HVAC Design for Architects
4 Cover and Book Design Yoshiki Waterhouse Copyright 2025, Building Technology Press, LLC All rights are reserved. No part may be reproduced in any form by any electronic or mechanical means without permission in writing from the publisher. Copyright for all photos are owned by the authors unless otherwise noted in the figure caption.
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6 Table of Contents Basics 10 1 HVAC System Selection 13 Ventilation Type 16 Fuel Source 21 Heat Delivery Medium 22 2 Performance Metrics 27 Systems 30 3 Natural Ventilation 32 Electric And Fossil Fuel Hot Water Radiator with Natural Gas Boiler 34 4 Minimum Outdoor Air 38 All Electric Variable Refrigerant Flow with Air Source Heat Pump 40 Electric and Fossil Fuel Fan Coil Unit with Natural Gas Boiler and Water-Cooled Chiller 44 5 All Air 48 Electric And Fossil Fuel Variable Air Volume with Natural Gas Boiler and Water-Cooled Chiller 50
7 Components 54 Air Handling Unit (AHU) 56 Cooling Tower 58 Fan Coil Unit (FCU) 60 Hot Water Radiator 62 Heat Recovery Ventilator (HRV) 64 Natural Gas Boiler 66 Variable Refrigerant Flow Indoor Unit 68 Variable Refrigerant Flow Outdoor Unit 70 Water-Cooled Chiller 72
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9 Preface Welcome to HVAC Design For Architects, an interactive guidebook for practitioners and students interested in better understanding the environmental, spatial and economic implications of different HVAC systems. The guide is divided into three parts, Basics, Systems and Components. Basics introduces a design decision framework for architects to identify suitable HVAC systems for their projects along with performance metrics to evaluate them. The framework is encoded in the interactive HVAC System Selector form at the bottom right of this screen. The form helps the reader navigate the different HVAC designs provided under Systems. Components reviews the building blocks of these systems such as air handling units, chillers and natural gas boilers. Going forward, we will continue adding additional systems based on reader feedback. The book complements the Climate-Driven Design I textbook, also by Building Technology Press. We have made every effort to ensure that the information presented is accurate and relevant. Should you nevertheless find any errors, typos or have divergent opinions, we welcome your feedback at hvac@BuildingTechnologyPress.com. Ali Irani, Ellen Reinhard and Christoph Reinhart Cambridge, Earth Day 2025 Fig 1.1 Medium pressure gas lines and associated controls for the gas turbine system at the MIT Central Utility Plant (CUP) in Cambridge, MA.
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11 Basics Part one introduces the HVAC System Selector form along with specific performance metrics to weigh the pros and cons of alternative HVAC designs. Fig 1.2 Architects and engineers working together
12 Natural Gas Connection Water-cooled chiller Electrical panel Electricity Natural gas boiler In Out Cooling tower Air handling unit VAV box Electrical panel tricity In Out DOAS Ground source heat pump Floor mounted fan coil unit All air | Electric and fossil fuel | VAV system with Natural gas boiler and water-cooled chiller System comparisons Minimum outdoor air | FCU system with GSHP Energy performance Thermal comfort Acoustic comfort First cost Operating cost Maintenance Space requirements
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
14 % Completion 50% completion Setting energy targets (n= 34 respondents) Developing energy model (n= 32 respondents) Developing MEP design narrative with system options model (n= 32 respondents) Developing space requirements for MEP systems (n= 31 respondents) Designing floor to floor section with ductwork (n= 32 respondents) Designing preliminary duct and system layout (n= 31 respondents) Producing design drawings with full ductwork (n= 29 respondents) Pre-concept design Concept design Schematic design Design development Construction documentation 100 75 50 25 0 100 75 50 25 0 100 75 50 25 0 100 75 50 25 0 100 75 50 25 0 100 75 50 25 0 100 75 50 25 0 Fig 1.4 According to a 2025 survey of North American practitioners, HVAC-related design services are worked on later than other environmental design tasks
15 the owner exceeds any design team's project fee.2 If there is so much value to be captured from effective building design/HVAC integration, what factors prevent this from happening? Low levels of intellectual engagement between architects and HVAC engineers remain a contributing factor.3 Many architects lack an interest in and understanding of the ever-increasing number of HVAC solutions available to them. It does not help that the field indulges in technical jargon as demonstrated by the system comparison at the beginning of this chapter. The resulting communication vacuum can delay HVAC discussions to the last possible moment (Fig 1.4). Relationships take time to build which makes it natural for "new" team members to promote standard, low risk solutions. With first and operational costs being the guiding performance metric for most project decisions, the cheapest solution will prevail unless client and design team agree on either binding performance targets or a project-internal price of carbon to align cost and greenhouse gas emission (GHG) goals.4 In the authors' Sustainable Design Lab at MIT, architects, engineers and programmers work together every day to develop design workflows for high performance buildings and neighborhoods (Fig 1.2). In the same spirit, we wrote this digital guidebook to promote a more informed, earlier dialogue between architects and engineers as well as to introduce design professionals and architecture students to the elusive world of HVAC system design. We came up with a three-step decision framework to identify suitable HVAC system solutions for a given project based on a set of deliberate