13 1 HVAC System Selection “We compared an all air system with heat recovery, powered by a natural gas boiler and a water-cooled electric chiller against an all-electric, minimum outside air system with fan coils units powered by a ground source heat pump. We recommend the all air system since it is more economical to operate and easier to maintain.” How should an architect respond to such a recommendation by a mechanical engineer who joined a mid-sized commercial building project during the late stages of design development? During earlier meetings, the design team had expressed an "interest" in sustainability, maybe even "going for" a net zero energy building. Now, faced with the prospect of a fossil fuel powered HVAC system, the architect may wonder how the team ended up with this disappointing outcome and – more pressingly – where all those air ducts can fit in the design? Nobody can be happy with a situation in which the overall building design and HVAC concept are not well integrated. Unfortunately, this happens all the time. Why is that? Timing is key. A 2025 survey of 52 North American design professionals reported that HVAC system discussions typically only come up during design development when daylighting/solar gain controls, program distribution and key structural elements have largely been set.1 Fig 1.4 summarizes survey results with the red, vertical lines indicating at what point 50% of a design deliverable have been completed. The figure reveals that HVAC-related deliverables were provided later in the design process. This practice is undesirable on multiple levels. Excessive solar gains can unduly increase cooling peak loads leading to larger than expected equipment sizes. Circulation spaces can be maintained at different temperatures and act as buffer spaces to shield regularly occupied areas from the elements. Such savings can only be realized if circulation areas have been properly located during early massing studies and independently adjustable thermal zones have been budgeted for. Finally, air ducts, mechanical rooms, cooling towers and air handling units take up significant space – up to 30% of a building's enclosed volume. Trying to fit all of this equipment after the fact may lead to conflicts with structural elements such as ceiling ducts running into beams. To preempt such spatial clashes, design teams routinely specify a 3ft high ceiling plenum space to accommodate any mechanical, electrical and plumbing (MEP) needs. If these services are instead coordinated so that the plenum height can be halved, an originally eight-story building of fixed absolute height can accommodate an extra floor. The resulting financial reward for Fig 1.3 Sections and comparisons of two HVAC systems
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