Basic Air Conditioning Operation

Usually, 400 square feet of area with an eight-foot ceiling can be cooled with one ton of air conditioning. One tone of air conditioning is equivalent to 12,000 Btu (British thermal unit). One Btu will raise the temperature of one pound of water one degree Fahrenheit.

When a unit is being sized for a structure, many factors have to be considered. Things such as wall thickness, what material is used in their construction. Color of building, amount of glass area, exposure, along with many other factors are considered in the engineering computations for the heat load. In a residence that is simple without any unusual configurations 400 square feet per 12,000 Btu formula can be used. The reason for it being mentioned here is in the event a homeowner has a complaint that his/her unit is not cooling enough, you will notice immediately if a 24,000 Btu unit is trying to cool a 1600-square foot house.

I’m going to discuss a split, straight cool, three-ton system. Placement of the condensing unit is very important. The first thing you should consider is noise factor to the home and neighbors. Some condensing units are more noisy than others. If a unit has objectionable noise, it should not be placed near sleeping quarters. If a replacement unit is being considered, it wouldn’t be a bad idea to find one of the intended units operating in the field and listening to any noise that it might generate. Condensing unit fans are de­signed to discharge air either in a vertical or horizontal plane. With this in mind, when placing a vertical discharge unit you wouldn’t place it under a low roof overhang or under a window awning. In the case of the horizontal discharge condensing units, there has to be ample room for air to enter the unit and room for it to exit. If being placed on a roof, consideration to the prevailing winds is prudent.

Depending upon the building codes in your area, the condenser unit should be raised above the ground. There should be freedom of air circulation under the unit to inhibit the formation of rust due to the unit constantly being wet. A couple of 4″ x4″ timbers would lift the condenser off the ground high enough for ventilation to take place.

The air handler (evaporator section) can be located in many locations depending upon the structure. In residences the air handler might be installed in a garage, interior closet, or in the attic. In a straight cool unit or heat pump unit, the air handler contains fan, coil, and controls. If the air conditioning system was added after the heating system was installed, a metal plenum is constructed usually above the heat exchanger of the furnace. The coil and metering de­vice is located in the plenum. The fan of the furnace is used for the air-conditioning unit. The controls are mounted in the furnace section. This can be either a gas or an oil furnace.

There must be an airway for air to get back to the unit. If the evaporator section is located in a closet, a louvered door is sufficient to allow air passage back to the unit. If the evaporator section is located in the garage or in the attic, a return air grill is installed. The grill is an entry to the duct that directs the conditioned air back to the evaporator coil. There should always be a filter to clean the air prior to its entering the evaporator coil. It could be located behind the return air grill on a filter rail, in the ductwork on a rail, or in the evaporator section itself on a rail.

In most of the systems today, the transformer that supplies the low voltage for the control circuit is located in the evaporator section. Few units still place the transformer in the condensing unit.

If the evaporator section is located in the attic, or a crawl space above a finished ceiling, an auxiliary condensate drain pan should be installed. In certain regions this is required by building code. The purpose of the auxiliary pan is to collect any water that might spill from the unit in the event the primary drain becomes restricted. If an auxiliary is not used, water can quickly damage the ceiling below the unit.


switch also has a manual reset. Smoke detectors can also be installed on ventilation fans to stop their operation when smoke is detected. Figure 18-2 shows this type of smoke detector.


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