Defining HVAC Systems

HVACR systems are comprised of the following mechanical systems: heating, ventilation, air conditioning, and refrigeration. In this section, we will break down each of these categories and distinguish how they are similar and different.

Heating Systems

There are many different heating systems. Central heating is popular in cold climates and gets utilized in private houses and public buildings. Boilers, furnaces, and heat pumps may be characterized as central heating systems. These units heat water, steam, and/or air in a central location, such as a furnace room in a home or a mechanical room in a large building.

Radiators and vents, which allow the heat to spread, are generally located in the coldest part of the room, typically next to the windows, to minimize condensation and offset the convective air current due to the window by the cold glass. Home builders and HVAC professionals can direct vents away from windows to prevent "wasted" heat. Similarly, energy auditors and home inspectors make a point to seal air leaks from the home's exterior to prevent cold drafts of air from coming inside.

One reason why HVAC training is of paramount importance is because indoor air quality directly affects the health and safety of occupants. Using furnaces, space heaters and boilers as means of indoor heating may result in incomplete combustion and the emission of harmful gases like carbon monoxide, NOx, formaldehyde, VOC’s and other combustion by-products. Incomplete combustion occurs when there is insufficient oxygen; the inputs are fuels containing various contaminants and the outputs are the harmful by-products, most dangerously carbon monoxide which is a tasteless and odorless gas that has serious adverse health effects when inhaled.

Ventilating Systems

Ventilation is the process of "changing" or replacing air in any space to control temperature or remove any combination of moisture, odors, smoke, heat, dust, airborne bacteria or carbon dioxide, and to replenish oxygen. Ventilation includes both the exchange of air with the outside as well as circulation of air within the building. It is one of the most important factors for maintaining acceptable indoor air quality in buildings. Methods for ventilating a building may be divided into mechanical and natural types.

Mechanical Ventilation - Mechanical, or forced, ventilation is provided by an air handler and used to control indoor air quality. Excess humidity, odors, and contaminants can often be controlled via dilution or replacement with outside air. Kitchens and bathrooms typically have mechanical exhausts to control odors and humidity. Ceiling fans and table/floor fans circulate air within a room for the purpose of reducing the occupants' perceived temperature. Because hot air rises, ceiling fans may be used to keep a room warmer in the winter by circulating the warm stratified air from the ceiling to the floor.

Natural Ventilation - Natural ventilation is the ventilation of a building with outside air without the use of fans or other mechanical systems. It can be achieved with windows or vents when the spaces to ventilate are small and the architecture permits. In more complex systems, warm air in the building can rise and flow out openings higher up in the building (called the stack effect) thus forcing cool outside air to be drawn into the building naturally through openings in the lower areas. These systems use very little energy, but care must be taken to ensure the occupants' comfort.

Air Conditioning and Refrigeration Systems

An air conditioning (AC) system provides cooling, ventilation, and humidity control for all or part of a house or building. Air conditioning and refrigeration are created through the removal of heat. Heat can be removed by means of radiation, convection, and heat pump systems through a process called the refrigeration cycle. The materials involved in the refrigeration cycle -- water, air, ice, and chemicals -- are referred to as refrigerants.

The refrigeration cycle is a process in which a gas goes through a series of compression and expansion in order to move heat energy from one location to another. The cycle starts by compressing the gas until it reaches a desired high pressure and temperature. From there, it enters the condenser. The condenser acts as a heat exchanger and removes the heat energy from the gas. As the energy is removed, the gas condenses into a liquid. The next step is to move the high pressure liquid through an expansion device and into the evaporator. Here, the pressure will drop, allowing the liquid to evaporate back into a gas. This change of state absorbs heat energy, thus cooling the ambient air. Lastly, the gas returns to the compressor and then restarts the cycle.

In cooler climates, this process can be reversed, allowing the same unit to both cool and heat the building efficiently.

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