choices regarding: 1. Ventilation type 2. Fuel sources 3. Heat delivery medium The framework can be used from the earliest design phases onwards and is encoded in the interactive HVAC System Selector form, that is linked on the bottom right of this screen. Based on a series of questions, the form helps the reader to identify HVAC systems that may be suitable for their project. These solutions are described in part two of this book. We will grow the number of systems over time with the intent to ultimately span the gamut of designs commonly found in mid-sized commercial buildings across North America. To accomplish this goal, we hereby solicit your input. Please share photos and descriptions of relevant case study buildings for consideration for this book at hvac@BuildingTechnologyPress.com. For illustrative purposes, each system in part two is applied to the same building, the Medium Office Boston from the Climate-Driven Design I textbook (Fig 1.5).5 The three story building has a conditioned floor area of 4935m2 (53,112ft2) and is mostly based on the U.S. Department of Energy‘s (DOE) commercial prototype building of the same name.6 Detailed
16 assumptions are provided in the appendix of Climate-Driven Design volume I. This booklet serves as a companion document to other Building Technology Press textbooks and includes references to relevant book sections for further information. For example, CDDI-10 stands for ClimateDriven Design I chapter 10. The remainder of this chapter presents the three step HVAC selector framework starting with the type of ventilation system desired. Ventilation Type The choice of HVAC system for a particular building or portion of a building is inextricably linked to its ventilation requirements, i.e. how much fresh, outdoor air per unit time is needed to properly operate all enclosed spaces in the building. Air flow rates are measured in volume per time such as liter per second (l/s) or cubic feet per minute (ft3/min). The American Society of Heating, Refrigerating and Air-Conditioning Engineers' (ASHRAE) standard ASHRAE 62.1 Ventilation and Acceptable Indoor Air Quality specifies ventilation requirements based on program type, peak number of occupants and floor area (CDDII-10).7 For the Medium Office Boston, the required area and person based flow rates are 0.3l/s/m2 and 2.5 l/s/person, respectively. Assuming a peak occupancy density of 0.057 person/m2, the required fresh air flow rate, Qfresh air, is Fig 1.5 Perspective of the Medium Office Boston from the Climate-Driven Design I textbook
17 Qfresh air = (0.3l/s/m2 + 2.5l/s/person × 0.057person/m2) × 4935 m2 = 2.2m3/s Equ 1-1 with 1m3 =1000l. This means that the building roughly requires the content of a British red telephone box per second to enter and leave the building (Fig 1.6). Such copious amounts of air can be delivered by a combination of infiltration, natural and mechanical ventilation. Infiltration happens via accidental cracks in the building envelope and should be reduced as much as possible, especially in new construction. Natural ventilations corresponds to deliberate outside air exchange through opened windows and doors. Mechanical ventilation happens via electric fans that deliver outdoor and exhaust return air, usually via a network of ducts. A natural ventilation concept has various benefits from simplicity and reduced costs to giving occupants agency over their fresh air supply. There are however several factors that determine whether natural ventilation is at all feasible in a building. Starting off, all floor plans have to be narrow enough for the air to reach all indoor spaces. As a rule of thumb, the maximum depth of a natural ventilated buildings is about five times the floor-to-floor height. Second, the outside air needs to be pollutant free all year round, a requirement that may be compromised by recurring nearby wildfires, heavy Supply fresh air Exhaust air 2.2m3/s 2.2m3/s Fig 1.6 The Medium Office Boston requires the equivalent of British telephone booth of air to enter and leave the building every second
18 traffic or industrial activities. Third, if the outside air is of sufficient quality, it also has to be of adequate temperature and relative humidity to maintain indoor comfort. Several early stage design tools have been proposed to help design teams with the final task, i.e. to decide whether natural ventilation is an option in terms of indoor temperature and relative humidity for a given climate, program and façade orientation. Fig 1.7 shows an example output from the ECOmpass tool for an sidelit office in Boston (CDDI-4).8 Each pie slice indicates for a given cardinal direction (north, south, east, west) whether an office can (green) or cannot (gray) be naturally ventilated assuming that occupants may dress lightly (knee length trousers/ short sleeved shirt) to avoid overheating. Yellow indicates a borderline situation. Columns correspond to varying window-to-wall ratios (WWR). Rows represent the combined use of increasingly more passive design strategies from bottom to top starting with daytime natural ventilation and adding external shading, nighttime ventilation and thermal mass construction. The figure demonstrates that a naturally ventilated building in Boston requires careful deployment of shading controls, nighttime flushing and ideally some active thermal mass (CDDI-8). Apart from Boston being a borderline climate for natural ventilation in offices, Fig 1.5 also shows that the Medium Office Boston is significantly deeper than five times its floor to ceiling height 40% 50% 60% 70% 80% 20% 30% XX% N W E S Daytime ventilation Shading Night ventilation Thermal mass No AC required (< 20kh) Might require AC (<1200kh) Requires AC Window-to-wall ratio Fig 1.7 Passive conditioning study for an office in Boston using ECOmpass (Visualization and simulation: Svenja Herb)
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.
