HVAC systems are often the largest consumers of energy in buildings (upwards of 40-50%) and are usually one of the first systems looked at for efficiencies in a green building project.
Photo credit: Stonehenge

The goal of a building’s heating, ventilating and air conditioning (HVAC) system is to provide thermal comfort to building occupants, meet indoor air quality requirements, and meet any specific temperature or humidity requirements. There are several design standards which HVAC systems must meet for new construction from the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) including (but not limited to) ASHRAE Standard 90.1 (Energy Standard for Buildings Except Low-Rise Residential Buildings), 62.1 (Ventilation for Acceptable Indoor Air Quality), and 55 (Thermal Environmental Conditions for Human Occupancy). The air in a building is cooled by transferring heat from one space to another, which can be accomplished by various methods. Heat from a room is typically transferred from the air to another medium such as water or refrigerant and rejected to the outside environment. Many buildings also require either dehumidification or humidification in order to keep the humidity levels in the building at the appropriate level for human thermal comfort. Depending on the season, most people feel comfortable with temperatures hovering between 68-80 degrees and a relative humidity level between 30-50%.

Basics of a Typical Air Conditioning System

Air conditioning systems use the theory of refrigeration to cool a space, and operate under the same principles as your home refrigerator. Air conditioners force liquid refrigerant to evaporate and condense over and over in a series of coils in what is referred to as the "Vapor Compression Cycle". A liquid can absorb heat when it changes phases from a liquid to a gas. Fans in the HVAC system push the warmer interior air through a set of ductwork (known as the return) to the cold coils in the evaporator filled with liquid refrigerant. The liquid refrigerant then absorbs the heat from the air and changes to a gas, and is sent to another piece of equipment outside (the compressor) to reject the heat to the environment. The compressor puts the gaseous refrigerant under high pressure, which also raises the temperature of the refrigerant. That excess heat is rejected to the outside through the condenser coil with the use of a fan. As the refrigerant cools, it is then converted back to a liquid state and is sent back to the evaporator inside the building to start the cycle all over again. The colder air then moves from the evaporator through a different set of ductwork (known as the supply) and back to the interior space. There are several variations on this basic type of air conditioning system and each serve different purposes. Some types are better able to meet the heating and cooling loads of larger buildings, while others are simply more energy efficient.

Constant Volume vs Variable Volume

Most HVAC systems moderate a buildings air temperature by either varying the temperature of the supply air (while keeping the amount of air delivered to the space constant) or by keeping the supply air temperature constant (typically 55 degrees F) and varying the volume of air that is delivered to the space. The former system is called a Constant Volume (CV) system, while the latter is called a Variable Air Volume, or VAV system. CV systems are more typical in residential buildings and smaller commercial buildings as they are simpler and often more cost effective than VAV systems at smaller sizes. VAV systems are common in larger commercial buildings because they tend to be more energy efficient when cooling larger spaces.

Free LEED Exam PreperationChillers and Cooling Towers

A water chiller is basically a box that contains all the components necessary to complete the vapor compression cycle (evaporator, compressor, condenser and expansion valve). This aptly named piece of equipment transfers heat from an incoming pipe filled with water to another pipe filled with refrigerant. The heat is then either rejected through the condenser in an air-cooled chiller, or is transferred to another pipe filled with water and sent to a cooling tower to be rejected to the environment via evaporation in what is called a water-cooled chiller. The chilled water (typically 40-45 degrees F) is then sent to the building through a series of pipes to numerous air handlers throughout the building where the air is blown across the chilled water coils to cool the interior spaces, similar to a refrigerant coil in the evaporator. This is a more efficient system for larger buildings because water can hold much more energy than air. A cubic foot of air can carry 0.02 BTU/degree F while a cubic foot of water can transport 62.4 BTU/degree F. Larger chiller systems are often water-cooled and thus use cooling towers as mentioned previously, as this is a more efficient means of rejecting heat. These systems are obviously more complex than the basic air conditioning described previously, but the energy savings in larger buildings more than pay for the additional initial and maintenance costs.

