Heat Airflow (CFM by Temperature Rise)

Show Formula Explanation

Formula:
CFM = BTU_output ÷ (1.08 × ΔT)

Where:

  • BTU_output = BTU_input × efficiency (e.g. 80% = 0.80)
  • ΔT = Supply air temp – Return air temp (°F)
  • 1.08 = Constant (0.24 Btu/lb-°F × 60 min/hr × 0.075 lb/ft³ air)

How to use:

  • Enter measured ΔT and select “Actual” → gives actual airflow.
  • Enter a rated ΔT from the furnace’s nameplate and select “Target” → gives required airflow.

Airflow (CFM by Temperature Rise) — Electric Heat



Airflow (CFM by Temperature Rise)





Airflow (CFM by Temperature Rise)

Static Pressure Total External (TESP)

Purpose: Shows total resistance blower is working against. Compare to equipment data plate rating (e.g., 0.5" WC).

 

  • Return Pressure (–) = Negative pressure measured in return duct before blower.

  • Supply Pressure (+) = Positive pressure measured in supply plenum after coil.

  • Both measured in inches of water column (in. WC) using a manometer.


 

Airflow (CFM) by Temperature Rise

Purpose: Checks actual delivered airflow without needing a flow hood.

 

  • BTU Input = Gas appliance input (from nameplate or meter clocking).

  • Efficiency = Appliance steady-state efficiency (e.g., 80% = 0.80).

  • 3.413 = Conversion factor: 1 watt = 3.413 BTU/hr.

  • 1.08 = Constant = 0.24 (specific heat of air) × 60 (minutes/hr) × 0.075 (density of air in lb/ft³).

  • ΔT (Temp Rise) = Supply air temp – Return air temp (measured with thermometer).


 

Gas Meter Clocking (Firing Rate / Input BTU)

This calculates actual furnace or boiler gas input. Compare to nameplate to check over/under-firing.

 

 

  • 3600 = Number of seconds in 1 hour.

  • Seconds per Dial Revolution = How long it takes for the gas meter’s test dial to make one full turn (measured with a stopwatch).

  • Dial Size (ft³) = Volume of gas that one revolution represents (stamped on the meter dial: ½ ft³, 1 ft³, 2 ft³).

        Use in decimal form in formula: (½ ft³ = 0.50,     1 ft³= 1.0,      2 ft³= 2.0, etc.) 

  • 1000 = Approximate BTUs in one cubic foot of natural gas. *

 * On a local gas bill, look for the “therm factor,” “heat content,” or “BTU per cubic foot” value — utilities list the actual energy content of one cubic foot of natural gas delivered, which often ranges from ~1,000 to 1,050 BTU/ft³, and that number should be used instead of the flat 1,000 BTU constant for the most accurate clocking calculations. 

Multiply "Therm Factor" by 1000. Example: Therm Factor = 1.066 x 1000 = 1,066 BTUs

Bottom line:

  • If the bill says BTU/ft³ → use directly.

  • If it gives a Therm Factor → multiply by 1000.

  • If it’s in MJ/m³ → multiply by 26.84.