20 As shown in Fig 1.8, mechanically ventilated systems move outdoor air via an air handling unit (AHU) from the rooftop into the building and distribute it via a main vertical shaft that splits off into horizontal ducts on each floor. A return air system brings the exhaust air back to the AHU with heat recovery inside. If there is no heat recovery system in place, the pressurized supply air may just be left to leave the building via haphazard exfiltration pathways. The cross-section of the main shaft is dictated by the maximum required air flow rate and the air speed in the duct. The latter is typically capped at 8m/s (1500ft/min) for resistance and noise reasons. Using Equ 1.1 for our Medium Office Boston, the shaft cross-sectional area for a minimum outside air system – including a 15% safety factor – is Aminimum air = (2 × 2.2m3/s ÷ 8m/s) × 1.15 = 0.63m2 Equ 1-2 The factor two accounts for supply and return air shafts. In addition to fresh air, an all air system has to also meet heating and cooling loads stemming from solar gains, envelope losses and internal gains. This means that these systems deliver air that is cooler than the cooling or warmer than the heating set points. Supply air may diverge from heating and cooling set points by up to 25ºF (14K) to avoid occupant discomfort. In other words, for an cooling set point of 75ºF (24ºC) the supply air may be as low as 24ºC − 14ºC = 10ºC or 50ºF. To calculate the air flow rate required to meet peak heating and cooling loads, both quantities need to be known. For the Medium Office Boston in climate zone 4a (CDDI-4), peak heating and cooling loads correspond to 353kW and 247kW, respectively. To meet these loads with an allowable temperature difference of 14K, the required air flow rate for heating, Qheating, is Qheating = Peak heating load ÷ (cp × d × DT) = 353kW ÷ (1006J/kgK × 1.2kg/m3 × 14K) = 20.9m3/s Equ 1-3 where cp and d are the specific heat capacity and density of air, respectively. An equivalent calculation leads to 14.6m3/s to satisfy peak cooling needs. Comparing the air flow rates required for fresh air, cooling and heating reveals that the latter dictates the overall size, i.e. the all air system needs to accommodate a flow rate of 20.9m3/s. Using Equ 1.2, the shaft size of the all air system, Aall-air,is Aall-air = (2 × 20.9m3/s ÷ 8m/s) × 1.15 = 6.0m2 Equ 1-4 The cross section of the all air system is thus an order of magnitude larger than that of its minimum outdoor air counterpart. The HVAC System Selector form provides similar calculations for commercial systems in different ASHRAE climate zones using reference peak loads from the Medium Office Boston and adjusting them by the conditioned floor area
21 provided by the user. As shown in Fig 1.9, ASHRAE climate zones divide the world into regions with similar temperature and annual precipitation profiles (CDDI-3). Based on the foregoing natural ventilation assessment and estimated main duct sizes for minium and all air systems, users of the HVAC selector form have to decide whether their project should indeed be naturally ventilated, hybrid, minimum outdoor air or all air. According to the severity of the climate and/or a load calculation, they also need to indicate whether the building requires active heating, cooling or both. Fuel Source After selecting a ventilation system along with heating and cooling requirements, the HVAC selector form presents with a list of HVAC systems. The next key design decision concerns the fuel sources for the building. Since equipment, electric lighting, fans ducts and air-conditioning are near-universally run on electricity, this choice boils down to the heating fuel. Without carbon sequestering, burning natural gas, oil or propane prevents carbon neutrality which is why there is a push among sustainable minded design teams, clients and government agencies to "electrify all heating." A key underlying assumption is that electric grids across the world 0A, 0B Extremely hot humid/dry 1A, 1B Very hot humid/dry 2A, 2B Hot humid/dry 4A, 4B, 4C Mixed humid/dry/marine 3A, 3B, 3C Warm humid/dry/marine 5A, 5B, 5C Cool humid/dry/marine 6A, 6B, 6C Cold humid/dry/marine 7 Very cold 8 Subarctic/arctic Fig 1.9 World divided by ASHRAE climate zone classification based of TMYx 2004–2018 (Simulation: Sam Letellier-Duchesne)
22 will eventually decarbonize via the proliferation of renewable energy carriers and/or nuclear energy (CDDI-1). The choice to electrify all heating by say switching from a natural gas boiler to a heat pump may initially seem "easy" since the latter has a coefficient of performance of around 3 compared to a natural gas furnace of around 0.95. As discussed in CDDI-1, the 2016 greenhouse gas (GHG) emissions of grid electricity and natural gas in New England were 0.256kgCO2e/kWh and 0.231kgCO2e/kWh, respectively. This means that for each kWh of heating load a building with a heat pump generated 0.256kgCO2e/kWhelectricity ÷ 3.0 = 0.085kgCO2e/kWhthermal load Equ 1-5 compared to 0.243kgCO2/kWhthermal load for the natural gas boiler. This means that the heat pump systems would have reduced carbon emissions by 65%.However, given the cost difference between electricity (US$0.225/ kWh) and natural gas (US$0.043/kWh) at the time, operating the the heat pump would have been 66% higher. To economically break even, the local carbon price would have had to be 178US$/tCO2e (CDDI-1). In the absence of a carbon tax, design teams and their clients need to agree on a project-internal carbon evaluation to avoid GHG emissions and/or install an adequately sized photovoltaic system. This type of analysis is highly context-specific. Up-to-date electricity emission numbers are provided by the Electricity Maps platform.9 By selecting the fossil fuel and electricity or all-electric system