4 Essential Factors to Consider When Designing an HVAC System

When designing an air conditioning system, it is essential to take into account a variety of factors. These include the size and design of the building, the local climate, energy efficiency, indoor air quality, maintenance and serviceability, cost, occupancy and use, zoning, noise, sustainability, regulatory and code requirements, and future expansion. In humid climates, air conditioning systems must be designed to maintain humidity control. This can be achieved by pressurizing the building with dehumidified air.

The Florida Solar Energy Center (FSEC) found that building pressures as low as +1 pascal (Pa) relative to outdoor conditions are sufficient to prevent outdoor air infiltration problems. On the other hand, even a slightly depressurized building (-1 Pa compared to normal outdoor conditions) in hot, humid climates can cause devastating moisture and microbial growth problems when the building envelope retains this moisture. Air conditioning systems that positively pressurize a building space by providing unconditioned or only partially conditioned outdoor air will prevent the infiltration of outside air through the building envelope. However, this same situation can cause moisture loads inside the building that exceed the dehumidification capacities of the system. The comparison of latent and sensitive loads in several major cities in different geographical regions (Peart and Cook, 1999) helps to illustrate the new definition.

Figure 2 (ABC) shows the monthly average of latent and sensitive loads coming from outside air in Orlando (Florida), Atlanta (Georgia) and Columbus (Ohio). During the cooling season in Orlando (Figure 2A), the latent load far exceeds the sensible load of outdoor air. The effect of these conditions is that any outside air entering the building envelope or the occupied space is likely to cause moisture accumulation and microbial growth problems. In addition, since this outdoor air is used to ventilate the occupied spaces of the building, it represents an enormous dehumidification challenge for the replacement air system. To avoid these issues, overemphasis on ventilation must be avoided at all costs. Achieving adequate pressurization of buildings is sometimes difficult.

The pressurization of buildings must overcome any depressurization caused by the chimney effect, the wind effect and the fan effect (figure). The design team must consider how exhaust air systems will affect space pressures. If the system cannot provide sufficient dehumidification while reacting solely to temperature control, it must continue to eliminate humidity without affecting the interior temperature or the comfort of the occupants. One way to achieve this is by reheating, a way of simultaneously cooling and heating to continue dehumidification without overcooling the occupants. The second method of dehumidification is through the use of a desiccant that attracts moisture from the air to its surface by introducing low vapor pressure to the surface of the desiccant.

The desiccant must then be recharged through a heating process. The best dehumidification strategies involve a combination of desiccant and cooling systems, especially for 100 percent outdoor air currents such as systems of replacement air. Since air leaves a cooling-based system when it becomes saturated, it only moves to a lower relative humidity once it mixes with room air and heat is added to it. When selecting and designing HVAC systems for buildings, energy efficiency should be optimized through ventilation, zone control, heat recovery and operations. Ease of maintenance and ease of service are also essential factors in HVAC system design.

Julia Hirliman
Julia Hirliman

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