Engineering
HVAC sensible cooling/heating BTU/hr (1.08 * CFM * DeltaT) & Air Changes (ACH)
Calculate HVAC sensible heat transfer in BTU/hr and cooling tons from airflow (CFM) and supply-to-room temperature delta, plus room Air Changes per Hour (ACH).
Where the HVAC 1.08 constant comes from—and how CFM drives BTU/hr & ACH
At standard sea-level air density (0.075 lb/ft^3) and specific heat (0.24 BTU/(lb·°F)) multiplied by 60 min/hr, sensible heat capacity equals 0.075 * 0.24 * 60 = 1.08 BTU/(hr·CFM·°F)! Delivering 1,200 CFM of 55 °F supply air into a 75 °F zone (DeltaT = 20 °F) removes 1.08 * 1,200 * 20 = 25,920 BTU/hr (2.16 cooling tons at 12,000 BTU/hr per ton).
The ACH arithmetic is airflow*60/room volume in consistent cubic-foot units. It is a nominal turnover rate under a mixing assumption, not a count of complete replacements. Recirculated supply air is not automatically outdoor ventilation or clean-air delivery. No cleanroom, laboratory or health compliance is established here.
| HVAC Design Metric (1,200 CFM, 75 °F Room, 55 °F Supply) | Formula | Output |
|---|---|---|
| Sensible Cooling Load (BTU/hr) | 1.08 * 1,200 * ABS(75 - 55) | 25,920 BTU/hr |
| Sensible Cooling Tonnage (Tons) | 25,920 / 12,000 | 2.16 Tons |
| Air Changes per Hour (9,000 ft^3 room) | (1,200 * 60) / 9,000 | 8.0 ACH |
| Required CFM for 12 ACH Target | (12 * 9,000) / 60 | 1,800 CFM |
Sensible cooling or heating output in BTU/hr
A2=CFM (1200), B2=RoomTemp_F (75), C2=SupplyTemp_F (55).
The coefficient 1.08 assumes the stated density and specific heat. ABS reports the magnitude and removes heating/cooling direction. This is sensible heat only: latent moisture loads and full building/equipment sizing are excluded.
=IF(OR(COUNT(A2:C2)<>3,A2<0),"Invalid inputs",ROUND(1.08 * A2 * ABS(B2 - C2), 0))Sensible heat expressed in refrigeration-ton equivalents
Dividing sensible BTU/hour by 12,000 gives a unit conversion, not a total cooling-equipment rating. Nominal ACH is shown separately.
=IF(OR(COUNT(A2:C2)<>3,A2<0),"Invalid inputs",ROUND((1.08 * A2 * ABS(B2 - C2)) / 12000, 2))Nominal airflow turnovers per hour
D2 is room volume in cubic feet. Specify whether the supplied airflow is total, outdoor or filtered air before interpreting the number.
=IF(OR(NOT(ISNUMBER(A2)),NOT(ISNUMBER(D2)),A2<0,D2<=0),"Invalid inputs",ROUND((A2 * 60) / D2, 2))Try the example
Use the original values or change them to test your understanding.