How HVAC (Heating, Ventilation, Air Conditioning) Systems Work


HVAC (pronounced either “H-V-A-C” or “H-vak”) is an acronym that stands for the closely related functions of “Heating, Ventilating, and Air Conditioning”- the technology of indoor environmental comfort. HVAC system design is a major sub-discipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer. Refrigeration is sometimes added to the field’s abbreviation as HVAC&R or HVACR, or ventilating is dropped as in HACR (such as the designation of HACR-rated circuit breakers). HVAC is particularly important in the design of medium to large industrial and office buildings such as skyscrapers and in marine environments such as aquariums, where safe and healthy building conditions are regulated with temperature and humidity, as well as “fresh air” from outdoors.

Background Heating

The invention of the components of HVAC systems went hand-in-hand with the industrial revolution, and new methods of modernization, higher efficiency, and system control are constantly introduced by companies and inventors all over the world. The three central functions of heating, ventilating, and air-conditioning are interrelated, providing thermal comfort, acceptable indoor air quality, within reasonable installation, operation, and maintenance costs. HVAC systems can provide ventilation, reduce air infiltration, and maintain pressure relationships between spaces. How air is delivered to, and removed from spaces is known as room air distribution.[1]

In modern buildings the design, installation, and control systems of these functions are integrated into one or more HVAC systems. For very small buildings, contractors normally “size” and select HVAC systems and equipment. For larger buildings, building services designers and engineers, such as mechanical, architectural, or building services engineers analyze, design, and specify the HVAC systems, and specialty mechanical contractors build and commission them. Building permits and code-compliance inspections of the installations are normally required for all sizes of buildings.

The HVAC industry is a worldwide enterprise, with career opportunities including operation and maintenance, system design and construction, equipment manufacturing and sales, and in education and research. The HVAC industry had been historically regulated by the manufacturers of HVAC equipment, but Regulating and Standards organizations such as ASHRAE, SMACNA, ACCA, Uniform Mechanical Code, International Mechanical Code, and AMCA have been established to support the industry and encourage high standards and achievement.

There are different types of standard heating systems. Central heating is often used in cold climates to heat private houses and public buildings. Such a system contains a boiler, furnace, or heat pump to heat water, steam, or air, all in a central location such as a furnace room in a home or a mechanical room in a large building. The system also contains either ductwork, for forced air systems, or piping to distribute a heated fluid and radiators to transfer this heat to the air. The term radiator in this context is misleading since most heat transfer from the heat exchanger is by convection, not radiation. The radiators may be mounted on walls or buried in the floor to give under-floor heat.

In boiler fed or radiant heating systems, all but the simplest systems have a pump to circulate the water and ensure an equal supply of heat to all the radiators. The heated water can also be fed through another (secondary) heat exchanger inside a storage cylinder to provide hot running water.

Forced air systems send heated air through ductwork. During warm weather the same ductwork can be used for air conditioning. The forced air can also be filtered or put through air cleaners.

Heating can also be provided from electric, or resistance heating using a filament that becomes hot when electricity is caused to pass through it. This type of heat can be found in electric baseboard heaters, portable electric heaters, and as backup or supplemental heating for heat pump (or reverse heating) system.

The heating elements (radiators or vents) should be located in the coldest part of the room, typically next to the windows to minimize condensation and offset the convective air current formed in the room due to the air next to the window becoming negatively buoyant due to the cold glass. Devices that direct vents away from windows to prevent “wasted” heat defeat this design intent. Cold air drafts can contribute significantly to subjectively feeling colder than the average room temperature. Therefore, it is important to control the air leaks from outside in addition to proper design of the heating system.

The invention of central heating is often credited to the ancient Romans, who installed a system of air ducts called a hypocaust in the walls and floors of public baths and private villas.


Ventilating is the process of “changing” or replacing air in any space to control temperature or remove moisture, odors, smoke, heat, dust and airborne bacteria. Ventilation includes both the exchange of air to 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/forced and natural types. Ventilation is used to remove unpleasant smells and excessive moisture, introduce outside air, and to keep interior building air circulating, to prevent stagnation of the interior air.

Mechanical or forced ventilation

“Mechanical” or “forced” ventilation is used to control indoor air quality. Excess humidity, odors, and contaminants can often be controlled via dilution or replacement with outside air. However, in humid climates much energy is required to remove excess moisture from ventilation air.

Kitchens and bathrooms typically have mechanical exhaust to control odors and sometimes humidity. Factors in the design of such systems include the flow rate (which is a function of the fan speed and exhaust vent size) and noise level. If the ducting for the fans traverse unheated space (e.g., an attic), the ducting should be insulated as well to prevent condensation on the ducting. Direct drive fans are available for many applications, and can reduce maintenance needs.

