Fresh Air System Basics: Ventilation & HVAC Guide

The Purpose of Ventilation:

• Health Protection: Purify indoor air and supply continuous fresh oxygen for occupants.

• Building Durability: Remove excess heat and moisture to maintain a stable thermal environment and protect the building structure.

Modern Buildings: Airtightness is stronger and sound insulation requirements are high.

Natural Ventilation: Opening windows for ventilation is outdated. Continuous 24-hour ventilation guarantees fresh air circulation indoors. This achieves an ideal, fresh home environment. It makes home life healthier.

Mechanical Ventilation: It provides fresh air that is filtered, targeted, and quantified.

Whole-House Ventilation vs. Local Ventilation:

• Whole-House Ventilation: There is no fixed pollution source. It ventilates the entire house, brings in outdoor fresh air to lower pollutant concentration. This is dilution ventilation which can be used in residential homes, offices, etc.

• Local Ventilation: There is a fixed pollution source, or the source is concentrated. It exhausts nearby pollutants outdoors as quickly as possible which is used in kitchens, bathrooms: smoking rooms, etc.

Mechanical Ventilation: Uses Ventilation Equipment as the driving force. It uses mechanical power to force ventilation and to guarantees a stable airflow.
Natural Ventilation: This means opening windows.

Types of Mechanical Ventilation:

1. Two-way Ventilation: Both air supply and air exhaust use mechanical fans.

2. Positive Pressure Ventilation: Mechanical air supply + natural air exhaust. It involves indoors, air supply vents, and ventilation fans.

3. Negative Pressure Ventilation: Natural air supply + mechanical air exhaust.

Residential ventilation: It is divided into natural ventilation and mechanical ventilation.

Mechanical ventilation: It is divided into two-way ventilation (both supply and exhaust are mechanical fans).

Positive pressure ventilation (mechanical supply + natural exhaust).

Negative pressure ventilation (natural supply + mechanical exhaust).

Airflow Volume:
For example: A room has an area of 40m², ceiling height is 3m, the volume is 120m³. You use a ventilation device of 120m³/h.
This means in 1 hour, you can replace the indoor air once.
Airflow volume means the amount of air exhausted (or sucked in) by the ventilation device in a unit of time. Its size directly represents the ventilation effect.

Pressure: 
Water column pressure gauge. Pressure is divided into: dynamic pressure, static pressure, and total pressure.
Total pressure = static pressure + dynamic pressure. Airflow. (Static pressure) S. (Dynamic pressure) V. (Total pressure). [Unit] Pa (Pascal).

Note: How long the pipes and parts of the fresh air system can be depends on how much static pressure there is. A home's P-Q curve is the standard comparing reference airflow and pressure loss.

• Dynamic Pressure (Converts to flow speed): This refers to the pressure created by speed when fluid flows inside a pipe. In simple terms: Dynamic pressure is the pressure that drives fluid forward.

• Static Pressure (Pressure gauge test value): This refers to the vertical pressure acting on the pipe wall when fluid flows inside. In simple terms: Static pressure is the pressure that overcomes pipe resistance.

Pressure Loss: 
The resistance of air flowing inside the pipe.

• Friction Pressure Loss (Straight pipe): When fluid flows through a straight pipe of a certain diameter, resistance is generated by internal fluid friction. The resistance size is proportional to the path length.

• Local Pressure Loss (Fittings): When fluid passes through parts like elbows, tees, or vents, gas movement is disturbed. This inevitably causes energy loss. This loss in a local area points out local resistance. U.

The Relationship Between Airflow and Static Pressure

Think of a fan's total capacity (Airflow + Static Pressure) as a fixed score of 100. If your ductwork creates a high resistance (e.g., a static pressure loss of 40), your airflow drops to 60, meaning slower wind speeds. If the resistance is massive (90), your airflow drops to just 10. However, with zero resistance, your airflow hits the maximum of 100.

In short: Higher pressure loss equals lower dynamic pressure, slower wind speeds, and less airflow. Conversely, minimizing pressure loss increases your dynamic pressure, speeding up the wind and delivering more air.

Noise

Using a fresh air system at 30~40 decibels is an ideal quiet environment.

Opening windows lets in dust, noise, and unfiltered air. Mechanical ventilation provides clean, filtered, and steady fresh air 24 hours a day.

Positive pressure pushes fresh air in mechanically, forcing old air out naturally. Negative pressure pulls old air out mechanically, letting fresh air in naturally.

Static pressure overcomes the resistance inside the air ducts. If the static pressure is too low, the air will not reach the vents at the end of the pipes.

It transfers temperature and humidity from the outgoing stale air to the incoming fresh air. This means your AC or heater does not have to work as hard.