Fundamentals of HVAC
This section provides a crash course on HVAC systems (Heating, Ventilation, and Air Conditioning). HVAC systems are important for maintaining comfortable and healthy indoor environments in buildings. They also represent a significant source of energy consumption and therefore many opportunities exist for analyzing energy efficiency opportunities in HVAC system selection, design, and controls. As a result, HVAC analysis is one of the key roles of a BEM practitioner.
For more information about specific system types, refer to the page HVAC system types.
Functions of HVAC systems
HVAC systems perform many functions in a building including:
- Thermal Comfort: HVAC systems are designed to maintain a comfortable temperature and humidity level inside the building, regardless of the weather outside. This is crucial for ensuring the occupants' comfort and productivity.
- Indoor Air Quality: HVAC systems regulate the air quality in the building by removing pollutants, such as dust and other contaminants This is important for maintaining a healthy environment and preventing respiratory problems.
- Building Preservation: HVAC systems help maintain the integrity of the building by controlling humidity levels and preventing mold growth. This is crucial for preventing damage to the building's walls, and other structural components.
Science of HVAC
Psychrometrics
Psychrometrics is the study of the properties of air such as its moisture content, temperature, and pressure. In HVAC systems, psychrometrics is used to determine the optimal conditions for maintaining comfortable and healthy indoor air conditions. This involves measuring and analyzing the various properties of air and designing systems to control and maintain these properties within specific ranges.
The most important properties of air that are studied relating to occupants' comfort include dry-bulb temperature, which is a measure of the air's sensible heat, and relative humidity, which is a measure of the air's moisture content. Other properties include dew point temperature, wet-bulb temperature, and enthalpy.
Refrigerants
The vapor compression cycle
HVAC systems produce cooling (and heating, in the case of heat pump systems) using the refrigerant's vapor compression cycle. Learn more:
Environmental impacts of different refrigerant types
Not all refrigerants are created equally. Older refrigerants have higher global warming and ozone depleting impacts than newer alternatives. Read more about this at the EPA website.[1]
HVAC system design concepts
As the name strongly implies, HVAC systems are designed to provide heating, ventilation, and air conditioning to many spaces inside of a building. There are many different system types that achieve these objectives in different ways and with varying levels of efficiency. This section provides some of the key concepts to understand, and system-specific details are provided in the lists below.
Thermal zones
A thermal zone is an area in the building that has its temperature controlled by an HVAC system. A thermal zone may be a single room, a set of rooms (that experience similar loads), or a portion of a large room (with loads that vary, e.g., at the perimeter near windows vs. interior area of a room).
Thermal zones may have heating and cooling (depending on conditioning needs, weather, etc.) or they may have either heating OR cooling in some cases. The temperature is controlled by a temperature sensor (usually a thermostat, but sometimes located within the HVAC system in a position where it can measure room temperature such as the return air stream).
The HVAC system is designed to maintain temperature setpoints within the thermal zone. If the zone's temperature drops below the heating setpoint, the system will respond by providing heating. If the zones's temperature exceeds its cooling setpoint, the HVAC system will respond by providing cooling. There are typically two different setpoints for heating and cooling, with a "buffer zone" or "dead zone" in between the setpoints so that heating and cooling are not constantly fighting against each other and wasting energy.
Zones per system (single zone systems vs. multi-zone systems)
HVAC systems may be either "single zone" or "multi-zone" systems. Single zone systems provide heating, cooling, and or ventilation to a single zone in the building - in these systems, one temperature sensor controls the HVAC systems response to heating or cooling. Multi-zone systems provide service to multiple zones in the building - in these systems, each zone has its own temperature sensor and can call for heating and cooling, independently from other zones. See below for more information on different system types and how they provide conditioning to the thermal zones in a building.
A "zonal" system is similar to a single zone system in that it provides conditioning to one thermal zone, however a key difference is that zonal systems generally provide conditioning only to a space. Single zone systems are typically located on the roof whereas zonal systems are located in the zone or above the ceiling of the zone. Zonal systems often do not provide outside air ventilation, and are often paired with a separate system that provides outside air (a dedicated outside air system, commonly called a "DOAS" system).
Note, however, that there is a wide range of names and terminology for different system types, so this distinction between single zone and zonal systems may not always hold true. For example, some systems are located within a zone and bring in ventilation air (sometimes called a "PTAC" or "through the wall unit"). This type of system may very well be referred to as a zonal system.
