HVAC Converter
By KAMP Inc. / UnitOwl · Last reviewed:
HVAC — heating, ventilation, and air conditioning — uses a distinctive set of units to describe thermal capacity, efficiency, and airflow. Understanding BTU/hr, tons of cooling, SEER ratings, and CFM is essential for sizing equipment, comparing energy efficiency, and interpreting the specifications on heating and cooling equipment. These units span the imperial and metric divide, with the US relying heavily on BTU-based measurements while the rest of the world uses kilowatts.
Popular HVAC Conversions
BTU, Tons of Cooling, and Kilowatts
The British Thermal Unit (BTU) is the energy required to raise one pound of water by 1°F. In HVAC, capacity is rated in BTU per hour (BTU/hr) or simply BTU. One ton of cooling equals 12,000 BTU/hr — derived from the heat absorbed by melting one ton of ice in 24 hours. A 2-ton central air conditioner has 24,000 BTU/hr of cooling capacity. In the metric world, the same capacity is expressed in kilowatts: 1 ton = 3.517 kW, so a 2-ton unit ≈ 7.03 kW. Rule of thumb for residential cooling: 12,000 BTU/hr cools approximately 450–550 sq ft (42–51 m²), though this varies with insulation, ceiling height, climate, and solar gain.
| Rating | Equivalent |
|---|---|
| 1 ton of cooling | 12,000 BTU/hr = 3.517 kW |
| 2 ton | 24,000 BTU/hr = 7.03 kW |
| 3 ton | 36,000 BTU/hr = 10.55 kW |
| 5 ton | 60,000 BTU/hr = 17.58 kW |
| 1 kW cooling | 3,412 BTU/hr / 0.284 tons |
| 1 MBH | 1,000 BTU/hr (heating loads) |
SEER, EER, and Efficiency Ratings
SEER (Seasonal Energy Efficiency Ratio) measures how much cooling a system delivers per unit of electricity consumed over a typical cooling season. SEER = total cooling output (BTU) / total electrical energy input (watt-hours). Higher SEER is better: a 14 SEER unit uses 14 BTU of cooling per watt-hour. Federal minimum standards in the US have risen over the years — the 2023 minimum is 15 SEER2 (a revised test procedure). High-efficiency units reach SEER 20–30. EER (Energy Efficiency Ratio) is a snapshot efficiency at a specific temperature (95°F outdoor, 80°F/50% RH indoor). For heating, AFUE (Annual Fuel Utilization Efficiency) measures furnace efficiency: a 96 AFUE furnace converts 96% of gas energy to heat. Heat pumps use HSPF (Heating Seasonal Performance Factor) for heating efficiency.
Airflow: CFM, CMH, and Duct Sizing
Airflow in HVAC is measured in cubic feet per minute (CFM) in the US or cubic meters per hour (m³/h) in metric countries. 1 CFM = 1.699 m³/h. Proper airflow is critical: too little causes poor temperature distribution and equipment damage; too much increases noise and energy use. A rule of thumb is 400 CFM per ton of cooling capacity for most residential systems. A 3-ton system should deliver around 1,200 CFM through the duct system. For ventilation, residential building codes often specify air changes per hour (ACH): a bedroom might require 0.35 ACH minimum. To calculate CFM from ACH: CFM = (room volume in ft³ × ACH) / 60. A 1,000 ft³ bedroom at 0.35 ACH requires about 5.8 CFM of outside air.
Sources & References
- NIST — Units and Conversion Factors — Official unit conversion factors from the National Institute of Standards and Technology.
- BIPM — The International System of Units (SI) — International SI unit definitions from the International Bureau of Weights and Measures.