Ceiling fans and table/floor fans circulate air within a room for the purpose of reducing the perceived temperature because of evaporation of perspiration on the skin of the occupants. 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. Ceiling fans do not provide ventilation as defined as the introduction of outside air.

Natural ventilation

Natural ventilation is the ventilation of a building with outside air without the use of a fan or other mechanical system. It can be achieved with operable windows or trickle vents when the spaces to ventilate are small and the architecture permits. In more complex systems warm air in the building can be allowed to rise and flow out upper openings to the outside (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. In warm or humid months, in many climates, maintaining thermal comfort via solely natural ventilation may not be possible so conventional air conditioning systems are used as backups. Air-side economizers perform the same function as natural ventilation, but use mechanical systems’ fans, ducts, dampers, and control systems to introduce and distribute cool outdoor air when appropriate.

Air conditioning


Air conditioning and refrigeration are provided through the removal of heat. The definition of cold is the absence of heat and all air conditioning systems work on this basic principle. Heat can be removed through the process of radiation, convection, and conduction using mediums such as water, air, ice, and chemicals referred to as refrigerants.

An air conditioning system, or a standalone air conditioner, provides cooling, ventilation, and humidity control for all or part of a house or building. The refrigerant provides cooling through a process called the refrigeration cycle. The refrigeration cycle consists of four essential elements to create a cooling effect. A compressor provides compression for the system. This compression causes the cooling vapor to heat up. The compressed vapor is then cooled by heat exchange with the outside air, so that the vapor condenses to a fluid, in the condenser. The fluid is then pumped to the inside of the building, where it enters an evaporator. In this evaporator, small spray nozzles spray the cooling fluid into a chamber, where the pressure drops and the fluid evaporates. Since the evaporation absorbs heat from the surroundings, the surroundings cool off, and thus the evaporator absorbs or adds heat to the system. The vapor is then returned to the compressor. A metering device acts as a restriction in the system at the evaporator to ensure that the heat being absorbed by the system is absorbed at the proper rate.

Central, ‘all-air’ air conditioning systems are often installed in modern residences, offices, and public buildings, but are difficult to retrofit (install in a building that was not designed to receive it) because of the bulky air ducts required. A duct system must be carefully maintained to prevent the growth of pathogenic bacteria in the ducts. An alternative to large ducts to carry the needed air to heat or cool an area is the use of remote fan coils or split systems. These systems, although most often seen in residential applications, are gaining popularity in small commercial buildings. The coil is connected to a remote condenser unit using piping instead of ducts.

Dehumidification in an air conditioning system is provided by the evaporator. Since the evaporator operates at a temperature below dew point, moisture is collected at the evaporator. This moisture is collected at the bottom of the evaporator in a condensate pan and removed by piping it to a central drain or onto the ground outside. A dehumidifier is an air-conditioner-like device that controls the humidity of a room or building. They are often employed in basements which have a higher relative humidity because of their lower temperature (and propensity for damp floors and walls). In food retailing establishments, large open chiller cabinets are highly effective at dehumidifying the internal air. Conversely, a humidifier increases the humidity of a building.

Air-conditioned buildings often have sealed windows, because open windows would disrupt the attempts of the HVAC system to maintain constant indoor air conditions.

Energy efficiency

For the last 20–30 years, manufacturers of HVAC equipment have been making an effort to make the systems they manufacture more efficient. This was originally driven by rising energy costs, and has more recently been driven by increased awareness of environmental issues. In the USA, the EPA has also imposed tighter restrictions. There are several methods for making HVAC systems more efficient.

Heating energy

Water heating is more efficient for heating buildings and was the standard many years ago. Today forced air systems can double for air conditioning and are more popular.

A couple of benefits of forced air systems, which are now widely applied in churches, schools and high-end residences,are 1) better air conditioned effect 2) up to 15-20% energy saving, and 3) evenly conditioned effect. A drawback is the installation cost, which might be slightly higher than traditional HVAC system.

Energy efficiency can be improved even more in central heating systems by introducing zoned heating. This allows a more granular application of heat, similar to non-central heating systems. Zones are controlled by multiple thermostats. In water heating systems the thermostats control zone valves, and in forced air systems they control zone dampers inside the vents which selectively block the flow of air. In this case, the control system is very critical to maintain a proper temperature.

Ventilation Energy recovery

Energy recovery systems sometimes utilize heat recovery ventilation or energy recovery ventilation systems that employ heat exchangers or enthalpy wheels to recover sensible or latent heat from exhausted air. This is done by transfer of energy to the incoming outside fresh air.

