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Understanding Heat Loss/HVAC

Updated: Jan 10

Heat loss is a significant component of any heating or cooling system, whether it be residential or commercial. The amount of heat that is lost in a building depends on many different factors, but there are two main components to consider. These are radiation and conduction. Both these processes have a profound impact on the efficiency of a home or business. Therefore, it is important to understand them so that you can determine how much heat is being lost and what you can do about it.

Understanding Heat Loss


The term conduction is commonly used to refer to the movement of heat from one region to another in a system. Conduction is the process of transmitting heat from a warmer object to a cooler object through physical contact.

It is the most important method of heat transfer between solids and liquids. In a solid, conduction occurs when the temperature gradient in a material is large enough to permit heat to flow from one side to the other. This is often a result of vibrations and collisions of molecules.

Conduction can also occur in a liquid medium when a molecule is in motion. For example, when a water molecule passes through a stream of air, the water expands, and heat is transferred.


Convection heat loss occurs when a hot body is exposed to a cooler surface. The temperature difference between the two is not significant at any pressure but becomes appreciable at higher pressures.

The rate of convection-heat transfer is proportional to the temperature difference between the two materials. This is determined by the physical properties of the fluid and the geometry of the surfaces.

The Nusselt number is a measure of convective heat loss. A high Nusselt number indicates that the convective loss is large.

It is difficult to accurately determine the magnitude of convection losses. Instead, they are usually calculated by subtracting the other heat losses from the energy balance.

For example, the amount of heat loss from a convective system can be reduced by using thin glass plates. In addition, thicker clothing and more insulating barriers prevent the transfer of heat.


When it comes to heat loss, the human body is not the only recipient. This is a good thing, albeit a bad one. To keep our cool we need not resort to air conditioning or forced air, both of which are non-conventional and thus subject to the whims of the aforementioned etacopter. In fact, the aforementioned human occupant is a hotbed of flammable liquids and not a welcome sight in a hot tub. Of course, you have to keep the temperature within a reasonable range, which is another story in and of itself. The aforementioned occupant is no stranger to heat-related accidents, the most notable being the oh-so-famous heat stroke. Thankfully, there is a robust set of controls in place to minimize such incidents, but a little heat loss is still a big deal. Your HVAC system must be up to code.


Evaporation and heat loss occurs when the surface of a body is wet. As the temperature increases, the water takes up the heat and evaporates. A liquid with a larger surface area can evaporate more quickly. During summer months, the sun warms the water, increasing the evaporation rate. In winter, the water takes on less heat.

Evaporation is an efficient heat loss mechanism. The energy needed to evaporate water is greater than the energy required to warm the water. However, evaporation can be very expensive. It is also very important in areas with seasonal droughts.

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