29 outdoor air system that provides fresh air to the zones, and distributed fan coil units for space conditioning. In contrast, the VAV system relies on one set of (larger) ducts to provide ventilation and space conditioning. As a consequence, the VAV is generally more common which further reduces design and procurement costs. Higher first costs can be offset with operational savings. Operating cost The operating cost accounts for expenses related to electricity and natural gas purchases as well as maintenance costs. A system with higher operating costs is typically less energy efficient, even though operating costs also depend on local utility prices. For example, 2025 prices for natural gas were higher in Europe than in the United States, making electricitybased heat pumps often more economical in Europe. In Fig 1.3, the fan coil unit system costs less than the VAV to operate due to its superior energy performance. In addition, the fan coil unit system is all-electric and so takes advantage of higher efficiency heat pumps for both heating and cooling. Maintenance This rating assesses how often an HVAC system has to be maintained as well as the effort level associated with this task. Systems with higher maintenance requirements typically have more pieces of equipment, more ducted and piped connections, or are harder to access. The fan coil unit system in Fig 1.3 has more maintenance requirements than the VAV system due to the additional pieces of equipment inside each space. Conversely, the VAV is maintained centrally at the AHU with less potential disruptions to building occupants. Spatial requirements Each system has different spatial requirements. As shown in chapter 1, all air systems require larger duct cross-sections than minimum outdoor air systems which leads to larger vertical shaft areas and potentially greater ceiling heights.
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