTUs.
• Sensible heat: 18,000 BTUs. $25,000 - 18,000 = 7,000 \, \text{BTUs of latent heat}$

Method 2: Using CFM and Moisture Difference

• CFM: 1000.
• ΔG (moisture difference from psychrometric chart): 20 grains per pound.
• Constant: 0.68. $\text{Latent Heat (BTUs)} = 1000 \times 0.68 \times 20 = 13,600 \, \text{BTUs}$

6. Latent Heat Removal: Calculating Water Extraction

Purpose: In dehumidification, calculating how much water is removed from the air helps determine the latent heat removed and informs dehumidifier sizing.

Formula: $\text{Latent Heat (BTUs)} = \text{Gallons per hour} \times 8.33 \times 1060$

Example:

• 3 gallons of water removed per hour. $3 \times 8.33 \times 1060 = 26,478 \, \text{BTUs per hour}$

This formula helps HVAC professionals assess the dehumidification performance of their systems.