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Heating Ventilation and Air Conditioning Full Course - HVAC Course (Part 38)

 

                            In this part of Full HVAC Course, We will learn about ventilation requirements of IAQ in HVAC.

Ventilation requirements of IAQ

Building code requirements

Most states require building codes to specify minimum air exchange rates per hour (AER/hr) for enclosed spaces. AER/hr refers to how much outside air enters a space over time. This number is based on the size of the room and the amount of people present. This value indicates how well a space filters out contaminants, including volatile organic compounds (VOCs). The higher the AER/hr, the lower the level of VOCs in a space.

Indoor Air Quality (IAQ)

Indoor air quality (IAQ) refers to the quality of the air inside buildings. In addition to building codes, the U.S. Environmental Protection Agency (EPA) recommends that facilities keep the concentration of airborne particles less than 0.1 microns (µm) in diameter (PM10), and the concentration of toxic chemicals below their respective EPA allowable limits. Other factors that influence IAQ are temperature, humidity, cleanliness, and proper ventilation.

Ventilation

For adequate ventilation to occur, an area's air exchange rate must exceed 1 cubic foot of fresh air per minute (cfm) for each person occupying the space. For example, if a room has 10 occupants, then the air flow should be at least 10 cfm/person. If the airflow rate is insufficient, the room may become stagnant and filled with harmful pollutants, especially when temperatures are high.

Supply-Demand Ratio

The supply-demand ratio is the relationship between the air exchanges in a space and the occupancy levels. The ideal range is 14 cfm/person during peak hours and 15 cfm/person at night. Most commercial buildings have a supply-demand ratio of 2 cfm/person.

Demand Factors

Factors that increase demand include activities such as cooking, using blow dryers, smoking, or cleaning with strong solvents. Activities that decrease demand include sleeping and watching television.

Moisture Content

Moisture content can affect the air exchanges in a structure and can cause mold growth. An unoccupied space should not have a relative humidity above 50%. Relative humidity measures the moisture content in a space; this measurement varies depending on the climate where a structure is located.

Humidity

In general, humidity levels in occupied spaces should be maintained between 30% - 60%, while humidity levels in unoccupied spaces should stay below 25%.

Temperature

Temperature affects the rate at which air moves through a space. An average indoor temperature of 75°F provides better air movement than an average indoor temperature of 65°F. To minimize the spread of disease, the EPA recommends keeping the temperature in a space below 78°F.

Heating Ventilation Air Quality (HVAC) systems are designed to provide proper airflow throughout a building. A properly functioning HVAC system provides heating and cooling comfort while maintaining healthy indoor air quality (IAQ). Properly sized vents allow adequate air flow while preventing drafts. In addition, they must have sufficient capacity to remove pollutants which may be present during operation. An air filtration system must be selected based upon the type of pollutant being removed. Most modern equipment has a HEPA filter to clean the air before it enters the ductwork. These filters need periodic cleaning to avoid clogging.

Humidification Humidifiers humidify outdoor air before venting it indoors by adding moisture back to the air. Humidifiers increase humidity levels which reduces the drying effect on skin caused by dry air. When selecting a humidifier, select one that will adequately control the amount of water added to the air. Too much humidity creates condensation problems and can cause mold.

Duct Cleaning While dust accumulates on duct surfaces, it does not necessarily indicate that ducts are dirty. Dust should be cleaned periodically using a vacuum cleaner equipped with a high-quality bagless vacuum. If dust buildup is persistent, duct cleaning should be performed.

Dehumidification Moisture removal from buildings is necessary because excessive moisture causes damage to insulation, wood framing, and finishes. Dehumidifiers do just that – remove excess moisture (or dew point) from the air in order to maintain comfortable indoor conditions. They work best if installed near the return air registers.

Carbon Monoxide Detectors Indoor carbon monoxide detectors are now required by law. They are inexpensive devices that help prevent injury or death due to CO poisoning. Their primary function is to sound an alarm when dangerous amounts of CO are detected. There are two types of CO detectors: those that detect carbon dioxide (CO2), indicating a fire or use of gas appliances; and those that detect carbon monoxide (CO), indicating a potential problem with combustion appliances. If a detector sounds an alarm, occupants should take appropriate action immediately to reduce risk of harm.

