19 meaning that the building anyhow needs mechanical ventilation, at least for its core area. It should be noted that the presence of a mechanical ventilation system does not imply that a building is inefficient. On the contrary, for new construction or retrofitted buildings with a tight envelope, a ventilation system is needed to meet fresh air requirements as frequent window openings during winter or summer lead to significant heat losses. To avoid these losses while still meeting ventilation requirements, mechanical systems can be outfitted with heat recovery ventilation (HRV). An HRV transfers sensible (and sometimes latent) heat from the conditioned exhaust air to incoming outdoor air (Fig 6.10). In many climates so-called hybrid systems are in place for which operable windows provide ventilation during the milder times in the year and extreme outside conditions are covered via mechanical ventilation. Once it is clear that a building requires mechanical ventilation, the design teams needs to decide whether the ventilation system should only meet fresh air requirements or whether forced air should also be used to heat and cool the building. The resulting two system types are called minimum outdoor air and all air. An advantage of the latter type is that one central system can meet all conditioning needs of a building. A key argument in favor of minimum outdoor air systems is that they usually require significantly smaller duct cross-sections. Why is that? Central plant Supply air Return air AHU Fig 1.8 Section of the Medium Office Boston with a rooftop Air Handling Unit (AHU) supply and return shafts and vertical ducts.
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