# How to Determine the Fan Curves for Each Efficiency Level and Product Class

In order to generate the fan performance data used in the analysis DOE(Department of Energy) applied the following procedure (the 3-ton baseline PSC motor for non-weatherized (non-condensing) gas furnaces is used as an example):

STEP 1: Using the coefficients to generate for airflow (cfm) vs. pressure and watts/cfm vs. pressure curves at each airflow speed (heating, cooling, and continuous fan), DOE found the airflow cfm and watts per CFM at DOE’s reference system curve external static pressure.For example, DOE’s reference system curve external static pressure is 0.65 in.w.g at the maximum cooling airflow speed for non-weatherized gas furnaces.For the 3-ton baseline PSC motor for non-weatherized (non-condensing) gas furnaces the fan airflow CFM was calculate to be 1158 cubic feet per minute.

STEP 2: Using the BE equation above, DOE multiplied the airflow times the watt/cfm at each pressure from Step 1 to calculate BE at each airflow speed (heating, cooling, and continuous fan) in terms of DOE’s reference system curve external static pressure. For example, for the 3-ton baseline PSC motor for non-weatherized (non-condensing) gas furnaces BE was calculate to be 382 watts at heating, 495 watts at cooling, and 382 watts at continuous fan.

STEP 3: Using the calculated maximum airflow CFM and BE values at DOE’s reference system curve external static pressure, DOE was able to calculate Furnace Efficiency Rating (FER) values. For example, for the 3-ton baseline PSC motor for non-weatherized (non-condensing) gas furnaces FER was calculated as 363.

STEP 4: The constant curve fit parameter (m0) in the pressure and watts/cfm vs. pressure curves was then adjusted using an adjustment multiplier in order to match the FER values derived in the engineering analysis. For example, for the 3-ton baseline PSC motor for non-weatherized (non-condensing) gas furnaces the FER value derived in the engineering analysis was 380, so the adjustment multiplier to convert the FER value calculated in step 3 was calculated as 1.04.

Table 1.1 shows the airflow (cfm) vs. pressure coefficients determined for non-weatherized (non-condensing) gas furnaces (3-ton) at each efficiency level (EL). Figure 1.1 to Figure 1.2 show the resulting curves at various pressures and operating modes.Table 1.1 Coefficients for CFM equation for Non-Weatherized (Non-Condensing) Gas Furnace Fan, 3-Ton

Figure 1.1 CFM Curves for Non-Weatherized (Non-Condensing) Gas Furnace Fan, 3-Ton (Heating Mode)

Figure 1.2 CFM Curves for Non-Weatherized (Non-Condensing) Gas Furnace Fan, 3-Ton (Cooling Mode)

Figure 1.3 CFM Curves for Non-Weatherized(Non-Condensing) Gas Furnace Fan, 3-Ton (Continuous Fan mode)

Table 1.12 shows the watts/cfm vs. pressure curves coefficients determined for non-weatherized (non-condensing) gas furnaces (3-ton) at each efficiency level. Figure 1.4 to Figure 1.6 show the resulting curves at various pressures. Figure 1.7 to Figure 1.9 show the resulting Watts vs. pressure curves.

Table 1.2 Coefficients for Watts/CFM Equation for Non-Weatherized (Non-Condensing) Gas Furnaces, 3-Ton

Figure 1.4 Watt/CFM Curves for Non-Weatherized(Non-Condensing) Gas Furnace Fan, 3-Ton(Heating Mode)

Figure 1.5 Watt/CFM Curves for Non-Weatherized (Non-Condensing) Gas Furnace Fan, 3-Ton (Cooling Mode)

Figure 1.6 Watt/CFM Curves for Non-Weatherized (Non-Condensing) Gas Furnace Fan, 3-Ton (Continuous Fan Mode)

Figure 1.7 Resulting Watt vs. Pressure Curves for NonWeatherized (Non-Condensing) Gas Furnace Fan, 3Ton (Heating Mode)

Figure 1.8 Resulting Watt vs. Pressure Curves for Non-Weatherized (Non-Condensing) Gas Furnace Fan, 3Ton (Cooling Mode)

Figure 1.9 Resulting Watt vs. Pressure Curves for Non-Weatherized (Non-Condensing) Gas Furnace Fan, 3Ton (Continuous Fan Mode)

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