option in the HVAC selector form, the number of viable systems is further filtered down. The final choice is concerned with the building's heat delivery medium. Heat Delivery Medium While an all air system meets any space conditioning needs via heated or cooled air, minimum outside air systems only satisfy fresh air requirements. This means that any space heating or cooling loads have to be met separately, for example via a fan coil unit as in Fig 1.10. As shown in the diagrammatic section in Fig 1.11, a fan circulates room air through the unit across a heat exchanger coil. The temperature of the coil is controlled via hot or chilled water that is pumped via two separate piping systems through the building, leading two pairs of supply and return pipes. A fifth pipe collects any condensed water that drips from the heat exchanger coil during the cooling season. For the fan coil unit in Fig 1.10, heat is delivered via water which is why the system is called "hydronic." Part three provides photo/ section pairs as in Fig 1.10 and 1.11 for a variety of HVAC components. Why does the heat delivery medium matter? Given that the volumetric heat capacity of water is 4000 times higher than that of air, water pipes take up
23 Conditioned air Hot water supply Hot water return Chilled water pipes Drain pan Condensate pipe Fan Supply grille Air intake Motor Heat exchanger coils Fig 1.10 Image of a floor mounted fan coil unit Fig 1.11 Diagrammatic section of floor mounted fan coil unit
24 significantly less space than air ducts (CDDI-6). Refrigerant lines can carry even more heat per volume and thus require even less space, a reason why they are becoming popular for retrofit projects. The heat delivery medium choice adds another filter to the HVAC selector form to hone in on a final system choice. Once the above laid out three choices have been made, the reader can compare the remaining system options via the performance criteria discussed in the next chapter. Summary The HVAC selector form, linked at the bottom right of each book spread takes the reader through a series of steps to identify suitable HVAC systems for their projects. The first step is concerned with identifying a suitable ventilation systems for the building such as natural, hybrid, all air or minimum outdoor air. The second step sets the fuel type for the heating system, for example electricity. For non all air systems, the final step requires the reader to select a heat delivery medium such as electricity, water or a refrigerant. References 1 A. Irani and C. Reinhart, 2025, "Bridging the gap – Understanding and Enhancing the HVAC Coordination Process from the Perspective of Architects and Sustainability Analysts through an Online Survey and Guided Interviews," under review 2 Y. Teo, J. Yap, H. An, S. Yu, L. Zhangand K. Cheong, 2022, "Enhancing the MEP Coordination Process with BIM Technology and Management Strategies," Sensors, 22:13 3 J. Landler, 2013, "The architect-MEP Engineer Disconnect," Interdisciplinary Environmental Review, 14(2), 121 4 S. Mollaoglu-Korkmaz,L. Swarup and D. Riley, 2013, "Delivering Sustainable, High-Performance Buildings: Influence of Project Delivery Methods on Integration and Project Outcomes," Journal of Management in Engineering, 29, 71-78 5 C. Reinhart, 2025, Climate-Driven Design I, Building Technology Press, Cambridge, MA, USA. 6 M. Deru et al., 2011, U.S. DOE Commercial Reference Building Models of the National Building Stock, Technical Report, National Renewable Energy Laboratory, NREL/TP-5500-46861, pp. 1–118, February 2011 7 The American Society of Heating, Refrigerating and Air-Conditioning Engineers, 2022, ANSI/ ASHRAE/IES Standard 962.1-2022, Ventilation for Acceptable Indoor Air Quality, Atlanta 8 S. Herb, S. Wolk and C. Reinhart, 2025, “Beyond the Bioclimatic Chart: An Automated Simulation-Based Method for the Assessment of Natural Ventilation and Passive Design Potential,” Building and Environment, 112362 9 Electricity Maps, https://www.electricitymaps.com, last accessed April 2025
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27 2 Performance Metrics This chapter introduces the performance specifications used for the system descriptions in part two including environmental efficiency, first and operating costs as well as ease of maintenance. Each system description consists of two spreads. The first spread introduces the main characteristics of the system including ventilation, heating and cooling sources, fuel type and heat delivery medium. On the second page, a Google Maps based axonometric view shows a case study building with the particular HVAC system installed. URL links in the figure caption take the reader to an interactive 3D viewport of the building. Outside components such as air handling units, cooling towers and supply air ducts are highlighted in the axonometric view and linked to their respective descriptions in part three. A diagrammatic section of the system shows major inside and outside components and their connections. Individual components are again linked to part three. The second spread tabulates additional system specifications along with high-level pros and cons. An interior photo of the case study building reveals how the system is visually experienced by the occupants. Line diagrams below the interior photos further explains the content shown above. Specifications The specifications table lists whether or not the system has a heating and cooling system along with typical coefficient of performance ranges. Humidity control capabilities are listed with the mechanical ventilation strategy, the quantity of fresh air provided and whether or not the system can support hybrid ventilation. The system fuel type is shown as either all-electric or fossil fuel and electricity. The table also lists the climate zones in which this type of HVAC system could be applied as well as regions in which it is currently widely used and for what building programs. Fig 2.1 An MIT Department of Facilities technician inspects supply air fans in an air handling unit on the MIT Campus
