Duct CFM Calculation Formula

CFM Formula:

\[ CFM = \frac{Heating\ Load\ (BTU/hr) \times 1.08}{\Delta T} \]

Required Airflow (CFM):

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1. What is the Duct CFM Calculation Formula?

The Duct CFM Calculation Formula determines the required airflow in cubic feet per minute (CFM) for HVAC duct systems based on heating load and temperature rise. This calculation is essential for proper HVAC system design and sizing.

2. How Does the Calculator Work?

The calculator uses the CFM formula:

\[ CFM = \frac{Heating\ Load\ (BTU/hr) \times 1.08}{\Delta T} \]

Where:

\( CFM \) — Cubic Feet per Minute (airflow)
\( Heating\ Load \) — Total heating requirement in BTU per hour
\( 1.08 \) — Constant factor for standard air conditions
\( \Delta T \) — Temperature rise in degrees Fahrenheit

Explanation: The formula calculates the airflow needed to deliver the required heating capacity while maintaining the desired temperature difference.

3. Importance of CFM Calculation

Details: Accurate CFM calculation ensures proper HVAC system performance, prevents under-sizing or over-sizing of ductwork, maintains comfort levels, and optimizes energy efficiency.

4. Using the Calculator

Tips: Enter heating load in BTU/hr and temperature rise in °F. Both values must be positive numbers. Typical temperature rises range from 15°F to 40°F depending on the application.

5. Frequently Asked Questions (FAQ)

Q1: What is the 1.08 constant in the formula?
A: The 1.08 constant represents the product of air density (0.075 lb/ft³) and specific heat (0.24 BTU/lb-°F) multiplied by 60 minutes/hour.

Q2: What are typical CFM requirements for residential HVAC?
A: Residential systems typically require 400-2000 CFM depending on home size, with approximately 1 CFM per square foot of living space.

Q3: How does altitude affect CFM calculations?
A: At higher altitudes, air density decreases, requiring adjustments to the calculation. The standard formula assumes sea level conditions.

Q4: Can this formula be used for cooling calculations?
A: While similar principles apply, cooling calculations typically use different constants and consider latent heat removal in addition to sensible heat.

Q5: What factors influence heating load calculation?
A: Heating load depends on building size, insulation quality, window area, climate zone, occupancy, and internal heat gains from appliances and people.