Air conditioning energy

The performance of vapor compression refrigeration cycles is limited by thermodynamics. These air conditioning and heat pump devices move heat rather than convert it from one form to another, so thermal efficiencies do not appropriately describe the performance of these devices. The Coefficient-of-Performance (COP) measures performance, but this dimensionless measure has not been adopted, but rather the Energy Efficiency Ratio (EER). EER is the Energy Efficiency Ratio based on a 95°F outdoor temperature. To more accurately describe the performance of air conditioning equipment over a typical cooling season a modified version of the EER is used, and is the Seasonal Energy Efficiency Ratio (SEER). SEER ratings are based on seasonal temperature averages instead of a constant 95°F outdoor temperature. The current industry minimum SEER rating is 13 SEER.


Heating and Cooling Tips:

(1) Don’t delay when you need air conditioner or heater repair If you smell gas, send everyone out of your home and call your gas provider’s emergency telephone number immediately. If your air conditioner won’t cool, makes odd noises or freezes up, turn your thermostat to “off” and call your service company without delay. Running your A/C system when it isn’t working properly can turn a small repair into a much larger one.

(2) For your health, comfort and financial well-being, change or clean your air conditioning filters regularly per the manufacturers specifications. Depending on your equipment, the type of air filters you use, and the quality of the air being filtered, air filters need to be changed (or cleaned) somewhere between twice a month to once per year. On the disposable air filters, changing them once per month when you pay your utility bill is a good way to remember. Never run your heating and air conditioning system without the correct air filter in place. The correct filter will be the correct size in length, width, and thickness, plus allow the amount of air required by your equipment to pass through.

(3) Have your air ducts checked periodically for leaks. Leaking ducts cause more than 20% of the losses of heating and cooling, according to the U.S. Department of Energy. If you are on a maintenance program with your air conditioning and heating company, leak detection could be included. If air ducts are located in a hot or cold space such as an attic, have the outside of the ducts insulated to reduce heat loss. Having leaky ducts saves wear and tear on both your heating and cooling equipment and lowers your monthly bills.

(4) Duct cleaning: the subject of diverging opinions Some service companies state they can clean the inside of your air ducts in a variety of ways. Prices vary accordingly. Other companies state that, unless your system has been run without a filter, moisture has entered the system, or your ductwork has a leak on the return side, duct cleaning should not be necessary. Other sources speculate that the cleaning process could cause or increase air leaks from the ducts. If your ducts do need cleaning, you should have the cause of the problem determined and removed first. We recommend that you ask the air conditioning and heating company you trust with the regular maintenance of your system—not a company that only offers duct cleaning.

(5) When you have work done on your system, ask about an electronic programmable thermostat The timer function saves unnecessary wear on your heating and cooling system plus saves energy. Thermostats that cover both cooling and heating can deliver more comfort and lower bills year around.

(6) As A/C and heating systems get older, they usually require more service calls. Since your system works hardest during periods of extreme heat or cold, when it does develop a problem, it is more likely to occur on a day you need cooling or heating the most.

(7) Annual maintenance agreements are often a good value in air conditioning and heating. Why? (a) Maintenance agreement costs are mostly for labor. Without regular maintenance, you are more likely to later pay for labor and parts in repairs. (b) Maintenance helps prevent unplanned, inconvenient repairs, which are more likely to be needed during the first hot or cold days of the season. (c) Maintenance includes cleaning and adjustments that cause your HVAC system to run more efficiently. This lowers your monthly cooling and heating energy costs. (d) Maintenance will help prevent related damage such as A/C drain line or pan water leaks through your ceiling or dangerous carbon monoxide gas producing malfunctions within your heating system.

(8) A well-maintained air conditioning and heating system can last up to 15 or 20 years, however… Due to improvements in energy efficiency, if your A/C system is more than 10 years old, it often makes sense to replace your old system before it quits or repair frequency increases. Some components in the heating system can last even longer. Much of the time, you are better off if you can afford to replace both the A/C and heating components together.

(9) Repair vs. replacement on an older air conditioning and heating system. If you are “on the fence” in your decision to repair or replace your HVAC system, you can ask your air conditioning and heating contractor if they can send a technician(s) who can both repair the system and give a price quote for a new one. If you do this, the company might refund the service call or trip charge on the repair if you buy a new system from them soon after. It should not hurt to ask. If the contractor agrees, have them write the arrangement on your receipt.

(10) When negotiating a contract to buy a previously owned home you may need a heating and cooling professional to evaluate the status of the air conditioning and heating system. Since you do not yet own the home, this creates some special circumstances. To get the information you need and keep your options open, you can request a combination repair and replacement estimate.

(11) Take a “whole house” approach to heating and air conditioning your home or business. Energy conservation measures cause your heating and A/C system to work less while keeping you comfortable. Be sure to consider energy conservation improvements whenever you review your heating and A/C needs. The contractor you choose might do all the related work or work closely with an energy conservation company. You might be able to get local rebates or federal tax credits on energy conservation improvements.

