Airflow Systems: Chapter 3
Ventilation
In this module we will learn what ventilation is and how it affects airflow systems. Skip to quiz!
Introduction
The V in HVAC stands for ventilation! Ventilation is a very important part of the HVAC system. So what is it?
Ventilation keeps fresh air flowing through a home or building. It keeps the indoor air quality healthy and comfortable. Let’s see how it keeps our air healthy.
Ventilation controls impurities. This means it keeps removing contaminants that build up indoors. A crowded room with poor ventilation quickly becomes overwhelmed with body odor.Good ventilation prevents this problem!
Ventilation makes sure you are not circulating too much or too little air. Too little air allows contaminants to build up. Too much air causes a high electricity bill.
Condensation is water that collects as droplets on surfaces. Condensation can lead to mold and rotten surfaces. Good ventilation reduces the risk of condensation.
When there are lots of people in a room it can become hot and stuffy. Good ventilation keeps air temperatures more comfortable.
Indoor air pollution with bad ventilation can lead to lots of health problems. This can include headaches, rashes, asthma, allergies and more. Let’s take a closer look at the effects of bad ventilation on health.
Bad ventilation means bad humidity control. Low humidity can cause a dry throat, dry skin, and static electricity build-up. High humidity contributes to bacterial and mold growth. This can make occupants very sick
Bad ventilation can result in uncomfortable odors and temperatures.Irritating temperatures and odors cause fatigue, discomfort, and distraction.
In some cases poor ventilation can cause Sick Building Syndrome (SBS). Symptoms include irritation of the eyes, nose, and throat, headaches, fatigue. Your susceptibility to colds and flu increases also.
Building Pressure
Recall that air both occupies space and exerts pressure.This is true for outdoor air as well as indoor air. The pressure exerted by air inside a building is called building pressure.
Recall that the standard atmospheric pressure is 1 atm. This varies from place to place. Positive pressure is when the building pressure is higher than the outside air. This causes air to flow from inside the building to outside.
Negative pressure is when the building pressure is lower than the outside air. This causes air to flow from outside the building to inside.
Building pressure also affects human comfort. Ventilation can also help control building pressure.
Airflow in Ventilation Systems
Once humans breathe in air, they use up the oxygen and release the CO2. New air is needed to replace the used up air without oxygen. This new air is called makeup air and is supplied by the ventilation systems.
The air in a building is not replaced all at once. A building can hold a certain volume of air. A ventilation system replaces that air slowly over time. When all of the original air in a building is replaced this is called an air change.
Air change is the number of times the air within a space is replaced during a specified period. Air change can be measured in changes/minute or changes/hour.
Ventilation Fans
Ventilation fans are used to circulate air through the building. There are two main types of fans:
Axial Fans, and
Centrifugal Fans
Axial fans move air parallel to the central point at which the blades rotate. Condenser fans are examples of axial fans. These are cheap and can move a lot of air very easily, but are noisy at high speeds.
Centrifugal fans are also called blowers and are are mainly used in ductwork. They move air perpendicular to the central point at which the blower wheel rotates. These are durable and can create high-pressure airflow, but are expensive.
Infiltration and Exfiltration
Infiltration is the accidental introduction of outside air into a building. Infiltration occurs due to pressure differences between inside and outside.
Infiltration causes air contaminants to enter the room. Air contaminants will affect the health of occupants.It also allows hot and cold air to enter. This affects the room temperature.
Exfiltration is the movement of air out of the building. It can leave through cracks between wall sections, wall-floor connections, and around windows and doors.
Exfiltration allows hot or cold air to escape from a building. This means HVAC systems have to work harder to maintain the conditions. Working hard means the electricity bill is higher! We want to avoid exfiltration and infiltration in ventilation systems.
Ventilation is a very important part of airflow systems. Poor ventilation leads to problems in comfort and health. A good ventilation system makes sure that pure air is always flowing through a building.
Types of Ventilation
In this module, you will learn about the different types of ventilation systems. The goal is to understand the different applications of each system and their pros and cons. Skip to quiz!
Overview
We are going to be exploring three types of ventilation systems.
Exhaust system,
Supply system, and
Balanced system
Each system has a different set of pros and cons.
Different rooms have different ventilation needs. For example, air needs to be removed from the kitchen because of smoke and cooking fumes. In the living room, we need to remove stale air and provide fresh air.
Exhaust Systems
Exhaust systems force indoor air out of the home. Exhaust systems remove unwanted air so that new air may fill the space. Kitchens operate with exhaust systems.
In exhaust systems, fans move indoor air out of building. Fresh air enters the building through windows and doors. However, more air leaves the building than air enters it. This means the amount of air inside the building decreases.
Less air inside the building reduces the air pressure inside. Recall that negative pressure means the building has a lower air pressure than its surroundings. So building pressure is negative in exhaust systems.
Exhaust systems are made up of fans, ductwork, air filters and air outlets. Air outlets are where the indoor air exits the build and is released outside.
