What is HVAC ?
What is HVAC ? (Heating, Ventilation & Air Conditioning) is a system of controlling temperature, humidity, & air quality in indoor spaces or vehicles. The goal of an HVAC system is to provide an acceptable level of indoor air quality & thermal comfort. The system’s designs are based on the principles of heat-transfer, thermodynamics, & fluid mechanics. There are a few variations that can include other aspects, like:
- HVAC&R or HVACR – The ‘R’ is used to indicate that the system includes refrigeration
- HACR – These are systems that include refrigeration, but not ventilation
HVAC systems are used in both commercial and residential establishments. They play a vital role in ensuring that the climate within the structure is comfortable and that stale air is replaced with fresh air from outside.
Heating, ventilation, and air-conditioning are all interrelated, especially when thermal comfort and indoor air quality need to be provided within reasonable costs for installation, operation, and maintenance. HVAC systems can increase ventilation, reduce unwanted air infiltration, and even maintain relative pressures between spaces within an establishment. Removing air from an enclosed space and replacing it through air delivery systems is known as Room Air Distribution.
In modern buildings, one or more HVAC systems are incorporated in the building’s design, along with control systems for manipulating their performance. For small buildings or residences, contractors usually estimate the required capacity based on the interior dimensions, and then select and incorporate the HVAC system accordingly.
Larger buildings, however, are thoroughly analyzed by building service designers or engineers who then specify the appropriate configuration of the HVAC systems needed, which are fabricated by specialty mechanical contractors. Buildings of all sizes must clear code-compliance inspections and obtain relevant building permits.
The heating system generates and distributes heat to the entire area covered by the HVAC. Heat generation can be achieved with boilers, furnaces, or heat pumps that circulate hot water, steam, or directly heat the air in a central location, like furnace rooms in houses or mechanical rooms in larger buildings. The heat is transferred by radiation, conduction, or convection.
- Generation – There is a wide range of heaters available, powered by a variety of solid, liquid, and gaseous fuels. Another popular power-source is electricity, which is typically used to heat ribbons of high-resistance wire in baseboard and portable heaters. One of the most efficient ways of generating heat is to use heat pumps. They extract waste heat from exhausts, or even free heat from the ground or the environment, and circulate it through the building. Electric heaters are typically installed as a backup for heat pumps or to provide supplemental heat. With recent improvements in efficiency and low-temperature operation, heat pump HVAC systems are steadily gaining popularity in all climates.
- Distribution – Regardless of the type of heating system, the heat needs to be distributed throughout the system’s operational area. The heat is transferred to a medium, generally air, water, or as steam:
- Water/Steam – Water can be heated and circulated in liquid form, or if there is enough heat converted to steam. A pump and system of pipes is used to transport the heated medium and the heat is transferred to the air using heat exchangers like radiators and hot water hydro-air coils. These can be wall-mounted or installed within the floors. Using water to transfer heat is known as hydronics. An additional advantage is the hot water can be used for bathing and washing too.
- Air – Warm air systems use a system of ducts to distribute heat. The warmer air is delivered and cooler air is returned to the heating system. Many systems can also use the same ducts for delivering cooler air from an air-conditioning system. Typically, a filtration or air cleaning system is installed within the ducting to remove pollen and dust particles.
- Dangers – Wherever there is fuel being burnt, there is always a chance of incomplete combustion that can result in the formation of Volatile Organic Compounds (VOCs), formaldehyde, carbon monoxide, and other dangerous substances. The dangers they pose at even low concentrations are well documented, so adequate ventilation must be provided.
Ventilation is the process of replacing air in an enclosed space with fresh air from outside, as well as circulating air within the space. It improves indoor air quality, removes odors, and can be used to control temperature and humidity. Additionally, it helps ‘vent’ and reduce:
- Carbon dioxide or monoxide and the like
- Microorganisms and pollen
Based on the methods used for creating ventilation, the systems can be categorized as:
- Mechanical or Forced Ventilation – Air handlers that mechanically push or force air to move provide this type of ventilation. Kitchens and bathrooms usually have mechanical exhausts that remove odors and sometimes excess moisture from the air. Some factors need to be considered to determine the size and speed of the fan like the size of the room, flow rate, and noise levels. Direct drive fans can be used in many applications and thus reduce maintenance costs and frequency. Ceiling, table, floor, and wall-mounted fans are preferred for circulating air within a room to reduce the perceived temperature. Since heat naturally rises, running ceiling fans during the colder months can keep a room warmer.