Performance Ratings The performance rating table offers readers a high-level system evaluation to facilitate cross comparison according to eight metrics. Each metric is scored on a five-point scale from "++" (best) to "--" (worst) performance along with a brief explanatory note. The eight metrics are described in the following. Energy performance Energy performance indicates of how much electricity and (if applicable) natural gas an HVAC system requires to condition a building relative to the same building outfitted with a different system. Systems with higher efficiency use less energy to operate than those with a lower efficiency. Energy performance is often expressed in terms of energy use intensity (EUI) which is defined as the sum of fuel uses divided by the conditioned floor area (CDDI-2). For example, the variable air volume (VAV) system in Fig 1.3 has a lower energy performance than room fan coil units with a dedicated outdoor air system, due to the larger volume of air that the VAV needs to centrally heat, cool and distribute through the building. Thermal comfort Thermal comfort describes the ability of a system to reliably provide individualized temperature and humidity control. HVAC systems that provide high thermal comfort allow for independent control of zone-level equipment and deliver conditioning and supply air directly to occupants. In Fig 1.3, the fan coil units provide more thermal comfort flexibility than the VAV system since individual fan coil units can be locally adjusted depending on zone conditions. To make up for this lack of control, some VAV systems include additional heating and cooling registers right at the room outlet at the expense of higher system costs and energy use. Acoustic comfort Mechanically ventilated or hydronic HVAC systems generate noise from to moving air and water which may create acoustic discomfort for building occupants. HVAC systems with good acoustic comfort make less noise since they have fewer pieces of equipment with less moving parts or slower moving air. In Fig 1.3, the fan coil units generate more noise than the VAV system since there is a fan located in each zone. The VAV system is not perfect, either as it generates noise from moving potentially large volumes of air at high speeds through the ducts. First cost First cost corresponds to the installation cost of the system. A higher first cost is associated with more pieces of equipment or the use of unconventional systems. In Fig 1.3, the fan coil unit system costs more than the VAV since it effectively requires two HVAC systems, the dedicated
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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31 Systems This part is divided into three chapters, dedicated to naturally ventilated, minimum outdoor air and all air systems. Each system is described via two spreads consisting of photographs, diagrams and system specifications. Fig 2.2 Rooftop air-cooled chillers and air-handling units on the roof of Fenway Park, in Boston, MA.
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33 3 Natural Ventilation This chapter describes HVAC systems that operate with natural ventilation via operable windows. These systems additionally need equipment for space conditioning, such as an fan coil unit, hot water radiator, or minisplit unit. Naturally ventilated systems have smaller spatial footprints than mechanically ventilated counterparts. Fig 3.1 Operable window with perimeter hot water radiator at MIT Campus.
34 Electric and Fossil Fuel Hot Water Radiator with Natural Gas Boiler This system is very common in European residential and commercial buildings that do not require air conditioning. Fresh air and cooling needs are met via operable windows. Hydronic radiators circulate hot water provided by a natural gas boiler in the basement. Circulation through the radiators is usually controlled via local valves, allowing for manual, room-by-room heating control. Flue gases are exhausted through the roof.
35 Boiler exhaust pipes Fig 3.3 Section through a typical medium sized commercial building with hot water radiator and natural gas boiler. The boiler exhaust flue is also shown. Fig 3.2 Axonometric view of a hotel building with a hot water radiators with operable windows in Olten, Switzerland (open in Google Maps)
36 Table 3.1 Specifications Condition Status Allocation Heating System Yes COP = 0.8 – 0.95 Cooling System No - Humidity Control No - Ventilation Control No Operable windows Fresh Outside Air Yes Operable windows Hybrid Ventilation No Only natural ventilation for spaces Fuel Type Yes Natural gas and electricity Climate Zones Multiple 3-8 Typical Regions Multiple Europe Applicable Building Programs Multiple Offices, schools, residential Table 3.2 System Performance Rating Metric Rating Comments Energy performance Water-based systems have higher energy performance Thermal comfort Limited thermal control, no active cooling Acoustic comfort Hot water radiator does not have any fans so noise is minimal First cost Fewer pieces of equipment in each space Operating cost High fossil-fuel prices penalize system Maintenance Limited maintenance requirements Space requirements Small area requirements for pipes, some large equipment for heating
37 Operable window Hot water radiator In Out Fig 3.5 Representative line diagram of a room with a hot water perimeter radiator and operable windows, showing supply of hot water. Fig 3.4 Interior view of room with hot water perimeter radiators and operable windows.
