Thermal Systems and HVAC Engineering

01Purpose and scope

• This manual supports the course Thermal Systems and HVAC Engineering. It describes how heating, ventilation, and air conditioning work is planned, designed, installed, verified, and operated in practice, and it is meant to be used as a working reference rather than a textbook.
• The scope covers building comfort systems, outdoor air ventilation, central plants with boilers and chillers, air distribution, and the thermal processes that connect them. It also touches on process cooling and heat recovery where those systems share equipment or controls with comfort HVAC. Data centers, clean rooms, and industrial refrigeration are only mentioned where their design logic differs in a useful way.
• Practice varies by building type, climate, owner requirements, and local code. A small office retrofit, a hospital operating suite, and a cold storage warehouse each use the same physics but reach very different decisions about redundancy, humidity control, and energy use. This manual points out where those differences matter and where the reader should consult the project specification or the authority having jurisdiction.

02Core principles

• Every HVAC system moves energy and air to keep a space within a target range of temperature, humidity, and air quality. The engineer starts by identifying what heat gains and losses the space experiences, then selects equipment and distribution that can meet those loads under realistic conditions rather than only at a single peak moment.
• Load calculation is the foundation. Sensible and latent loads come from envelope conductance, solar gains through glazing, infiltration, occupants, lighting, plug loads, and outdoor air ventilation. Peak cooling loads often occur in the late afternoon, not at noon, because of thermal mass and glazing orientation. Heating loads are usually driven by design outdoor temperatures from recognized weather data sets rather than by a comfortable average day.
• Air and water are the two main transport media. Air systems depend on fan power, duct sizing, pressure drop, and leakage. Water systems depend on pump head, pipe diameter, flow balance, and temperature difference across coils. Poor balance in either network causes comfort complaints, wasted energy, and equipment stress, and these problems often look like equipment faults when the real cause is distribution.
• Control strategy ties the whole system together. Sequences of operation determine when economizers open, when pumps stage, and how setpoints reset. A well-designed plant can still perform poorly if its sequences are vague or the sensors that feed them are miscalibrated.

03Planning and preparation

• Begin with the owner's project requirements and the basis of design. These documents state the occupancy, operating hours, indoor setpoints, humidity limits, redundancy expectations, and budget constraints. If they are missing, write them down with the owner before any sizing work starts, because later changes to these assumptions are expensive.
• Gather the building information that drives the load model: architectural drawings, wall and roof assemblies, window schedules, occupancy schedules, lighting and equipment power densities, and any existing mechanical records for retrofits. Site visits matter, especially for existing buildings, where the as-built condition often differs from drawings. Measure ceiling plenum space, mechanical room access, roof structure capacity, and utility service size.
• Select the applicable codes and standards early. In the United States, many jurisdictions adopt the International Mechanical Code or a state mechanical code, and energy requirements often follow a model energy code or a state adoption of ASHRAE Standard 90.1. Ventilation rates are commonly based on ASHRAE Standard 62.1. Adoption status and edition differ by location, so confirm the current requirements with the authority having jurisdiction before finalizing the design.

04Recommended workflow

1. 1
   : Confirm the scope, occupancy, schedules, and setpoints with the owner in writing. Record any assumptions that are not confirmed so they can be revisited during review.
2. 2
   : Build a zone-level load model using the building envelope, internal gains, and outdoor air requirements. Calculate heating and cooling loads for each zone and for the building as a whole, and check that the results make sense against known benchmarks for similar buildings in your own records.
3. 3
   : Select the primary system type, such as variable air volume with reheat, dedicated outdoor air with fan coils, variable refrigerant flow, or a water-source heat pump loop. The choice depends on zoning needs, floor plate shape, available mechanical space, and the owner's maintenance capability. Document why the alternatives were rejected.
4. 4
   : Size equipment and distribution. Select air handlers, chillers, boilers, pumps, and terminal units from manufacturer data at the design conditions, then size ducts and pipes to keep velocities and pressure drops within normal limits for the system type. Allow for reasonable margins without oversizing, since oversized equipment cycles poorly and wastes energy.
5. 5
   : Develop the control sequences and points list. Write sequences in plain language that a controls technician can program, define alarm limits, and specify sensor accuracy and location. Check that each sequence has a clear owner, such as the building automation vendor or the mechanical contractor.
6. 6
   : Review the design against the basis of design, codes, and constructability. Coordinate with structural, electrical, architectural, and plumbing disciplines on shafts, clearances, power, and roof loading. Resolve clashes before the drawings are issued for construction.
7. 7
   : Plan installation verification and commissioning. Define the tests for airflow, water flow, temperature, controls, and safety devices, and specify who performs them and how results are recorded. Schedule seasonal testing where the design depends on heating or cooling conditions that cannot be simulated on site at completion.