- Natural Ventilation – Air can also be circulated and vented without the use of fans or any other mechanical devices. Natural ventilation can be achieved using a combination of operable windows and louvers. In smaller spaces, trickle vents can be installed if the architecture permits it. There are quite a few systems that use different methods to achieve natural ventilation, like stack-effect ventilation and air-side economizers.In any natural ventilation system, the air change rate needs to be monitored and mechanical systems can be used to supplement the ventilation if needed. This is basically hourly volume of air vented divided by the entire volume of the enclosed space. An air change rate of four changes per hour is usually the most comfortable for humans, but this might need to be higher or lower depending on the number of people, external temperature and humidity, odor levels, etc.
Air conditioning systems are used to reduce the temperature and control the humidity of the air being circulated. Most of the air is recirculated, but fresh air from outside is also drawn in, creating positive pressure in the space. All the air being moved is passed over a heat exchanger, which is mechanically cooled, and then circulated to the areas that need it.
Both air conditioning and refrigeration are achieved by removing the heat through radiation, conduction, or convection. The mediums used to conduct the heat are called refrigerants, which are circulated through a refrigeration system or a free cooling system.
The refrigeration cycle consists of four essential elements:
- The refrigerant starts in a gaseous state and is pressurized by a compressor, which increases its temperature.
- It enters a condenser that cools it to liquid state, usually by blowing cool air over it.
- An expansion valve is used to regulate the flow of the liquefied refrigerant.
- The cooled liquid is released into another heat exchanger where it depressurizes.
- When the refrigerant is depressurized, it absorbs energy from its surroundings in the form of heat. This rapidly cools the heat exchanger and the air being circulated through it. Some systems have a ‘reversing valve’ that converts the cooling heat exchanger into a heat source so it can be used to increase the indoor air temperature.
Systems that operate on the principles of free cooling are extremely efficient. They are often combined with storage mediums that can capture the higher or lower temperatures in various seasons and release them in the months they are needed. Common storage mediums used for free cooling systems include deep aquifers and even underground rocks that are accessed using boreholes equipped with heat exchangers.
Some hybrid systems use free cooling when the required temperature is close to the outside temperature, and a heat pump is used to chill the circulated air further if needed. The free cooling storage functions like a heat sink during summers and can be used as a heat source in winters.
Many HVAC systems include an ‘economizer mode’ which manipulates external vents automatically. When the temperature outside the enclosed system is lower and the air inside needs to be cooled, the vents can be partially or completely opened to enable free cooling. This can dramatically reduce air conditioning costs for spaces that generate a lot of heat.
Central vs. Split Systems
Central systems that consist of the condenser, compressor, & evaporator packaged into a single unit are often used in modern buildings, but they are bulky and difficult to install if they weren’t part of the initial design.
In most residential buildings, the alternate ‘split’ systems are more popular. The evaporator & condenser are connected using thermally insulated piping, which makes them a lot more flexible. The condenser can be installed in a convenient location where the heat being vented won’t be a concern and multiple evaporators can be installed in different areas.
Dehumidification is the process of removing moisture from the air, which is achieved by the evaporator. The temperature of the evaporator’s heat exchanger is much lower than the dew point, so as air passes over it the moisture condenses on the surface and flows into a collection plan. Dehumidifiers are like air conditioners, but their primary objective is to remove moisture instead of cooling. Humidifiers do just the opposite by heating water and circulating air around it. The water then evaporates and increases the humidity.
Every type of HVAC system will require some form of maintenance at some point in its serviceable life. The most frequently needed maintenance is cleaning and replacing the air filters, since they are constantly filtering out particles and eventually get clogged, which can reduce the efficiency of the system tremendously.
If the circulation is inadequate, it can cause the evaporator coils to ‘ice-up’, which reduces the airflow even further. This can be extremely dangerous since electronic parts can overheat and cause a fire. In addition to replacing the air filters, the condensing coils should also be cleaned periodically. They help dissipate heat from the motor that drives the compressor. Overheating reduces the life of the motor, leading to hefty repair bills.
HVAC equipment manufacturers invest heavily in research and development to increase the efficiency of their systems, so there is a huge range of systems and methods that can reduce operating costs and environmental impact.
Forced air systems are preferred over water heating due to their higher efficiency and the fact that they can double as air conditioners. The benefits include:
- Better air conditioning/cooling
- Energy savings of 15-20%
- Even cooling and heating within an enclosed space
The drawback for many people is the higher installation cost, but the savings in the long run are usually worth it.
Using zoned heating can increase the energy efficiency of central heating systems. Multiple thermostats are installed to control the temperature in different zones, allowing for a more granular heating, like non-central heating systems. Zone valves or dampers are used to control the flow of the heat medium to each zone and selectively block airflow, which makes the control system critical for maintaining the right temperature.
Geothermal Heat Pump
Below the surface of the Earth, the temperature remains almost constant throughout the year. Geothermal heat pumps use this natural system to provide cooling, heating, and in many cases hot water. The temperature varies with latitude, but at a depth of just 6 feet it remains between 45 to 75 °F.
