Selection of HVAC Equipment
The climate conditions in most parts of the country require the addition of HVAC systems to provide acceptable thermal comfort for building occupants, necessitating the heating and cooling of outdoor air. Selecting equipment to heat, cool, and ventilate the building involves balancing several factors, including:
- Heating and cooling needs
- Energy efficiency
- Humidity control
- Natural ventilation potential
- Adherence to codes and standards
- Quantity and quality of outdoor air
- Indoor air quality
- Cost
Where possible, use central HVAC air handling units (AHUs) that serve multiple rooms instead of unit ventilators or individual heat pumps. Although there are many different types of air handling units, they can generally be categorized into two groups for their overall IAQ impacts in buildings: unit ventilators and individual heat pump units serving a single room via ductless operation; and central HVAC air handling units serving multiple rooms via ductwork.
Unit ventilators and heat pumps have the advantage of requiring reduced floor space and do not circulate air between rooms. However, ensuring proper maintenance of multiple units over time can be more challenging, and they present additional opportunities for humidity issues due to wall penetrations, drain pan, and drainage problems.Central HVAC air handling units (AHUs), on the other hand, offer several advantages over unit ventilators and heat pumps serving individual rooms. These include:
- It is quieter and therefore more likely to be switched on or off by teachers and staff;
- Less airflow due to multiple consumables and a turn away from passengers;
- It is better at controlling humidity and intensive moisture drainage;
- Maintenance is easier due to the reduced number of components and fewer access units;
- More space around the units and accessible without interfering with classroom activities;
- Space for higher efficiency air filters and more surface area;
- Made of heavier duty components;
- Unintentional reduction of the amount of external air supply is less likely.
The following features are important for all air handling units:
Double sloping drain pan and drain siphon depth
Double sloping drain pan – A double sloping pan prevents water from standing and stagnating in the pan.
Non-corrosive drain pan – Made of stainless steel or plastic. Prevents corrosion that would cause water leakage inside the AHU.
Easy access doors – All access doors are hinged and use quick release latches that require no tools to open. Easy access to filters, drain pans and cooling coils is mandatory.
Double wall cabinet – The inner wall protects the insulation from moisture and mechanical damage, increases sound damping and is easier to clean.
Tightly sealed cabinet – Small but continuous air leaks in and out of the AHU cabinet can affect IAQ and energy. The largest pressure differentials that trigger leaks occur in the AHU.
Double skin doors with seals – Double skin doors provide better thermal and acoustic insulation and remain flatter, providing a better seal against door frame seals.
Minimum 2-inch thick filter housings – Filter housings must be able to accommodate filters 2 inches or thicker for better protection of equipment and ductwork as well as the indoor environment.
Expanded surface area filter bank – Designed to allow for more filter area, such as through a deep V approach or bags, to reduce filter maintenance frequency and fan energy costs.
Air filter arrangements designed for minimum leakage (racks and housings) – In the filter bank, all points where air can easily pass through the air filters, such as filter racks and access doors, should have gaskets and sealing materials for minimal leakage. Use appropriate gaskets and manufacturer-provided filter rack spacers with seals.
Air filter monitor – A differential pressure gauge that indicates the static pressure drop across the filter bank. This feature can be easily installed optionally in the field.
Corrosion-resistant dampers and connections – All moving parts such as pivot pins, damper actuators, and connections are designed to withstand corrosion due to weather conditions and moisture throughout the system’s lifetime.
