MAJOR MECHANICAL SYSTEMS EXPLAINED: HVAC

The major systems that make today's buildings livable are the heating, air conditioning, plumbing and electrical systems. 

Ventilation

Ventilation is the intentional movement of air from outside a building to the inside. Ventilation air is that air used for providing acceptable indoor air quality. When people or animals are present in buildings, ventilation air is necessary to dilute odors and limit the concentration of carbon dioxide and airborne pollutants. Ventilation air is often delivered to spaces by mechanical systems which may also heat, cool, humidify and dehumidify the space. Many heating and cooling systems are referred to as HVAC (Heating, Ventilation, and Air Conditioning) systems.  Ventilation systems have exhaust fans and ducts to remove stale air from the building while introducing fresh air to replace what has been exhausted.

Heating Systems

Residential heating systems typically fall into one of these categories: hot water, steam, forced air or electric. 

·      Hot Water Systems

Hydronics is the name for the use of water as the heat-transfer medium in heating and cooling systems. Some of the oldest and most common examples are steam and hot-water radiators. In any hot water system, the water is heated in a boiler and circulated by means of one or more circulator pumps. A system may be broken up into individual heating zones using either multiple circulator pumps or a single pump and electrically operated zone valves. 

• Steam Systems
  
  A steam system delivers steam to radiators or other heating elements where the steam gives up its heat and is condensed back to water. The radiators and steam supply pipes are pitched so that gravity eventually takes this condensed water back down through the steam supply piping to the boiler where it can once again be turned into steam and returned to the radiators. 
• Forced Air
  
  Forced air heating is probably the type most commonly installed in North America.  The three types of air furnaces are electric, oil-fired, and gas-fired.  A forced-air system draws room air through ductwork and a filter into a furnace, where the air is heated. The warmed air is then blown back to rooms through ductwork. The system includes an air filter, blower, heat exchanger/element/coil, and various controls. Like any other kind of central heating system, thermostats are used to control forced air heating systems. 
• Electric Heat
  
  Electric heating in homes is usually in the form of baseboard heaters along the walls of a room, although it can be in the form of a heat pump (which we'll talk about on an upcoming screen). Electric heating has several advantages: it can be precisely controlled to allow a consistency of temperature within very narrow limits; it is cleaner than other methods of heating because it does not involve any combustion; it is considered safe because it is protected from overloading by automatic breakers; it is quick to use and to adjust, and it is relatively quiet. 
• Air Conditioning Systems
• Central air conditioning and HVAC systems are now common in new construction. Most air conditioning units are situated outside the building, either near a wall or on the roof. However, in some instances, the unit is located in the basement of the structure.
• Air conditioning systems are combined with forced-air heating units to deliver heat through the ductwork system in cold seasons and cool, dehumidified air for air conditioning through the same ducts during warm seasons.
• The air-conditioning unit pumps liquid refrigerant (a liquid that cools something) into the air-handling unit (AHU), the unit inside the home that generates heated or cooled air. The most common refrigerant used in air conditioners is Freon. The AHU generates cool air and forces it through the home through the ductwork and into rooms. The ductwork system also returns air from the rooms back into the AHU for re-circulation. Meanwhile, the slightly warmed refrigerant travels back outside to the air conditioner, to be cooled and re-circulated.
• Air-cooled systems use air as the condensing medium and have a fan that moves the air over the coils. Water-cooled systems use water as the condensing medium and use a pump to circulate the water through the condenser and out to a cooling tower.
• Most central air conditioning units can be expected to last about 15-20 years. However, this number may be affected by the climate in the geographical location of the unit. For example, a central air conditioning unit in Florida probably has a life expectancy of closer to 10 years. 
• Heat Pumps
• For climates with moderate heating and cooling needs, heat pumps offer an energy-efficient alternative to furnaces and air conditioners. Like a refrigerator, heat pumps use electricity to move heat from a cool space into a warm space, making the cool space cooler and the warm space warmer. During the heating season, heat pumps move heat from the cool outdoors into the warm house. During the cooling season, heat pumps move heat from the cool house into the warm outdoors. Because they move heat rather than generate heat, heat pumps can provide up to 4 times the amount of energy they consume.
• The most common type of heat pump is the air-source (air-to-air) heat pump, which transfers heat between the building and the outside air. Higher efficiencies are achieved with geothermal (ground-source or water-source) heat pumps, which transfer heat between the building and the ground or a nearby water source.
• BTU Ratings
• A British Thermal Unit (BTU) is the amount of heat energy needed to raise the temperature of one pound of water by one degree F. BTUs measure the amount of heat an air conditioning unit can remove from the room. As the BTU rating increases so does the size, weight and cost of the unit.