Heating and Air Conditioning

In the late nineteenth and early twentieth centuries, heating a home was a matter of tossing another log or more coal into the fireplace or stove. "Air conditioning" was the shade of a backyard tree. Twenty-first century homes use far more sophisticated technological tools—tools that a homeowner is likely to take for granted.

A heating and air conditioning contractor has to be as much of a scientist as a builder or technician. The course requirements of typical heating and air conditioning programs at community or junior colleges place a heavy emphasis on chemistry, physics, engineering, and mathematics, including geometry and algebra . Good heating and air conditioning contractors are able to compute formulas and equations in order to arrive at volumes, pressures, and degrees. They must be able to accurately measure distances, angles, circles, arcs, temperatures, weights, and volumes. They also must identify and interpret geometric figures, graphs, scales, and gauge indications. Further, they must know the scientific principles that are central to their work, including heat transfer, combustion , temperature, pressure, electricity, and magnetism. They should also know the physical and chemical properties of commonly used substances such as refrigerants and hydrocarbons .

The Science of Cooling

The science of cooling is rooted in the Second Law of Thermodynamics, which states that heat only flows from higher to lower temperature levels, and never the other way around. Using this law, physicists can explain exactly how an air conditioner (or a refrigerator, which uses the same process) works. They can also use related principles, such as those of exergy and anergy , to design better, more efficient cooling systems.

How does a typical air conditioner work? It lowers the temperature by continuous extraction of heat energy using a thermodynamic cycle. The most common of these cycles is called the vapor-compression refrigeration cycle, sometimes called the Rankine cycle. In this cycle, a substance known as the "working fluid," or refrigerant, goes through cyclical changes of state in a closed loop. This loop is made up of four parts: an evaporator, a compressor, a condenser, and a throttle valve. The evaporator is installed in the space to be cooled while the other parts are installed outside of the space.

Before it enters the evaporator, the working fluid is a liquid or liquid-vapor mixture. Its pressure is low, and its temperature is below that of the space to be cooled, also called the "cold room." In the evaporator, the fluid takes up heat from the cold room because of the temperature difference. The fluid-vapor is then brought from low to high pressure in the compressor, which increases its temperature. In a well-designed system, the temperature of the fluid-vapor leaving the compressor should be above that of the surroundings, providing the temperature difference necessary for removing heat from the fluid to the surroundings. This occurs in the condenser, where the fluid undergoes a phase change from vapor to liquid because heat has been removed. The loop is closed by the throttle valve, where the fluid is expanded from the high condenser pressure to the low evaporator pressure.

One issue that scientists struggle with is finding a good working fluid. Water would be, at first glance, the ideal fluid because it is inexpensive and safe. Its thermodynamic properties, though, prevent it from being the best choice. Water vapor has a very low density, so using it would require huge piping volumes and a lot of work on the part of the compressor. Chlorofluorocarbons (CFCs) work better, but unfortunately they contribute to ozone depletion in the stratosphere. In large industrial refrigeration plants, ammonia (NH₃) is the most common choice.

Contractors who install air-conditioning systems are probably not thinking about the vapor-compression refrigeration cycle. They are, however, thinking about what size air conditioner is needed to keep you comfortable on hot summer days. Factors that must be measured and taken into account in deciding on the size of an air-conditioning system are:

• the geographical location, and therefore average temperatures during the cooling season;
• the length of walls in the rooms, including walls not exposed to direct sunlight, those that are exposed to direct sunlight, and interior walls;
• the type of wall frame construction (framing, masonry, etc.);
• the ceiling height;
• the ceiling area, and the presence and amount of insulation above the ceiling;
• the space's floor area;
• the width of doors and arches;
• the window area, the orientation of the windows (north, south, etc.), and the type of glass (single-pane, double-pane, block);
• the number of people who normally occupy the room (giving off body heat);
• the amount of heat given off by lights and appliances; and
• the hours of operation.

Taken together, these factors determine the cooling capacity that is needed, usually measured in British thermal units (BTUs); one BTU is the amount of energy needed to raise the temperature of a pound of water one degree Fahrenheit.

The Science of Heating

In many respects the science of heating a home is much simpler than cooling: Unless you have solar heat or a windmill, something somewhere gets burned, and the heat is transferred into your living space either directly (as is in the case of natural gas, propane, or heating oil) or indirectly (as in the case of electricity). When a furnace runs, it ignites the fuel with burners that heat up the heat exchanger. A blower moves air across the heat exchanger, and the warm air is then circulated through the living area by a duct system. Fumes from the burned fuel are expelled through a flue (a pipe designed to remove exhaust gases from a fireplace, stove, or burner).

To determine how big a furnace needs to be, the same information listed for air conditioning is necessary. The result of these calculations is what a heating contractor calls a "heat load," which is measured in BTUs. For example, a contractor might determine that a house's heat load is 61,000 BTUs and that an 80,000 BTU furnace running at 80 percent efficiency, providing 64,000 BTUs, is a good approximate fit. A problem with a higher efficiency furnace is that it can often "short cycle," meaning that it will turn off before all the cold air in the home has cycled through the furnace.

The simplest type of furnace is called a "single-stage" furnace. This means that the furnace is either on or off and the fan blower is adjusted to a single setting that provides the optimum amount of heat based on the home's heat load. Most furnaces, however, are "two-stage" furnaces, and these provide more comfort and typically a higher efficiency.

When a two-stage thermostat senses that a room is cold and sends an electrical signal to the furnace to run, the furnace operates at two-thirds strength. If after a set amount of time the thermostat is still calling for heat, the furnace switches to 100 percent capacity. This gives the furnace's blowers time to circulate warm air throughout the house. If the furnace operated at 100 percent right away, it might circulate enough hot air to satisfy the thermostat but not enough to warm colder pockets in other areas of the house.

Michael J. O'Neal

Bibliography

Bell, Arthur A., Jr. HVAC: Equations, Data, Rules of Thumb. New York: McGraw-Hill, 2000.

Haines, Roger W., and C. Lewis Wilson. HVAC Systems Design Handbook. New York: McGraw-Hill, 1998.

Mull, Thomas E. HVAC Principles and Applications Manual. New York: McGraw-Hill, 1997.

FUZZY LOGIC AND THERMOSTATS

Early generations of electronic controls, including the thermostats on furnaces and air conditioners, were always either on or off. Today's controls are computerized, thereby enabling them to use "fuzzy logic," or a computer's ability to "learn" and "think" in shades of gray rather than black-and-white.

Thus, instead of turning on a furnace all the way when the temperature in a building falls to a certain point, a fuzzy logic thermostat turns the heat on just a little as the temperature approaches a specific setting. This feature helps maintain the temperature at a steady, constant value and avoids cycles of chilliness followed by blasts of hot air.