Checking Airflow | Ferguson HVAC, Exams of Aerodynamics

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Checking Airflow

When checking Air Flow:

• Make sure that all duct

dampers and registers are

open and un-blocked

• Ensure that a CLEAN filter is in

place

• Make sure that all interior

doors are open

Temperature Rise

Temperature Rise

Airflow Calculations

For Furnaces

BTUH 100, Supply Air Temp: 120 F Return Air Temp. 70 F 120 F – 70F = 50F (Delta T) 100, 1.08 X 50 100,000 = 1851 CFM 54 _________________________________ BTUH OUTPUT 1.08 X Temperature Rise ( Delta T) CFM =

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Example

BTUH = 70,000 x 90% (AFUE) = 63000 BTUH Return Temperature = 70°F Supply Temperature = 125°F ∆T = 55°F

63000 BTUH

Sensible Heat Method

1. Place the unit in the HEATING mode of operation
2. Move the blower speed tap for cooling to the heating terminal
3. Be sure to disconnect the Heating speed lead and secure it safely
4. Subtract your measured return air temperature from your supply air temperature. This is the Delta-T value.
5. Multiply the Delta-T value by 1.08. Record this value.
6. Divide your BTUH Output by the value you obtained in Step 5.
7. The answer is your actual system airflow.

Airflow Calculations for Furnaces

(Cooling Mode)

Formulas:

Heating Output = KW x 3413 x Corr. Factor Actual CFM = CFM (from table) x Corr. Factor BTUH = KW x 3413 BTUH = CFM x 1.08 x Temperature Rise (T) CFM = KW x 3413 1.08 x T T = BTUH CFM x 1.

• Static pressure is the resistance to airflow from objects in the air stream
• Filters, coils, heat exchangers, registers, grills, balancing dampers and, of course, the duct itself, are just a few
• After the resistance of these objects is subtracted, the balance is what is available for the duct system

Static Pressure

Furnace Static Pressure

Most Residential Systems

Including

Goodman/Amana

Are designed To Operate

At 0.5” Water Column maximum

Total System Static Pressure

Static Pressure

Magnehelic or Inclined Manometer