7.1: Residental Heat Loss (I)

Houses are heated to keep the temperature inside at about 65°F when the outside temperature is lower. A house requires heat continuously because of the heat loss. Heat can escape from a house through various places; some are well known and some are not noticeable. Heat can escape from the roof, walls, doors, windows, basement walls, chimney, vents, and even the floor, as shown in Figure 7.1.1.

Figure 7.1.1. Heat loss examples in a house

As shown in Figure 7.1.2, the more heat the house leaks, the more the furnace has to put out to make up for the loss. For the furnace to generate more heat to compensate the heat loss, more fuel needs to be put into the furnace, hence higher fuel or heating costs.

Figure 7.1.2. Heat loss flowchart

As you will recall from Chapter 3, not all energy conversion devices are efficient. Thus, it is important to note that furnaces are not 100 percent efficient. When a furnace’s efficiency is lower, the fuel consumption for the same amount of heat output will be even higher.

Mechanisms of Heat Loss/Transfer

As shown in Figure 7.1.3, heat escapes (or transfers) from inside to outside (high temperature to low temperature) by three mechanisms (either individually or in combination) from a home:

• Conduction
• Convection
• Radiation

Figure 7.1.3. Heat transfer by conduction, convection, and radiation

Conduction

Conduction is a process by which heat is transferred from the hot area of a solid object to the cool area of a solid object by the collisions of particles.

In other words, in solids the atoms or molecules do not have the freedom to move, as liquids or gases do, so the energy is stored in the vibration of atoms. An atom or molecule with more energy transfers energy to an adjacent atom or molecule by physical contact or collision.

In the image below, heat (energy) is conducted from the end of the rod in the candle flame further down to the cooler end of the rod as the vibrations of one molecule are passed to the next; however, there is no movement of energetic atoms or molecules.

The following video illustrates conduction.

With regard to residential heating, the heat is transferred by conduction through solids like walls, floors, and the roof, as shown in the video below.

Convection

Convection is a process by which heat is transferred from one part of a fluid (liquid or gas) to another by the bulk movement of the fluid itself. Hot regions of a fluid or gas are less dense than cooler regions, so they tend to rise. As the warmer fluids rise, they are replaced by cooler fluid or gases from above.

In the video below, heat (energy) coming from candle flame rises and is replaced by the cool air surrounding it.

In residential heating, convection is the mechanism by which heat is lost by warm air leaking to the outside when the doors are opened, or cold air leaking into the house through the cracks or openings in walls, windows, or doors. When cold air comes in contact with the heater in a room, it absorbs the heat and rises. Cold air, being heavy, sinks to the floor and gets heated, and thus slowly heats the whole room air. This is depicted in the video below.

Radiation

Radiation is transfer of heat through electromagnetic waves through space. Unlike convection or conduction, where energy from gases, liquids, and solids is transferred by the molecules with or without their physical movement, radiation does not need any medium (molecules or atoms). Energy can be transferred by radiation even in a vacuum.

In the video below, sunlight travels to the earth through space, where there are no gases, solids, or liquids.

Reducing Energy Consumption

There are two ways in which we can reduce energy consumption.

1. The most cost-effective way is to improve the home’s “envelope”—the walls, windows, doors, roof, and floors that enclose the home—by improving the insulation (conduction losses) and sealing the air leaks with caulking (convection losses).
2. The second way to reduce the energy consumption is by improving the efficiency of the furnace that provides the heat.

Conduction Heat Loss

Most heat is lost through a house's walls through conduction. As you learned from the activity on the previous screen, the amount of heat loss depends on three factors:

• Size of the house (area through which the heat can escape)
• Local weather or climatic conditions:
  • The inside temperature is often constant at a comfortable temperature of 65°F.
  • As the outside temperature falls lower than 65°F, the heat is lost to the outside.
  • The higher the temperature difference, the higher the heat loss to outside.
  • By calculating the Heating Degree Days (HDD), we can determine how many degrees the mean temperature fell below 65ºF for the day.
• Wall's capacity to resist heat loss.
  • Insulation is rated in terms of thermal resistance, called R-value, which indicates the resistance to heat flow.
  • The higher the R-value, the greater is the insulating effectiveness.