Ventilation (Leaving Cert Construction Studies): Revision Notes
Ventilation
What is ventilation?
Ventilation is the process of circulating and replacing air within a building or other environment. This system removes stale, moisture-laden air and replaces it with fresh air to maintain healthy indoor conditions.
Historically, buildings were naturally ventilated through small gaps in construction materials, windows, and doors. However, modern construction methods create much more airtight buildings, making purpose-designed ventilation systems essential for maintaining air quality and preventing moisture problems.
The choice of ventilation system affects a building's Building Energy Rating (BER), so it must be considered during the design stage of any construction project.
Requirements of ventilation systems
Ireland primarily uses two main ventilation approaches: natural ventilation and mechanical ventilation with heat recovery. Regardless of which system is chosen, all ventilation systems must meet several functional requirements.
Essential functions
Every ventilation system must:
- Remove or dilute pollutants from indoor air to maintain healthy conditions
- Provide sufficient air change per hour (ACH) to control condensation throughout the building
- Quickly ventilate wet areas such as kitchens, bathrooms, and utility rooms to remove pollutants and excess moisture
- Include purge ventilation in habitable rooms to rapidly remove pollutants and excess moisture when needed
- Supply fresh air to address any remaining air quality problems that extraction alone cannot solve
Understanding pollutants
Pollutants are harmful substances that can cause both immediate and long-term health problems. Common pollutants found in homes include:
- Odours from cooking, cleaning, or other activities
- Aerosol chemicals from cleaning products or air fresheners
- Carbon monoxide from faulty heating appliances
- Carbon dioxide from human respiration
- Radon gas, which can be effectively removed through proper ventilation
Air change per hour (ACH)
ACH measures how many times the entire volume of stale air in a room gets replaced with fresh air during one hour. This measurement helps determine whether ventilation is adequate.
For most buildings, an ACH rate between and provides sufficient air exchange to control condensation problems throughout the entire structure. This means the air in each room is completely replaced with fresh air every 40 minutes to 2 hours.
Practical Example: Understanding ACH Rates
If a room has an ACH of 1.0:
- The entire air volume is replaced once per hour
- Complete air replacement occurs every 60 minutes
If a room has an ACH of 0.5:
- The entire air volume is replaced every 2 hours
- Complete air replacement occurs every 120 minutes
Mechanical ventilation systems
Mechanical extract fans
Mechanical extract fans are installed in wet or very humid areas like bathrooms and kitchens. These fans rapidly extract moisture-laden air and direct it outside the building before condensation can occur.
Extract fans are typically mounted on ceilings and can operate either manually (switched on when needed) or automatically (activated by humidity sensors). In kitchens, these fans also remove heat and steam from cooking activities.
Purge ventilation
Purge ventilation, previously called rapid ventilation, provides a quick method for removing pollutants and excess moisture from habitable rooms. This system works by opening windows or doors to create direct air circulation between indoor and outdoor environments.
Purge ventilation is particularly useful when indoor air quality suddenly deteriorates, such as after cooking strong-smelling foods or when cleaning with chemical products.
Wall vents
Wall vents create controlled air circulation between rooms and the outside environment. These vents pass through the wall cavity and include closeable openings at both ends, allowing occupants to adjust ventilation levels as needed.
Wall vents provide a simple way to maintain background ventilation without relying on open windows, making them useful during cold weather when opening windows would cause excessive heat loss.
Natural ventilation
Natural ventilation relies on weather conditions, particularly wind and temperature differences, to drive air movement through buildings. While this approach can be effective, it creates challenges because outdoor conditions vary constantly.
Problems with natural ventilation
Over-ventilation occurs when too much warm indoor air is lost to the outside. This problem increases heating costs and can create draughts that make occupants uncomfortable. Over-ventilation typically happens during windy weather or when large temperature differences exist between indoor and outdoor environments.
Under-ventilation happens when insufficient air exchange occurs. This leads to several problems including:
- Mould growth due to excess moisture
- Condensation on windows and walls
- Generally poor indoor air quality
- Build-up of pollutants and odours
Under-ventilation commonly occurs during calm weather conditions or when buildings are too airtight without adequate ventilation provisions.
Key Points to Remember:
- Ventilation systems must remove pollutants, provide adequate ACH, and supply fresh air to maintain healthy indoor conditions
- An ACH rate of 0.5-1.5 is sufficient to control condensation in most buildings
- Mechanical extract fans are essential in wet areas like kitchens and bathrooms to prevent moisture problems
- Natural ventilation depends on weather conditions and can cause both over-ventilation and under-ventilation problems
- Modern airtight buildings require purpose-designed ventilation systems, unlike older buildings that relied on natural air leakage