Gas Furnace CFM Calculation

CFM Formula:

\[ CFM = \frac{Furnace\ BTU/hr}{1.08 \times \Delta T} \]

Blower Airflow (CFM):

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is Gas Furnace CFM Calculation?

The Gas Furnace CFM Calculation determines the required cubic feet per minute (CFM) of airflow for a gas furnace based on its BTU input and temperature rise. This calculation is essential for proper furnace sizing and efficient HVAC system operation.

2. How Does the Calculator Work?

The calculator uses the CFM formula:

\[ CFM = \frac{Furnace\ BTU/hr}{1.08 \times \Delta T} \]

Where:

\( CFM \) — Cubic feet per minute (blower airflow)
\( Furnace\ BTU/hr \) — Furnace input capacity in British Thermal Units per hour
\( \Delta T \) — Temperature rise in degrees Fahrenheit
\( 1.08 \) — Constant factor for air density and specific heat

Explanation: The formula calculates the airflow needed to transfer the furnace's heat output while maintaining the desired temperature rise across the heat exchanger.

3. Importance of CFM Calculation

Details: Proper CFM calculation ensures efficient furnace operation, prevents overheating, maintains comfort levels, and extends equipment lifespan. Incorrect airflow can lead to poor performance and premature system failure.

4. Using the Calculator

Tips: Enter furnace BTU/hr (found on the unit's rating plate) and temperature rise (typically 35-65°F for most systems). Both values must be positive numbers for accurate calculation.

5. Frequently Asked Questions (FAQ)

Q1: What is the typical temperature rise for gas furnaces?
A: Most gas furnaces operate with a temperature rise between 35-65°F. Consult manufacturer specifications for your specific model.

Q2: Why is 1.08 used in the formula?
A: The constant 1.08 accounts for the specific heat of air (0.24 BTU/lb-°F) and air density (0.075 lb/ft³) at standard conditions.

Q3: What happens if CFM is too high or too low?
A: Too high CFM can cause poor humidity removal and noise; too low CFM can lead to overheating and reduced efficiency.

Q4: How do I measure actual temperature rise?
A: Measure supply air temperature near the furnace and return air temperature, then subtract return from supply temperature.

Q5: Can this calculation be used for heat pumps?
A: No, this formula is specific to gas furnaces. Heat pumps require different calculations based on their operating characteristics.