HVAC fundamentals: heat, air and moisture

Heating, ventilation and air conditioning systems keep people comfortable and healthy, protect buildings from moisture damage and keep equipment within safe operating ranges. They are also the largest energy users in most buildings outside of process loads. Before looking at the equipment in lesson 5, this lesson covers the physics that drives HVAC design: how heat moves, how loads are estimated, what ventilation is for, and how air carries moisture as well as heat.

You will not size systems in this course, but a constructor who understands these fundamentals can follow a load calculation, read equipment schedules intelligently, see why an envelope change or added occupancy matters to the mechanical engineer, and recognize problems such as condensation or under-ventilated spaces before they become claims.

Heat and how it moves

Heat flows from warm to cold by three mechanisms. Conduction moves heat through solids, such as a wall or roof. Convection moves heat with a moving fluid, such as air leaking through gaps or blowing across a coil. Radiation moves heat as infrared energy between surfaces, such as sun through glass or a cold window chilling a person nearby. HVAC loads are the sum of these flows plus the heat that people, lights and equipment release inside.

US practice measures heat flow in British thermal units per hour (Btu/h), thermal resistance as R in h·ft²·°F/Btu and transmittance as U in Btu/h·ft²·°F. Cooling equipment is rated in tons of refrigeration (1 ton = 12,000 Btu/h, about 3.517 kW).

Heating and cooling loads

A heating load calculation estimates the heat a building loses on a cold design day; a cooling load calculation estimates the heat it must reject on a hot design day. Heating loads are dominated by conduction through the envelope, air leakage and outdoor air brought in for ventilation. Cooling loads add solar gain through windows, heat from people, lighting and equipment, and latent loads from moisture.

Design temperatures for US locations come from the ASHRAE Handbook of Fundamentals climatic data, such as the 99.6 or 99 percent heating design temperature and the 0.4 or 1 percent cooling design conditions; houses are often sized with ACCA Manual J.

Engineers use software that models each space hour by hour. The construction manager rarely sees the detail, but does see the results in equipment schedules, and needs to know that changes to glazing, insulation, occupancy, lighting or equipment can change those results. A tenant who adds a server room after design is a cooling load problem, not just an electrical one.

Sensible and latent heat

Sensible heat changes air temperature and can be read on a thermometer. Latent heat is the energy involved in adding or removing moisture, which does not show on a thermometer but matters greatly for comfort and for cooling equipment, because condensing water vapor on a cooling coil takes a lot of energy. The ratio of sensible to total cooling is the sensible heat ratio; humid climates and crowded rooms have more latent load.

For airflow calculations, a simplified formula links sensible heat to air volume and temperature difference.

In US units, sensible heat in Btu/h is approximately 1.08 × airflow in cubic feet per minute × temperature difference in °F, based on standard air.

Ventilation

Ventilation brings in outdoor air to dilute pollutants from people, materials and activities: carbon dioxide, odors, volatile organic compounds and airborne pathogens. It is different from air circulation, which only moves indoor air around. Too little ventilation causes stuffiness and health complaints; too much wastes energy, especially in cold climates.

The International Mechanical Code sets ventilation rates in its chapter 4, closely based on ANSI/ASHRAE 62.1, which many jurisdictions also reference directly; dwellings follow ASHRAE 62.2 or the IRC.

ASHRAE 62.1's Ventilation Rate Procedure adds a rate per person to a rate per unit floor area. For office space the breathing-zone rates are 2.5 L/s per person plus 0.3 L/s per square meter (5 cfm per person plus 0.06 cfm per square foot), before adjustments for air distribution effectiveness and system efficiency.

Air changes per hour (ACH) express airflow relative to room volume. They are used for labs, health care and some industrial spaces, and for checking that a design is reasonable.

Air and moisture: psychrometrics in brief

Psychrometrics is the study of moist air. Its key properties are dry-bulb temperature (the ordinary temperature), wet-bulb temperature (measured with a wetted sensor, showing evaporative cooling potential), relative humidity (how close the air is to saturation) and dew point (the temperature at which water vapor starts to condense). A psychrometric chart plots these relationships, and HVAC processes such as heating, cooling, humidifying and dehumidifying appear as lines on it.

Two consequences matter on every project. First, warm, humid indoor air that reaches a surface colder than its dew point will condense, which is how windows sweat, walls grow mold and cold pipes drip. Insulation and vapor control on chilled water pipes and ducts, and airtight, well-detailed envelopes, prevent this. Second, cold winter air holds little moisture, so heating it lowers relative humidity and many buildings add humidifiers, which in turn raise condensation risk at windows.

Comfort and indoor environmental quality

Thermal comfort depends on air temperature, radiant temperature, humidity, air speed, clothing and activity, as set out in ASHRAE Standard 55, which designers in both countries use. A space can meet its thermostat setting and still feel uncomfortable if a large cold window radiates at occupants or a diffuser blows directly on a desk. Noise from HVAC equipment, rated by noise criteria (NC) curves, and filtration levels, rated by MERV, are also part of indoor environmental quality and part of what gets tested at handover.

Energy codes and HVAC

Energy codes shape HVAC design. The IECC 2024 and ASHRAE 90.1-2022 set equipment efficiencies, economizer and energy recovery requirements, fan power limits, controls and envelope performance, and some states and cities go further with stretch codes or building performance standards.

For the constructor, energy code requirements show up as specific equipment efficiency ratings in submittals, as controls sequences to be implemented and tested, and as envelope airtightness targets that may be verified by blower door testing. Substituting a cheaper unit with a lower efficiency rating can make the building non-compliant.

Loads, equipment size and the construction manager

Load calculations belong to the mechanical engineer, but the construction manager lives with their consequences. Oversized equipment costs more, cycles on and off, controls humidity poorly and wastes energy; undersized equipment leaves spaces uncomfortable on design days. Changes during construction, such as a tenant adding a server room, more glass in the curtain wall or higher lighting loads, can change the loads and should be sent back to the engineer rather than absorbed quietly.

Envelope quality also affects loads. Air leakage, missing insulation and thermal bridges add heating and cooling demand that the design assumed away. Airtightness testing of the envelope, now required or encouraged by several energy codes and green building programs, gives early warning that the HVAC system will struggle.