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CFM Calculation Guide
Proper airflow is crucial for ensuring your HVAC system performs efficiently in both heating and cooling modes. This guide covers calculating CFM (Cubic Feet per Minute) for heating systems, air conditioning systems, and includes wet bulb measurement guidance for precise cooling calculations.
Heating CFM Calculation
Temperature Rise Method
Temperature rise is an important check after installing or servicing heating equipment. The acceptable range is generally 30-60°F, but always refer to the equipment's data tag or installation manual for specifics.
Key Principle: Lower temperature rise requires higher CFM.
Formula: CFM = BTU Output ÷ (1.08 × Temperature Rise)
Example Calculation: A 95% efficient furnace with a 70,000 BTU input and a measured temperature rise of 50°F:
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Convert BTU input to BTU output: 70,000 × 0.95 = 66,500 BTU Output
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Calculate CFM: 66,500 ÷ (1.08 × 50) = 1,231 CFM
This result shows that a 3-ton blower (capable of roughly 1,200 CFM) is appropriate. If targeting a lower temperature rise of 40°F:
66,500 ÷ (1.08 × 40) = 1,539 CFM
In this case, the blower would struggle to meet the airflow requirements, requiring adjustments or acceptance of a higher temperature rise.
Customer Considerations:
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Higher temperature rise leads to:
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More frequent filter changes.
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Increased risk of system limit trips if filters are neglected.
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Premature heat exchanger wear if airflow remains insufficient.
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Advise customers to keep all supply and return vents open to prevent airflow restrictions.
Sizing Furnace Example: To calculate the required furnace output for a specific airflow target:
BTU Output = (1.08 × Desired Temp Rise) × CFM
For example, targeting a 45°F temperature rise with 1,400 CFM:
BTU Output = (1.08 × 45) × 1,400 = 68,040 BTU Output
CFM Calculation for Cooling Systems
Accurately determining airflow in a cooling system is crucial for achieving optimal performance. This includes calculating:
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Sensible Heat (Temperature Control)
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Latent Heat (Moisture Control)
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Total Cooling Load (Combined Performance)
Step 1: Sensible Heat Calculation
The sensible heat portion deals with temperature reduction and is calculated as follows:
CFM = (Sensible BTU Load) ÷ (1.08 × ΔT)
Where:
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Sensible BTU Load = The heat load dedicated to changing air temperature.
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1.08 = Air constant (based on standard air properties at sea level).
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ΔT = Temperature difference (Supply Air Temp - Return Air Temp).
Step 2: Latent Heat Calculation
The latent heat portion addresses moisture removal and is calculated using:
CFM = (Latent BTU Load) ÷ (0.68 × ΔW)
Where:
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Latent BTU Load = The heat load required to remove moisture from the air.
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0.68 = Air constant for latent heat (based on air properties).
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ΔW = Difference in humidity ratios (grains of moisture per pound of air).
💡 Humidity ratio is typically determined using a psychrometric chart or a digital hygrometer.
Step 3: Total Cooling Load Calculation
The combined effect of both heat types is calculated using this formula:
Total Cooling BTU = 4.5 × CFM × (h1 - h2)
Where:
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4.5 = Constant (standard air density × 60 minutes).
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h1 = Enthalpy of return air (found using the wet bulb temperature).
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h2 = Enthalpy of supply air (found using the wet bulb temperature).
Step 4: Example Calculation
Scenario:
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System requires 36,000 BTU (3 tons) of cooling.
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Desired supply air temp is 55°F.
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Return air temp is 75°F with 50% humidity.
Step 1: Calculate Sensible CFM:
CFM = (36,000 × 0.7) ÷ (1.08 × (75 - 55))
CFM = 25,200 ÷ (1.08 × 20)
CFM = 25,200 ÷ 21.6
CFM = 1,167
Step 2: Calculate Latent CFM:
(Assuming ΔW = 0.009 from psychrometric data)
CFM = (36,000 × 0.3) ÷ (0.68 × 0.009)
CFM = 10,800 ÷ (0.68 × 0.009)
CFM = 10,800 ÷ 0.00612
CFM = 1,765
Step 3: Calculate Total Cooling Load:
(Assuming h1 = 28.3 and h2 = 22.5 from wet bulb data)
Total Cooling BTU = 4.5 × 1,167 × (28.3 - 22.5)
Total Cooling BTU = 4.5 × 1,167 × 5.8
Total Cooling BTU = 30,489
Step 5: Airflow Adjustments
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If calculated CFM is too low: Increase blower speed or improve duct design.
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If calculated CFM is too high: Adjust damper positions or reduce blower speed.
Key Tips for Accurate Measurements
✅ Use a reliable psychrometer or digital hygrometer for precise wet bulb readings.
✅ When calculating latent heat, ensure you account for high humidity loads in humid climates.
✅ Confirm blower settings align with calculated CFM to avoid over/underperformance.