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39 4 Minimum Outdoor Air Systems This chapter presents different minimum outdoor air systems. Minimum outdoor air systems are HVAC systems that provide only ventilation air to the building and rely on additional equipment for space conditioning. Fig 4.1 Minimum outdoor air system with fan coil units in the MIT Campus.
40 All Electric Variant Refrigerant Flow with Air Source Heat Pump This all electric system is gaining traction an many new buildings, especially in temperate climates such as Washington State, where heat pumps operate at high coefficients of performance throughout the year. According to a 2022 survey by AMCA International and Applied Marketing Knowledge, VRF units were installed in 14% of new, non-single family home construction projects in the United States, a number that is expected to continue to grow.1 Fresh air requirements for this dedicated outdoor air system are met through the rooftop air handling unit (AHU) in Fig 4.2. Fresh air enters the building through the AHU, is distributed via vertical shafts and horizontal ducts and enters individual spaces through supply grilles (Fig 4.4.) The outside air is conditioned through electric resistance heating and direct expansion (DX) cooling coils. The AHU system can accommodate heat recovery. Heating and cooling loads are met through a series of roof-mounted variable refrigerant flow (VRF) outdoor units and delivered through refrigerant lines to the VRF indoor units in each space (Fig 4.4). Indoor air is circulated through the units to provide heating and cooling. Any condensate is collected within the units and pumped out. Multiple indoor units can be served by one outdoor unit with some modern systems allowing for simultaneous heating and cooling. References 1 Applied Marketing Knowledge, 2022, HVAC Terminal System Selections: A Real-World Project Data Survey, AMCA International
41 VRF Outdoor Units Air handling unit (AHU) for DOAS Fig 4.3 Section through a typical medium sized commercial building with a ground level VRF system and a Dedicated Outdoor Air System (DOAS) Fig 4.2 Axonometric view of an office building with a rooftop Variable Refrigerant Flow (VRF) system in Seattle, Washington (open in Google Maps)
42 Table 4.1 Specifications Condition Status Allocation Heating System Yes COP = 2.5 – 3.0 Cooling System Yes COP = 2.5 – 3.5 Humidity Control Yes Dehumidification and humidification Ventilation Control Yes Mechanical ventilation with exhaust Fresh Outside Air Yes Min. outside air only Hybrid Ventilation Possible - Utility Connections Yes Electricity Climate Zones Multiple 1-6 Typical Regions Multiple Asia Applicable Building Programs Multiple Offices, schools Table 4.2 System Performance Rating Metric Rating Comments Energy performance Systems with DOAS have high efficiency Thermal comfort High degree of control over thermal comfort Acoustic comfort More pieces of equipment in zone making noise First cost More pieces of equipment to install Operating cost High energy efficiency decreases utility costs Maintenance Generally higher maintenance requirements Space requirements Small area requirements for air shafts and equipment
43 Refrigerant supply Refrigerant return Outside air VRF indoor unit Space conditioning Fig 4.5 Representative line diagram showing distribution and supply of air within the space, return duct not shown Fig 4.4 Interior view of office space with exposed ceilings showing VRF indoor units and outdoor air supply grilles (Image courtesy of Cody Lodi, Weber Thompson )
44 Fan Coil Unit with Natural Gas Boiler and Water-Cooled Chiller In the United States, this is currently the most common dedicated outdoor air system. According to a 2022 survey by AMCA International and Applied Marketing Knowledge, Fan Coil Units are installed in 23% of new, nonsingle family home construction projects in the United States.1 For this system, fresh air requirements are met through the rooftop air handling units (AHU) in Fig 4.6. Fresh air enters the building through the AHU, is distributed via vertical shafts and horizontal ducts and enters individual spaces through supply grilles (Fig 4.8.) The outside air is conditioned through electric resistance heating and direct expansion (DX) cooling coils. The AHU system can accommodate heat recovery. Heating and cooling loads are met through a natural gas boiler and watercooled chiller that are both located in the basement (Fig 4.7). Heat from the chiller is transported by water pipes to the rooftop and rejected via cooling towers to the ambient air. Hot and chilled water loops connect the boiler and chiller to fan coil units (FCU) in each room. A thermostat-controlled fan recirculates indoor air through the FCU to adjust local temperature conditions. References 1 Applied Marketing Knowledge, 2022, HVAC Terminal System Selections: A Real-World Project Data Survey, AMCA International
45 Cooling towers Air handling unit for DOAS Air handling unit for DOAS Fig 4.7 Section through a typical medium sized commercial building with an FCU system with Dedicated Outside Air. Fig 4.6 Axonometric view of an office building with an FCU and DOAS system in the Kendall Square neighborhood of Cambridge, Massachusetts (open in Google Maps)