General comments about HVAC contractor services:

The company you choose will make a big impact on your satisfaction and purchase of a new air conditioning and heating system. Why? They determine the size and type of equipment for your situation and they control the quality of the installation. If you have not yet seen them, in the navigation menu on top of this page you will find quick tips and expanded topics on two essential topics: how to select the right HVAC contractor and how to buy a new central heating and cooling system. Due to relationships with their equipment distributors or ownership by a manufacturer, some air conditioning and heating contractors install a limited number of equipment brands. Many independents have a brand they prefer or for which they are dealers, but they will usually order and install any brand of new equipment you prefer to buy. However, most HVAC companies will repair and maintain any brand of central heater, furnace and air conditioner you have. In addition to heating and cooling for indoor comfort and energy savings, an increasing number of HVAC companies perform air quality work such as air filters, dehumidifiers and humidifiers, ventilation and prevention of toxic carbon monoxide (CO) gas from your furnace. However, not all of them perform energy efficiency improvements to houses or buildings, such as the addition of solar screens or radiant barriers. If the service company you contact does not perform energy conservation improvements on homes or buildings, they should be able to recommend another Spokane area company that does. How to compare air conditioning and heating equipment price quotes Air conditioning and heating systems are made of components or units, and these can be interchanged to yield many possible combinations of performance and efficiency. Additionally, based on the particular situation in your home or building, these components must be chosen and installed to accommodate them. For these reasons, comparing proposals, features, and prices can be confusing indeed. To help you quickly sort through the maze of options, use our free and printer-friendly cost comparison organizer found in the navigation on the top of this page. If you get multiple price quotes, it will save you a lot of time and headaches. This grid allows you to compare the essential features “apples-to-apples” from contractor bids including: main components; equipment efficiency in SEER and AFUE; manufacturer’s and installer’s warranties; and equipment brand. How the Spokane area climate and resources affect your HVAC needs and decisions Local climate exerts one of the most significant influences on the type of indoor climate control you need. On the heating and cooling list of the USA listing 5 zones, Spokane sits in cooling zone 1 and on the borders of heating zones 1 and 2. These require approximately 584 cooling hours and about 3784 degree-days per year for heating. As a relative comparison, the average home or building here needs about 6.5 times more heating than A-C. Differences in elevation, proximity to oceans, lakes or other geographical influences often make a difference. Be sure to rely on actual measurements and service from a certified heating and air conditioning company. Spokane is located where the flat arid farming areas meet the Selkirk Mountains and forests of the foothills of the Rockies. The climate of Spokane and eastern Washington state brings four distinct seasons typical of its continental classification. Winters tend to be cold with more precipitation. Summers tend to be warm, sunny and drier. The hottest months, June, July and August, have average highs reaching the low 80s Fahrenheit. The coldest months of December and January dip to an average low around 20°F. This type of climate has numerous days throughout the year in which you may not need to run your air conditioner or furnace. However, more than heating and cooling contribute to comfort. On these temperate days, a whole house air filter or fresh air ventilation could make a major improvement in your indoor comfort and well being. Throughout the year humidity averages 78% before noon and 52% after noon. Pollen, allergies and air filters In Spokane air filters are a necessity to some homeowners. Certain eastern Washington plants can strongly influence what type of air filter you and your A/C system need. Some local examples include the birch, alder and cedar (juniper) trees, ragweed, and other types of allergy producing pollen and mold. The daily pollen count and allergy forecast for the Spokane and the surrounding area can be found at Weatherbug. Local utility and conservation providers For the Spokane area and also Washington state, the DSIRE website lists current energy efficiency rebates and incentives. In many cases, your local A/C and heating company can coordinate all available energy conservation and rebate programs, from federal tax credits to local rebates and incentives, saving you research time. The availability of certain fuels such as natural gas or heating oil can determine which HVAC equipment will give you the best combination of comfort and lower utility bills. Since natural gas is available in some Spokane neighborhoods, those consumers often choose gas central furnaces rather than heat pumps. If natural gas prices climb faster than electricity rates, this trend might change. In situations where a dual fuel heat pump makes the best choice, some of the factors that affect air source heat pumps include the average daily temperature, fluctuations in temperature, and humidity. Other topographical conditions, such as solid rock outcrops located close to the surface, can limit the use of ground source heat pumps. For more information on this topic, go to the top of this page and visit the heat pump page under Expanded A/C and Heating Details. How a whole house approach includes the “envelope” in the load calculation If you plan to get price quotes to buy a new A-C and heating system, look for contractors who will insert the heating and cooling demands of the Spokane climate along with the size and condition of your home or building “envelope” into the necessary load calculation. For many reasons, a rule of thumb load calculation based only on square footage or using the same size system as the old one without verification should be avoided. This calculation requires considerable training and expertise, and is not a good place to attempt to cut costs or save time.
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