Calculate the BTU/h of a natural-gas fired upflow furnace. Measured data: 1,532 cfm, return air 67°F, supply air 113°F.

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Multiple Choice

Calculate the BTU/h of a natural-gas fired upflow furnace. Measured data: 1,532 cfm, return air 67°F, supply air 113°F.

This question tests how to estimate the heating rate the furnace delivers to the air from the measured airflow and temperature rise. The sensible heat added to moving air is given by Q = 1.08 × CFM × ΔT, where ΔT is the temperature rise (supply minus return) and 1.08 comes from combining air density, specific heat, and unit conversions. Here, ΔT = 113°F − 67°F = 46°F, and CFM = 1,532. So Q = 1.08 × 1,532 × 46 = 1.08 × 70,472 ≈ 76,110 BTU/h (approximately 76,109 BTU/h with precise rounding). This is the furnace’s heat delivered to the air under these conditions (the sensible output). If you needed the gas input, you’d divide by efficiency, but the data given reflect the delivered BTU/h.

This question tests how to estimate the heating rate the furnace delivers to the air from the measured airflow and temperature rise. The sensible heat added to moving air is given by Q = 1.08 × CFM × ΔT, where ΔT is the temperature rise (supply minus return) and 1.08 comes from combining air density, specific heat, and unit conversions.

Here, ΔT = 113°F − 67°F = 46°F, and CFM = 1,532. So Q = 1.08 × 1,532 × 46 = 1.08 × 70,472 ≈ 76,110 BTU/h (approximately 76,109 BTU/h with precise rounding).

This is the furnace’s heat delivered to the air under these conditions (the sensible output). If you needed the gas input, you’d divide by efficiency, but the data given reflect the delivered BTU/h.

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