46 Table 4.3 Specifications Condition Status Allocation Heating System Yes COP = 0.8 – 0.95 Cooling System Yes COP = 4.0 – 6.0 Humidity Control Yes Dehumidification and humidification Ventilation Control Yes Mechanical ventilation with exhaust Fresh Outside Air Yes Min. outside air only Hybrid Ventilation Possible - Fuel Type Yes Natural gas and electricity Climate Zones Multiple 1-8 Typical Regions Multiple North America and Asia Applicable Building Programs Multiple Offices, retail, schools Table 4.4 System Performance Rating Metric Rating Comments Energy performance Systems with DOAS have high efficiency Thermal comfort Moderate degree of control over thermal comfort Acoustic comfort More pieces of equipment in zone making noise First cost More pieces of equipment to install Operating cost High energy efficiency decreases utility costs Maintenance Generally higher maintenance requirements Space requirements Small area requirements for pipes, some large equipment for heating and cooling
47 Space conditioning 4-Pipe fan coil unit Outside air Condensate line Hot water supply Hot water return Chilled water return Chilled water supply Fig 4.9 Representative line diagram showing distribution and supply of air within the space, return duct not shown Fig 4.8 Interior view of MIT Department of Architecture studio space with exposed ceilings showing FCU with outside air ducts
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49 5 All Air Systems This chapter presents different types of all air systems. All air systems combine fresh air supply and space conditioning via a single air duct system. Fig 5.1 All-air system used in the MIT Stata Center. Here air is supplied from an Underfloor Air Distribution (UFAD) system with a return visible above the door
50 Electric and Fossil Fuel Variable Air Volume with Natural Gas Boiler and Water-Cooled Chiller This system ranks among the most common HVAC configuration in U.S. commercial buildings. According to a 2022 survey by AMCA International and Applied Marketing Knowledge, variable air volume (VAV) systems could be found in 32% of new, non-single family home construction projects in the United States.1 Its popularity stems from low initial costs and easy of maintenance as a single entry point provides heating, cooling, fresh air and humidity control to the building. As shown in Fig 5.1, an large air handling unit (AHU) delivers conditioned air to this classroom building. The supply air is distributed via a system of shafts and ducts and enters the classrooms through ceiling integrated diffusers(Fig 5.4). Return air is then moved back to the rooftop AHU where a heat recovery unit transmits heat to or from the incoming fresh air during heating or cooling season, respectively. Heating and cooling is provided via two water loops that connect the AHU to a natural gas boiler and watercooled chiller. Boiler and chiller are usually located in a maintenance room in the basement. To reject heat from the chiller, an additional hydronic loop transports warm water to the roof and transmits it via cooling towers to the ambient air (Fig 5.2). In select cooling dominated buildings, cold supply air delivers to all spaces and heating registers in each zone reheat the cold air to maintain a comfortable indoor temperature. This practice is predictably wasteful. References 1 Applied Marketing Knowledge, 2022, HVAC Terminal System Selections: A Real-World Project Data Survey, AMCA International
51 Cooling tower units Air handling unit (AHU) Supply air Fig 5.2 Axonometric view of an office building with a Variable Air Volume (VAV) system at the MIT campus in Cambridge, Massachusetts (open in Google Maps) Fig 5.3 Section through a typical medium sized commercial building with a VAV system with hot water reheat
52 Table 5.1 Specifications Condition Status Allocation Heating System Yes COP = 0.8 – 0.95 Cooling System Yes COP = 4.0 – 6.0 Humidity Control Yes Dehumidification and humidification Ventilation Control Yes Mechanical ventilation with recirculation Fresh Outside Air Yes Min. outside air included in supply air Hybrid Ventilation Possible - Utility Connections Yes Natural gas and electricity Climate Zones Multiple 1-8 Typical Regions Multiple North America and Asia Applicable Building Programs Multiple Offices, retail, schools, large assembly spaces Table 5.2 System Performance Rating Metric Rating Comments Energy performance All-air systems have lower efficiency Thermal comfort Limited occupant controls on comfort Acoustic comfort Fewer pieces of equipment in zones making noise First cost Common system, fewer piecs of equipment Operating cost Low energy efficiency increases utility costs Maintenance Reliable system with less maintenance in zones Space requirements Large area requirements for air shafts and equipment
53 Supply air Return air Fig 5.5 Representative line diagram showing distribution and supply of air within the space Fig 5.4 Interior view of classroom space with finished ceilings showing VAV supply air grille and return air grille
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55 Components This part presents additioal infomation on common HVAC components that were referenced in previous sections. A brief description is provided for each component along with an image and illustrative diagram. Fig 6.1 Water-cooled chiller and pipework in the basement of an MIT Campus building in Cambridge, MA.