Providing ventilation to the occupants
A key function of HVAC systems is to provide fresh air (ventilation) in order to maintain a healthy environment and prevent respiratory problems for the occupants. Ventilation air can be brought into the building in different ways, as discussed below.
Mixed air system
A mixed air HVAC system combines outdoor air and recirculated indoor air to create the desired temperature and air quality within a building. In a mixed air system, outdoor air is brought into the building through an intake and mixed with indoor air that has been heated or cooled by the HVAC system. The mixed air is then filtered to remove any contaminants and distributed throughout the building via air ducts and vents.
The mixing of outdoor and indoor air in a mixed air HVAC system helps to improve indoor air quality by diluting any pollutants or contaminants present in the indoor air. Additionally, the use of outdoor air can help to reduce the energy consumption of the HVAC system by reducing the load on the system during periods of moderate outdoor temperatures (if equipped with an economizer).
DOAS system
A Dedicated Outdoor Air System (DOAS) is a type of HVAC system that is designed to provide a constant supply of fresh, conditioned outdoor air to spaces in a building. Unlike mixed air systems, DOAS does not mix outdoor and indoor air but rather provides fresh outdoor air separately from the recirculated indoor air.
In a DOAS system, outdoor air is typically brought into the building through a dedicated intake and is conditioned to the desired temperature and humidity level before it is distributed throughout the building using ductwork separate from other HVAC systems in the building. This conditioning can include heating, cooling, dehumidification, and humidification as based on the climate. Generally, the DOAS system is designed to provide enough air to meet the ventilation requirements, but it is not designed to meet all of the heating and cooling needs of the thermal zones, so supplemental conditioning is provided separate from the DOAS, often in the form of zonal systems.
After the outdoor air is conditioned, it is distributed throughout the building using separate ductwork from any recirculating systems in the building. This approach allows for better control of the indoor air quality.
100% outside air system
A 100% outside air HVAC system is an HVAC system that provides 100% fresh outdoor air to a building's indoor space. In a 100% outside air HVAC system, all of the air supplied to the building is brought in from the outside, and none of the indoor air is recirculated. This provides several benefits, including improved indoor air quality and reduced risk of airborne contaminants.
The system typically includes an intake system to bring outdoor air into the building, a heating and/or cooling system to condition the air to the desired temperature, and a distribution system (ductwork and vents) to circulate the air throughout the building.
One downside of a 100% outside air HVAC system is that it can be less energy-efficient than systems that recirculate indoor air. This is because the system has to work harder to condition the incoming outdoor air to the desired temperature and humidity levels. However, some systems use energy recovery ventilation to capture and reuse energy from the outgoing air stream, which can help to improve the systems' efficiency.
100% outside air HVAC systems are commonly used in buildings where indoor air quality is a priority, such as hospitals and laboratories.
Natural ventilation
Natural ventilation in buildings is the process of using natural air flows, such as wind or temperature differences, to provide fresh air to indoor spaces. It is a passive ventilation strategy that relies on the building's design and orientation, as well as the external climate conditions, to create air movement.
Natural ventilation systems can be designed to operate in a variety of ways, such as through operable windows, vents, and roof openings. The system allows outdoor air to flow through the building, reducing the need for mechanical ventilation systems, which can be energy-intensive.
Properly designed natural ventilation systems can help to improve indoor air quality, reduce energy consumption, and enhance occupant comfort. However, natural ventilation is not always suitable for all building types or climates, and should be carefully evaluated at the earliest phases of design.
Distribution systems for heating and cooling
Heating and cooling energy can be distributed throughout the building using either air (moved fans), fluids (such as hot or cold water, moved pumps; or pressure-driven in the case of refrigerants), or a combination of the two. The configuration will vary depending on the system type. Refer to the detailed system type pages for more information about how each system type distributes heating and cooling energy.
Cooling generation types
Cooling is generated with electricity by moving a refrigerant through the vapor compression cycle. The refrigerant can be used to cool air, or water, depending on the system type.
Direct expansion (DX)
DX systems have a refrigerant coil in the supply air stream that produces cool air before it is distributed into the building zones by a fan. The coil will absorb heat from the air as it passes over the coil, cooling the air before it's distributed into the occupied spaces. Heat is rejected to a condenser located outside of the building.
