On the Distribution screen, there are 2 radio buttons that can be used to calculate Distribution Efficiency, BPI and Duct Testing (Pressure Pan). This article discusses how the Duct Testing (Pressure Pan) determines distribution efficiency:

**Part 1: Calculate the Q50Ext**

Q50Ext is a calculation that relates the duct leakage to the outdoors

The formula is Q50Ext= (Ai)b * (square root (sum: ppan readings (pa)))

where

Ai = Total free area of all supply & return registers in sq in

B = empirically derived exponent = .668

Sum: ppan readings = Sum of pressure pan pressures (pa)

**Part 2: Use the Q50Ext value to Calculate the Efficiency**

Efficiency: 1-(Q50Ext/Fan Air Flow)

Where the Fan airflow comes from a table of Fan Airflow Multipliers:

Electronic Draft Furnaces = 130 CFM *(System Size, (BTU/Hr)/10,000 BTU)

Condensing Furnaces = 150 CFM *(System Size, (BTU/Hr)/10,000 BTU)

Central Air Conditioners & Heat Pumps = 400 CFM *(System Size, (BTU/Hr)/12,000 BTU)

Other Draft Furnaces = 100 CFM (System Size, (BTU/Hr)/10,000 BTU)

(note: When using P-Pan distribution calculations documented above, per DOE advised standards, the lowest efficiency allowed is 58%. If the calculation is lower than 58% we report it as 58%.)

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