Air Filtration and Cleaning
Air cleaning and filtration are important parts of maintaining the indoor environment because the air filters remove particles that can affect our lungs like dust, contaminants, spores, vapors, and gases. The filtered air is then heated or cooled, mixed with fresh air, and then circulated. Cleaning and filtration should always be taken into account when designing an HVAC system.
Clean Air Delivery Rate (CADR) and Filter Performance
CADR is the volume of clean and filtered air a system provides in a room, relative to the size of the room. When it comes to indoor air quality, filter performance is extremely important. This is affected by the size of the fiber or particles used in the filter, the packaging density of the filtering medium, and the rate of airflow through the filter.
Selection of HVAC Equipment
While selecting an HVAC system and its component equipment, a number of factors need to be taken into account:
- Natural Ventilation – Designers should consider using natural ventilation and installing operable windows to provide supplemental ventilation, in addition to mechanical ventilation. However, keep a few things in mind:
- If there is a lot of pollution and dust, it might actually lower indoor air quality
- Operable windows should be between 3-6 feet above the floor
- The windows should be adjustable and form a tight seal when closed
- To maximize the amount of natural ventilation, install windows on opposite faces of the building to create cross-ventilation
- Wherever it is feasible, install central HVAC air handling units (AHUs) that can serve multiple rooms, instead of individual unit ventilators or heat pumps. Look for these features in AHUs:
- Double-sloped drain pans to prevent standing water
- Hinged access doors that are easy to reach and operate
- Double wall cabinets to prevent moisture or mechanical damage
- Tight-sealing cabinets which reduce air leaks and improve air quality
- Gasket sealing double-walled doors for thermal and acoustic insulation
- Thick filter slots which can accommodate 2 inch or thicker filters
- Extended surface area filter bank to reduce filter maintenance and increase fan efficiency
- Air filter assemblies designed to minimize leakage
- Air filter monitors to gauge static pressure drop
- Corrosion resistant links, dampers, and all moving parts
Designing for Efficient Operations and Maintenance
To ensure better efficiency and accessibility for maintenance work, incorporate these aspects as part of your design scheme:
- Ensure all the system components, like AHUs, controls, and exhaust fans can be accessed easily, without ladders, removal of roof tiles, or crawling into tight spaces.
- Rooftop equipment should be installed where it is easily accessible by stairs instead of a ladder, as well as an adequately sized door for easy ingress and egress.
- Label all the HVAC system components to improve operations and maintenance with easily legible and weatherproof labels.
At the minimum, the components of HVAC systems should have labels that correspond with the drawings and diagrams:
- The name or number of the AHU
- All the connections with arrows indicating the direction of air flow
- Access doors for air filters and minimum filter dust-spot efficiency
- Filter pressure gauges along with the pressure at which a filter change is recommended
- The access doors or panels for the drain/drip pans
- The minimum volume of outdoor air during occupied times for each AHU
- Outdoor air and relief dampers with marks indicating fully open and closed positions, and the minimum designed position
- Access door for outdoor air controls
- Breakers for each component
- Inspection and maintenance access doors
- Air-side economizer dampers and controls, if any
- Names and numbers of each exhaust fan, along with the exhausted air volume
HVAC Industry and Standards
In the US, HVAC members are generally members of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), certified EPA Universal CFC, or locally certified engineers. The ASHRAE is an international society for individuals and organizations involved or interested in HVAC systems. It provides many opportunities for engineers to participate in research and development related activities through its many technical committees.
The ASHRAE publishes handbooks that are updated every four years, the most general of which is Fundamentals that includes calculations for estimating heating and cooling requirements. Typical building codes like the Uniform Mechanical Code (UMC) and International Mechanical Code (IMC) do not provide much information regarding HVAC design practices, so design professionals need to consult ASHRAE data for design and care standards. SMACNA, ACGIH, and some technical trade journals can also provide useful reference material.
American design standards for HVAC systems are legislated in the IMC and UMC. In some locations, either of the codes can be adopted and amended through legislative processes. The International Association of Plumbing and Mechanical Officials (IAPMO) and the International Code Council (ICC) update these codes on a 3-year cycle for code development. Local building permit departments typically administer the enforcement of these standards.
In the US and Canada, the National Air Duct Cleaners Association (NADCA) represents qualified member companies that provide assessment, cleaning, and restoration services for HVAC systems. NADCA also provides certifications for Certified Ventilation Inspectors (CVI), Air Systems Cleaning Specialists (ASCS), and standards for Assessment, Cleaning, and Restoration (ACR).
The design standards include:
- ASHRAE Standard 62.1-2007 (Design in accordance with ASHRAE standards)
- ASHRAE Standard 55–1992 with 1995 Addenda (Thermal Environmental Conditions for Human Occupancy)