The fan draws air from the room through ductwork. The air filter in the ductwork traps the pollutants. Filtered air is released through the air outlet.This creates negative pressure in the room.
Supply Systems
The supply ventilation system draws outdoor air into the home. The supply system does the exact opposite of the exhaust system.
These systems use fans to draw outside air in. The air inside the building already, escapes through cracks and leaks. In this case, the air entering is more than the air leaving. So, the amount of air inside is increasing.
More air inside the building increases the air pressure inside.Recall that positive pressure means the building has a higher air pressure than its surroundings. So building pressure is positive in supply systems.
The components for supply systems are almost the same as exhaust systems. Supply systems are made up of fans, ductwork, air filters and air inlets. Air inlets are where the air enters the building.
The fan pulls outdoor air into the ductwork. The air filter in the ductwork traps the pollutants. Filtered air enters the room air outlet. This creates positive pressure in the room. Positive room pressure does not allow outdoor air into the room.
Balanced Systems
The balanced ventilation system draws outdoor air into the home and pushes indoor air out.
The air in and air out processes occur at the same rate.
A balanced ventilation system combines the exhaust and supply systems. The air coming in is the same as the air going out. This means the amount of air inside the building is never changing.
The positive and negative pressure of the other systems cancel out. This system is a neutral pressure system.
A balanced system combines the components of the other two systems:
Supply fan & Exhaust fan,
Supply ductwork & Exhaust ductwork,
Supply air filter & Exhaust air filter, and
Air inlet & Air outlet
If we combine supply and exhaust systems, the balanced system comes into existence. The supply fan pulls outdoor air into the room through the ductwork by air inlet.
The exhaust fan draws air from the room through the ductwork by air outlet. Air filters are used to trap air contaminants in these processes.
System Comparison
Exhaust systems work well in cold climates and are inexpensive. Supply systems minimize pollutants from outside and are inexpensive. Balanced systems work well in all climates.
Exhaust systems can bring in pollutants and are not good in hot climates. Supply systems cause moisture problems and are not good in cold climates. Balanced systems are very expensive and do not remove moisture.
All three systems improve the conditions inside a building. However, all three systems raise the heating and cooling costs.
There are pros and cons to all three ventilation systems. Climate plays an important role on deciding which system to implement. All three ventilation system are helpful in keeping the indoor climate comfortable!
Question #1: Which of the following is a benefit of ventilation systems?
Controlling Impurities
Air Regulation
Stopping Condensation
Reducing Temperature
All of the above.
Scroll down for the answer...
Answer: All of the above.
A good ventilation system will provide all of the mentioned benefits. Additionally, there are many health benefits.
Question #2: A good ventilation system will keep the humidity as low as possible.
True
False
Scroll down for the answer...
Answer: False
When the humidity is too low, it can result in dry skin and rashes. When the humidity is too high, mold can form more easily.
Question #3: In which direction does the air flow with positive pressure?
Inside to Outside
Outside to Inside
Scroll down for the answer...
Answer: Inside to Outside
When there is positive pressure, the inside has higher pressure than the outside. Air flows from high pressure to low pressure.
Question #4: A building takes 30 minutes to cycle out all of the air. What is the air change of the building?
30 air changes/hour
0.5 air changes/hour
1/30 air changes/hour
2 air changes/hour
Scroll down for the answer...
Answer: 2 air changes/hour
It takes 30 minutes for one full air change. This means the building can complete two in one hour.
Question #5: Which fan would you use to move air through ductwork?
Axial
Centrifugal
Scroll down for the answer...
Answer: Centrifugal
Centrifugal fans are more powerful. This is good for moving large volumes of air through ductwork.
Question #6: Which process affects the temperature inside a building?
Infiltration
Exfiltration
Both
Scroll down for the answer...
Answer: Both
Both exfiltration and infiltration change the temperature of a building. Infiltration releases cool and warm air out. Exfiltration intakes cool and warm air.
Question #7: Which of the following is not a ventilation system?
Retrieval
Supply
Exhaust
Balanced
Scroll down for the answer...
Answer: Retrieval
Retrieval systems are not a standard form of ventilation systems.
Question #8: Exhaust systems have negative pressure because the inside has _____ pressure than the outside
Higher
Lower
Equal
Scroll down for the answer...
Answer: Lower
Negative pressure means the building has a lower air pressure than its surroundings.
Question #9: Which of the following is not part of a supply system?
Fans
Ductwork
Air filters
Air outlets
Scroll down for the answer...
Answer: Air outlets
Supply systems use air inlets. Exhaust systems use air outlets.
Question #10: A balanced system is a combination of the supply and exhaust systems.
True
False
Scroll down for the answer...
Answer: True
A balanced system both brings air in like supply and pushes air out like exhaust.
Question #11: Which is true of all ventilation systems?
Works best in cold climate
Raises heating and cooling costs
Cause moisture problems
Remove pollutants
Scroll down for the answer...
Answer: Raises heating and cooling costs
All systems require power to bring air in or remove it. This increases the heating and cooling costs.