Ventilation Requirements:

* Air changes per hour (ACH) - For ventilation, we recommend at least four ACH. That's about 10% ACH.

* Ceiling Height - At least ten feet off the floor. Three to five feet is preferred.

* Floor Areas - About 1 sq ft per person.

* Perimeter - Ventilation should extend outside of the house perimeter to ensure adequate airflow.

* Number of Stories - One story homes require two ACH; 2+ stories require three to four ACH.

* Window Size & Type - Smaller windows and larger openable windows have higher radon levels. Double paned windows reduce radon penetration.

Ventilation requirements are defined as those air changes per hour (ACH) necessary to maintain acceptable levels of indoor air quality (IAQ). These standards are based upon the average concentration of contaminants in outdoor air and are specified in terms of “hours” rather than “volumes” or “masses” of air. Thus, a ventilation system designed to provide 10 ACHs will remove approximately 10 cubic meters of total volume of contaminated outdoor air per hour. In addition, there are two categories of ventilation systems: mechanical and natural. Mechanical ventilation involves moving air indoors using fans, blowers, ductwork, etc., while natural ventilation uses naturally occurring air currents (e.g. wind, ceiling vents, and doorways) as a means of bringing fresh air into dwellings.

The first step in determining whether or not your home requires mechanical ventilation is to calculate the number of occupants and their typical activity level. Occupants of homes may vary greatly in their individual need for ventilation. Generally speaking, however, children and elderly people require more ventilation than adults and persons who spend time sleeping. An average home should have between 6 and 8 ACHs. Homes with three bedrooms or less should have at least 5 ACHs, while homes with four or more bedrooms should have no less than 9 ACHs. To determine the minimum amount of ventilation needed for your specific household, use the following formula:

Number of Persons x 24 hr/day Number of ACH Required

For example, if there are five occupants and they work full-time throughout the year, then the following calculation could be performed to determine the ideal number of ACHs needed:

5 x 24 hrs/day 120 hours/week x 52 weeks 6,240 hours

120 divided by 60 minutes per hour 2 hours per person

Therefore, the recommended number of ACHs would be 6,240 / 2 30 ACH.

If you think that these numbers sound low, consider the fact that many homes exceed the minimum number of ACHs. Many states have laws requiring ventilation of public buildings and homes built before the 1950's. Ventilation requirements increase with increases in temperature. Therefore, homes located in subtropical and tropical climates need considerably more ventilation than homes in temperate zones. If you live in a climate that is prone to extremely hot or cold weather, then you may want to add additional ACHs to accommodate daily fluctuations in temperature.

After calculating the number of ACHs required for your home, you need to decide what type of ventilation system is best suited for your situation. There are basically three options: ducted, whole-house fan, and window fan. Ducted ventilation systems move room air through a series of ductwork and distribute it evenly throughout the house using high velocity fans; whereas, whole-house fan systems move all of the air throughout the house at low velocities. Window fan systems do both—they bring air into the home via windows and exhaust unwanted air out of the house through windows.

Ducted ventilation systems are generally considered to be the most effective when the entire house is occupied and the occupant's activities remain relatively static. Whole-house fans, on the other hand, are good choices when the occupants of the house change frequently and there is a variety of different types of activities going on simultaneously. Window fans are useful when the house contains only one or two rooms. If your home has several small rooms and there are often times when one or more rooms are unoccupied, then a whole-house fan might be the best choice for you.

While choosing the right kind of ventilation system for your home is the first step toward proper IAQ, you must also make sure that it is installed properly. Make sure that any HVAC contractor installs the correct size fan. Failure to do so can lead to serious problems including overheating and carbon monoxide poisoning. Also, make sure that any venting is done correctly and that all vents are accessible so that you can easily check them for blockage. Finally, seal all cracks around doors and windows, especially under exterior stairs or inside walls where humidity tends to accumulate. Cracks allow moisture to enter the space as well as insects and rodents.

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Heat Load Calculation Guide 1

Heat Load Calculation Guide 2

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