As HVACR professionals, we are in the business of heat. We install systems in buildings and homes that either add heat or remove it. We maintain, tune, and repair these systems for our customers. We use our knowledge of how heat travels from one substance to another to enhance our customers' comfort and to help keep their utility expenditure to a minimum.

In this article, we will examine what heat is, the various types of heat, the dynamics involved in transferring heat from one substance to another, and the measurement of heat.

Heat - A form of energy that, when added to a substance, causes an increase in the rate of motion of its molecules. Molecular motion increases as the temperature of the substance increases. Temperature is a level of heat. Conversely, as heat is removed from the substance, molecular motion decreases. The point at which molecular motion stops and no traces of heat exist in a substance is said to occur at -460 degrees F.

Types Of Heat

Latent heat- The heat added to a substance to cause a change of state without changing its temperature.

There are two types of latent heat. The latent heat of fusion is the heat involved when a substance changes from a liquid to a solid and from a solid to a liquid. Ice melting or freezing is a common example.

The latent heat of vaporization is the heat involved when a substance changes from a liquid to gas and from a gas to liquid. An example is a steam boiler. Steam produced by the boiler travels through pipes to radiators. The steam condenses and turns back to a liquid when it hits the cooler radiator.

Sensible heat - The heat that is added to or subtracted from a substance to change its temperature. Raise the temperature in a room from 50 degrees to 70 degrees. The 20 degree difference is an example of sensible heat.

Heat Transfer

Understanding why heat transfers from one substance to another is one of the most important concepts for the HVACR professional to learn. This concept is rooted within the second law of thermodynamics: heat travels from hot to cold. For heat to travel, a temperature difference must be maintained.

Methods Of Transfer

Convection- The transference of heat from one location to another by means of a fluid, most commonly air and water. An example of natural convection is baseboard heating. Cooler air in the room falls, displacing the lighter, warmer air emitted from the baseboard heaters. This produces natural air currents and warms the room. Forced convection occurs when a furnace blower moves heated air through ductwork and into the area to be conditioned.

Conduction - The transference of heat between parts of a substance or separate substances within direct contact of each other. Burners in a furnace first warm the part of the heat exchanger that is closest to it. The entire heat exchanger warms as heat travels to its furthest extremities. Warm air passing over a cool evaporator is an example of conduction between the separate substances of air and the metal cooling coils.

Radiation - The transference of heat via heat rays. The most common example of radiation is the sun's rays. The rays will travel through space and heat the first object that they come in contact with, not necessarily the surrounding atmosphere. This explains why a car sitting in direct sunlight in the middle of winter is warmer than the surrounding air.

Heat Measurement

Temperature- The degree or level of heat within a substance. Measured with a thermometer, temperature is expressed in Fahrenheit or Celsius degrees.

British thermal unit - Known as Btu, the British thermal unit is the amount of heat required to raise the temperature of 1 pound of water 1 degree F.

Specific heat - The amount of Btu necessary to change 1 pound of a substance 1 degree F.

Whether you install, service, or design heating and air conditioning systems, a basic understanding of the fundamentals of heat is an absolute, solid, liquid, vapor kind of necessity.

David E. Rothacker is a member of the National Comfort Institute's Advisory Board and a founding member of the National Comfort Team. For questions or comments on Tech Basics, contact Rothacker at

Publication date: 05/31/2004