## UNITS OF HEAT ENERGY AND HEAT POWER BASIC INFORMATION

A British thermal unit (Btu) is used to describe the quantity of heat contained in a given substance. Because the Fahrenheit scale is the standard system of temperature measurement used in the United States, a Btu is defined as the amount of heat required to raise the temperature of 1 pound (lb) of water 1 degree Fahrenheit (°F). A Btu is a unit of heat energy.

The rate of heat consumption, called power, can be determined with a unit of heat energy and a unit of time. For example, Btu per minute or Btu per hour, written: Btu/m or Btum and Btu/hr or Btuh are common units of power used in HVAC work. Btu/hr is a unit of heat power.

Other expressions of heat power are:

1000 Btuh (can be expressed as MBh).
1,000,000 Btuh (can be expressed as MMBh).
(Roman numeral “M” = 1000, Roman numeral “MM” =
1,000,000).

UNITS OF ELECTRICAL ENERGY AND ELECTRICAL POWER
Watt-hours (Wh) or kilowatt-hours (kWh,) are units of electrical energy. A kilowatt-hour is equivalent to 1000 Watt-hours.

Watts (W) and kilowatts (kW) are units of electrical power. A kilowatt is equivalent to 1000 Watts (kilo (k) = 1000).

POWER EQUALS ENERGY TIMES TIME, P = E × t.
Btu/hr (a unit of power) = Btu (a unit of energy) × hr (a unit of time). kW (a unit of power) = kWh (a unit of energy) × hr (a unit of time).

Mathematically, kWh is also expressed kW/hr.

ELECTRICAL POWER AND HEAT POWER EQUIVALENTS
1 W = 3.413 Btuh
1 kW = 3413 Btuh

MOTOR HORSEPOWER AND HEAT POWER EQUIVALENTS
1 hp (horsepower) = 2545 Btuh

MOTOR HORSEPOWER AND ELECTRICAL POWER EQUIVALENTS
1 hp = 746 W
1 hp = 0.746 kW

BOILER HORSEPOWER AND HEAT POWER EQUIVALENTS
1 boiler horsepower = 33,475 Btuh

HEAT CONTENT AND SPECIFIC HEAT
The heat content of a substance is called enthalpy. The symbol is h. The units of enthalpy are Btu/lb or Btu/l °F. Specific heat (Fahrenheit scale) is the amount of heat necessary to raise the temperature of 1 lb of a substance 1°F. Every substance has a different specific heat. Some examples of specific heat values for HVAC substances are: The specific heat of water is 1.0 Btu/lb°F. The specific heat of air is 0.24 Btu/lb°F and the specific heat of steam or ice is approximately 0.5 Btu/lb°F.