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What does BTU stand for and how to calculate it?

What is a Btu (British Thermal Unit) air conditioner? The British Thermal Unit, or Btu, is still used to measure heat in North America and many other nations. The SI metric system Joule is used in various nations. Because many product data sheets use both units, knowing the equivalent of both units is crucial.

The amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit is measured in Btu.

To raise the temperature of 1 pound of water from 63 to 64 degrees Fahrenheit, 1 Btu is required. The equivalent in SI metric is 4.187 kJ to raise the temperature of 1 kilogram of water from 17 to 18 degrees Celsius.

Example 1: Find the amount of heat needed to increase the temperature of 50 lb. of water from 50 °F to 80 °F.


Heat required = weight in lb. X temperature change in °F

= 50 lb. X (80-50)°F

= 50 X 30 Btu

= 1,500 Btu

Example 2 (SI metric): Find the amount of heat needed to increase the temperature of 2kg of water from 5 °C to 30 °C.


Heat required in KJ = 4.817 X mass in kg X temperature change in °C

= 4.817 X 2kg X (30-5)°C

= 4.817 X 2kg X 25 °C

= 240.85 kJ

If the water is cooled, heat is removed from the it.


1 Btu = 1.055kJ

1 kJ = 0.948 Btu

Refrigeration Unit

Ton of refrigeration unit has been used for a long time and it represents the heat that is absorbed when 1 ton or 2,000lb. of ice melts in 24 hours. The ice is assumed to be at 32 °F (0 °C) and melted to become water at the same temperature. The more commonly used unit to rate a HVAC equipment is in Btu/hr.

1 ton of refrigeration effect = [Weight of 1 ton of ice (2000 lb.) X latent heat of melting of ice (144 Btu/lb)]/24 hours = [(2,000 X 144)/24] Btu/hr = [288,000/24] Btu/hr = 12,000 Btu/hr Hence, a 12,000 Btu/hr cooling capacity is equivalent to 1 ton of refrigeration.

Calculation In SI Metric Units Latent heat is 335 kJ/kg 1 ton is about 907 kg 1 ton of refrigeration effect = [907kg X 335kJ/kg]/[24 X 3600 sec] = 3.5167 kJ/sec = 3.5167 kW

Air Conditioner Btu Equivalents Summary

1 Btu/hr = 0.29W

1 kW = 3415 Btu/hr

1 ton = 12,000 Btu/hr

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