56 Return air duct Cooling coil Heating coil Outside air intake Supply air Drain pan Fig 6.2 Image of an air handling unit on a building roof at MIT Fig 6.3 Diagrammatic section of the air handling unit for Fig 6.2
57 Air Handling Unit (AHU) An air handling unit provides conditioned supply air to building spaces. The equipment can either deliver the minimum fresh air requirement for a dedicated outdoor air system (DOAS) or meet all fresh air and conditioning needs of a building (all-air or VAV). An air handling unit passes fresh and/ or recirculated air through air filters, heating coils, cooling coils, and heat recovery before delivering the air to all zones. Table 6.1 Specifications Fuel Source Electricity Produces Conditioned air or minimum fresh air Requires Hot and chilled water connection Back to: VAV System with Hot Water Reheat FCU with Dedicated Outside Air VRF System with Dedicated Outside Air
58 Dry air in Cold water out Fan unit Warm air out spray nozzles Water distribution system Hot water inlet water collection basin PVC film fill Fig 6.4 Rooftop cooling tower (image from the Creative Commons Attribution-Share Alike 4.0 International license, original author Aloofmanish, English Wikipedia) Fig 6.5 Diagrammatic section of the cooling tower from Fig 6.4
59 Cooling Tower A cooling tower is an outdoor piece of equipment that rejects heat from an indoor chiller to the environment. Incoming warm condenser water from the chiller passes through the cooling tower, in which fans and sprays lower the water temperature via evaporative cooling. The cooled condenser water is returned to the chiller to repeat the cycle. Note that a sizable amount of water is evaporated and lost during the process. Table 6.2 Specifications Fuel Source Electricity Produces Condenser water Efficiency COP = 0.7 – 0.75 Back to: VAV System with Hot Water Reheat FCU with Dedicated Outside Air
60 Drain pan Fan Filter Motor Air intake Conditioned air Heat exchanger coils Hot water supply Hot water return Supply grille Fig 6.6 Image of a ceiling mounted fan coil unit Fig 6.7 Diagrammatic section of the ceiling mounted fan coil unit from Fig 6.6
61 Back to: FCU with Dedicated Outside Air Fan Coil Unit A fan coil unit provides conditioned the air within a space by circulating it over heated or chilled water coils to achieve a desired local thermostat set point. Circulation happens via a fan within the unit. Hydronic FCUs have either 4-pipes if separate heating and cooling loops are available or 2-pipes if either heating or cooling can be provided. In both cases an additional condensate line and drain are needed. Table 6.3 Specifications Fuel Source Electricity Produces Conditioned air Requires Hot and chilled water connection
62 Hot water radiator Operable window Fig 6.8 Image of a hot water radiator in a residential building Fig 6.9 Diagrammatic drawing of hot water radiator from Fig 6.8
63 Hot Water Radiator A hot water radiator provides space heating via radiation and local convection. Hot water is circulated through the radiator, with the geometry of the radiator designed to maximize the surface area for heat transfer. Hot water radiators have a 2-pipe connection, with a supply and return pipe for hot water. Most radiators come with a valve to control the amount of heating provided to each space. Table 6.4 Specifications Fuel Source Natural gas (hot water produced by boiler) Produces Heat Efficiency COP = 0.8 – 0.95 Back to: Hot Water Radiators with Operable Windows
64 Supply fan Steelhousing Exhaust fan Exhaust air Supply air Filter Stale air from inside Outside air Heat exchanger Filter Fig 6.10 Image of an heat recovery ventilator, with cover open Fig 6.11 Diagrammatic section of the heat recovery ventilator from Fig 6.10 Photo: Zach Berzolla
65 Heat Recovery Ventilator (HRV) A heat recovery ventilator (HRV) is a type of air-to-air heat exchanger that transfers heat between incoming and outgoing streams of air. A HRV dramatically reduces heat losses from mechanical ventilation with efficiencies ranging from 70% to 85% . An energy recovery ventilator (ERV) is a specific type of HRV that exchanges heat and moisture between air streams (sensible and latent portion). The energy or heat recovery may occur at the space level (unit ventilators) or at building level in an air handling unit using enthalpy wheel or heat exchanger. Table 6.5 Specifications Fuel Source Electricity Produces Conditioned air Requires Intake and exhaust to outside Back to:
66 Safety relief valve Water backed furnace Hot water return Fully automated force draft burner Hinged access doors Exhaust air Tubes Hot water supply Fig 6.12 Commercial hot water boiler (image from the Creative Commons Attribution-Share Alike 3.0 Unported license, original author Dunnd74, English Wikipedia) Fig 6.13 Diagrammatic section of the hot water boiler from Fig 6.12
67 Natural Gas Boiler A hot water boiler is a widely used piece of equipment that produces hot water. A natural gas hot water boiler combusts natural gas in an integrated fire box, generating heat, which is then transferred to circulating water via a heat exchanger. The efficiency of the boiler can be increased by using heat from condensate to preheat water. Table 6.6 Specifications Fuel Source Natural gas Produces Hot water Efficiency COP = 0.8 – 0.95 Back to: VAV System with Hot Water Reheat FCU with Dedicated Outside Air Hot Water Radiators with Operable Windows
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