05Quality checks, risk controls and compliance

• Quality checks happen at several points: design review, submittal review, pre-installation inspection, start-up, functional performance testing, and post-occupancy review. At each point, compare the installed or proposed condition against the basis of design. Measured airflow should fall within a stated tolerance of design airflow, and water flows should be balanced to the values on the test and balance report. Stated tolerances should come from the specification or recognized commissioning guidance, not from habit.
• Risk controls focus on refrigerants, combustion, electrical hazards, moving equipment, and indoor air quality. Refrigerant handling in the United States is governed by EPA Section 608 regulations, and technicians who open refrigerant circuits need proper certification. Gas-fired equipment requires combustion air, venting, and carbon monoxide considerations under the applicable fuel gas and mechanical codes. Outdoor air intakes, filtration, and humidity control affect occupant health, and design decisions here should follow the ventilation standard in force for the building.
• This guidance does not replace qualified professional judgment, approved procedures, or required authorization. Licensed engineers, certified technicians, and the authority having jurisdiction hold the final responsibility for life safety, code compliance, and permits. Verify the current editions of all codes and standards with the adopting authority, since requirements change and local amendments are common.

06Collaboration and documentation

• HVAC work involves owners, architects, structural and electrical engineers, controls vendors, contractors, test and balance agents, and facility staff. Each group needs a clear set of documents and a named contact. Define responsibilities in the specification and the design narrative so that gaps, such as who supplies the control valves or who sets the variable frequency drive parameters, are settled before installation.
• The core deliverables are the load calculation package, equipment schedules, drawings showing ductwork, piping, and controls, the sequences of operation, the points list, and the basis of design narrative. Keep calculation files with their assumptions and input values so a reviewer can trace each number. Use version control for drawings and revision logs so that field changes are captured and carried back into the record drawings.
• Commissioning documentation includes the test plan, completed test forms, the test and balance report, controls checkout sheets, training records, and an operations and maintenance manual. Facility staff should receive a written schedule for filter changes, belt inspection, coil cleaning, and refrigerant leak checks. These records are what allow the next engineer or operator to understand the system without guessing.

07Measures and continuous improvement

• Track measurable indicators both during commissioning and after occupancy. Useful measures include supply air temperature and pressure against setpoint, space temperature and humidity within the comfort band, fan and pump energy per unit of conditioned area, chiller plant efficiency in kilowatts per ton under typical load, filter pressure drop trends, and the number of occupant comfort complaints per season. Choose measures that the building can actually collect, since trend logs that nobody reviews do not help.
• Compare energy and comfort results against the design intent and against the building's own history. A plant that uses more energy than the model predicted may have a control sequence that overrides economizer operation, a schedule that runs fans overnight, or a sensor that reads high. Investigate before assuming that the model was wrong, and record the cause and fix.
• Continuous improvement comes from scheduled review. Retro-commissioning, periodic controls audits, and operator feedback sessions can recover performance that drifts over time. Keep changes small, test them one at a time, and record the before and after values so the improvement can be defended.

08Common mistakes and how to prevent them

• A frequent mistake is relying on a single peak-day load without checking part-load behavior. Systems sized only for peak conditions often run at very low load for most of the year, which causes short cycling and poor humidity control. Prevent this by reviewing part-load performance, using variable speed drives where appropriate, and checking minimum turndown of the equipment.
• Another common error is using outdoor air quantities copied from a previous project without checking the current occupancy and ventilation standard. Outdoor air rates that are too low can leave spaces stuffy or with elevated carbon dioxide, while rates that are too high waste heating and cooling energy. Calculate the ventilation requirement from the actual occupancy and area for each space.
• Many field problems come from skipping water and air balancing or from accepting an unverified sensor reading. Require measured balancing reports, check sensor calibration against a reference instrument, and confirm control valve stroke and actuator direction during checkout. Small errors at this stage are much cheaper to fix before turnover than after a full season of complaints.

09Practical checklist

• Confirm the owner's project requirements, occupancy schedule, and indoor setpoints in writing.
• Identify the applicable mechanical code, energy code, and ventilation standard with the authority having jurisdiction.
• Build a zone-level heating and cooling load model with documented assumptions.
• Check part-load performance and minimum turndown for the selected equipment.
• Verify outdoor air quantities against the current occupancy and ventilation requirements.
• Coordinate ductwork, piping, electrical, and structural clearances before issuing drawings.
• Write sequences of operation and a points list that a controls technician can program without interpretation.
• Require test and balance reports, controls checkout records, and an operations and maintenance manual before turnover.

★Learning sources

• Micro Degree: Thermal Systems and HVAC Engineering
• Micro Degree: Data Analytics Applications
• Micro Degree: Azure Cloud Fundamentals
• Micro Degree: Google Cloud Platform Basics