Chilled water
Chilled water is produced in a chiller, and the chilled water is distributed to cooling coils located at single-zone, multi-zone, or zonal systems. Supply air passes over the coil, which cools the supply air. The chilled water coil absorbs heat from the supply air as it passes over the coil, and this heat is then rejected outside of the building by a condenser. Many types of chillers and condensers are available (illustrated in the diagram) and depending on the choice of these chillers and heat rejection approaches, efficiency will vary.
Heating generation types
Heating can be generated by electricity of fossil fuels and distributed to thermal zones by air, water, or directly via radiant systems.
Electric resistance
Electric resistance heat is a method of heating that uses an electric current to generate heat directly within a high resistance heating element. When an electric current is passed through the heating element, the resistance of the material causes it to heat up, and this heat is then transferred to the thermal zones. Electric resistance heat can be used for a variety of applications, including baseboard heaters, wall heaters, and electric furnaces.
A disadvantage of electric resistance heat is that it can be relatively expensive to operate, as it consumes a lot of electricity compared to other heating methods.
Heat pump
A heat pump is a heating and cooling system that transfers heat from one place to another. It can be used to heat or cool a building, depending on the direction of heat transfer. Much more detail about heat pump operation can be found on the Heat pumps page.
Heat pumps also use electricity as their fuel source but, unlike electric resistance, heat pumps are considered to be an energy-efficient heating and cooling option because they move heat effectively rather than generating it through electrical resistance.
Furnace
A furnace consists of a heat source, such as a burner or heating element, and a heat exchanger that transfers the heat to the air and then distributes it through ducts to various areas in a building. Furnaces typically use natural gas or electricity (electric resistance) as the fuel source, but may also use oil or propane.
Hot water
Hot water can be generated by a boiler (gas fuel source) or by air-to-water heat pumps (electricity fuel source) and then distributed to hot water coils at single-zone, multi-zone, zonal, or terminal air units.
Heating and cooling performance and efficiency characteristics
HVAC system performance characteristics are variable depending on many criteria. The system efficiency is typically a function of the outdoor weather conditions and the percent of load. The system capacity will often also vary based on the outdoor conditions. These performance characteristics are generally represented as a set of curves which can be input into the BEM simulation software.
Because system performance is so variable under different conditions, it is important to capture this when modeling the system, especially because systems spend most of their time operating in part-load conditions.
Some factors that affect the shape of these curves may be the type of system, or specific components of the system such as fans, chillers, heat rejection. Additional factors may include whether the system operates at a constant speed or variable speed, the type of refrigerant used, and many others.
Many HVAC systems's efficiency is represented by a single value such as COP or EER, but it is important to note that these are typically full-load efficiency values, or in some cases average efficiency values accounting for different seasonal performance. These simple values are not adequate for representing system performance in a BEM model, but detailed curve data (or detailed data that can be used to generate a curve) is usually available for download from manufacturers.
HVAC controls and operation
Basic design setpoints
The most basic settings for the HVAC system include when the system should operate, and what temperatures are to be maintained for heating and cooling operation. These are typically defined as the operating hours of the system, and heating and cooling temperature (and sometimes) humidity setpoints during both operating hours (occupied) and when unoccupied. Typically, temperature setpoints differ during unoccupied periods to save energy. During occupied periods, the setpoints are maintained to ensure the occupants remain comfortable, but, during heating mode in the winter, the building can be kept cooler when unoccupied (often at night) and, during cooling mode in the summer, the building can warmer when unoccupied. The differing/ more relaxed setpoints for the unoccupied periods are often referred to as "setback" temperatures.
During unoccupied periods, the HVAC system can also turn off completely. However, if the setback temperature setpoints fall out of range, the HVAC system will often cycle back on to maintain temperature.
Sequences of operation
In order for the HVAC system to meet the design setpoints and provide adequate ventilation to the occupants, the system must manage many of its operational features to do so effectively. Sensors in the building zones or inside the HVAC systems will send a signal to the HVAC system or a central control system (also called a Building Automation System or BAS) to determine how the system should react to changes in temperature, humidity, CO2 levels and other factors.
Depending on the system type, the system controls will meet the design setpoints by adjusting operating conditions such as:
- Supply air temperatures
- Fan speeds
- Compresser operation
- Airflow damper positions
- Pump speeds
- Water loop temperatures
- And others
The order in which these operating conditions are controlled is referred to as the system's sequence of operation or its control sequence. These will vary greatly depending on the system type and system complexity. Fine-tuning of the control sequences is an opportunity to improve the system's performance so, even for a given system type, there will be variability in control sequences from project to project. This topic is covered in more specific detail in the specific HVAC system pages.
References
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