Determining HVAC Efficiency

Energy efficiency by definition is the ratio of the energy output of a piece of equipment to its energy input. In the HVAC world, this is known as the Coefficient of Performance, or COP. It is the ratio of energy transfer (Q, in watts) performed by the equipment to the work required to run the machine (W, in watts). The Energy Efficiency Ratio (EER) is a variation of COP that allows for the conversion from watts to British Thermal Units, or BTUs. Most HVAC systems are measured in BTUs or “tons”. A "ton"equals 12,000 BTUs/hr of heat removed from a conditioned space. (Note: Back in the 19th century, before there were sophisticated HVAC systems, the term "ton" was used to refer to the amount of cooling power from one ton of ice in a 24 hour period.) Therefore the EER of an air conditioner is simply the COP multiplied by 3.412 to allow for that conversion (for example, if a two ton air conditioner consumers 2,400 watts, its EER would be 10.) The Seasonal Energy Efficiency Ratio, or SEER rating is similar to the EER rating, however the cooling output and input is measured over a typical cooling season instead of at a single point in time. SEER ratings are typically used for HVAC systems for residential and small commercial buildings (less than 5 tons), while EER is used for larger equipment. Chillers typically use COP for their efficiency rating. The more energy efficient a piece of equipment is, the more COP/EER/SEER rating increases. In 2006, the US Federal government established the minimum SEER rating for air conditioners to be 13. In order to be considered high efficiency, the air conditioner must have a SEER rating of at least 14. ASHRAE Standard 90.1 is the standard used in building codes to determine the minimum acceptable energy efficiencies for HVAC equipment. The LEED rating systems also uses this standard to meet its energy prerequisite (Energy & Atmosphere (EA) Prerequisite 2: Minimum Energy Performance) as well as EA Credit 1: Optimize Energy Performance.

Ground Source Heat Pump Systems (Geothermal)

A Ground Source Heat Pump (GSHP) is similar to a conventional air-source heat pump as described above, with an evaporator inside the building and refrigerant lines rejecting the heat from the interior air to the outside environment. But instead of rejecting the heat to the air through a condenser, the refrigerant lines run into a well field in the ground, taking advantage of the near constant temperature of the earth. In the summer the heat in the refrigerant is rejected into the cooler earth soil, while in the winter the system is reversed and heat from the earth is used to warm the building. It is claimed that GSHP’s use 25-40% less energy than a traditional air source heat pump system, and the US Environmental Protection Agency recently called GSHPs “the most energy-efficient and environmentally sensitive of all space conditioning systems.”

Variable Refrigerant Volume

A Variable Refrigerant Volume, or VRV system, is a variation on the air-source heat pump. With a VRV system, instead of varying the temperature of the supply air, or the volume of supply air through one evaporator, multiple evaporators are employed throughout the space, all connected to an outside condensing unit. The volume of refrigerant is then varied to each evaporator depending on the cooling or heating needs of the space. What makes VRVs such an efficient type of system is that they obviate the need for extra ductwork to move air around. As we discussed previously, air can only transport 0.02 BTU/degree F of energy. Other fluids such as water or refrigerant are much more efficient at transporting energy. The only ductwork required is what is needed to provide outside air to meet the ventilation requirements of ASHRAE Standard 62.1. VRV systems have been very popular overseas, in Japan and Europe in particular. As many older buildings do not have enough physical space to accommodate all of the necessary ductwork needed for conventional HVAC systems, VRV systems are especially practical when retrofitting an HVAC system into an older building that was not previously equipped with an HVAC system. VRVs are also beneficial in buildings that have simultaneous heating and cooling loads. Numerous studies have shown energy savings between 30-40% over a traditional chilled water/VAV HVAC system, where much of this efficiency comes from the fact that there are no duct energy losses.

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