HVAC Basics and Thermal Comfort
What HVAC does, the comfort zone, how a building is conditioned, the main system types and the numbers to remember.
What HVAC does
HVAC stands for heating, ventilation and air conditioning. Its job is to keep the people, the equipment or the product in a space within a range of temperature, humidity and air quality, using as little energy as possible. It does this with four kinds of equipment: sources of heat and cold (boilers, chillers, heat pumps), distribution (ducts and pipes, fans and pumps), terminals (diffusers, radiators, fan coil units) and controls that match output to the load. The rest of this guide takes each in turn: the properties of moist air (Moist Air and the Psychrometric Chart), what happens to air in a coil or a humidifier (Psychrometric Processes), how large the loads are (Heating and Cooling Load Calculations), and the equipment that meets them.
Thermal comfort
People feel comfortable when the heat they produce can leave the body at the rate they make it. A seated adult makes about 105 W (1 met, about 58 W per square meter of skin), roughly 75 W as dry heat and 55 W as moisture in a typical office, and loses it by convection, radiation and evaporation. Comfort depends on six things:
- Air temperature and mean radiant temperature (cold windows and walls matter as much as the air). Their average is the operative temperature.
- Air speed: moving air cools; drafts above about 0.2 m/s are noticed at neck and ankles.
- Humidity: matters less than temperature in the middle range, but 30 to 60 percent is the target. Below about 30 percent the air feels dry and static builds up; above 60 percent it feels clammy and mold grows.
- Clothing (clo; 0.5 in summer, 1.0 in winter) and activity (met).
| Season | Operative temperature (typical office) | Relative humidity | Air speed |
|---|---|---|---|
| Winter (1.0 clo) | 20 to 24 °C | 30 to 50 % | below 0.15 m/s |
| Summer (0.5 clo) | 23 to 26 °C | 40 to 60 % | below 0.25 m/s |
ASHRAE Standard 55 and ISO 7730 give the methods (the predicted mean vote and percentage dissatisfied) and the official comfort zones. A vertical air temperature difference over 3 K between head and ankle, or a radiant asymmetry over 5 K across a cold window, causes complaints even when the average is perfect.
The heat balance of a room
A room gains heat from the sun, people, lights and equipment and from warm outdoor air, and loses it through the envelope and by ventilation. HVAC adds or removes the difference. Loads are split into two kinds, because they need different treatment:
- Sensible heat changes the air temperature. Air carries it at ṁ cp ΔT, about 1.2 kJ per cubic meter per kelvin (1.2 × 1.006 kJ/m³·K), so a liter per second at 1 K carries 1.2 W.
- Latent heat is the energy in water vapor that changes the humidity: ṁ ΔW hfg, with hfg about 2 501 kJ/kg. A person adds 55 W of latent load; a hot shower adds a great deal more.
Total load = sensible + latent, and the sensible heat ratio SHR = sensible ÷ total tells you how the supply air must be treated: an SHR near 0.7 means a cooling coil has to dehumidify as well as cool (see Psychrometric Processes).
System types
| System | How it works | Typical use | Notes |
|---|---|---|---|
| Constant air volume (CAV) | one air handler, constant flow, temperature varies | small buildings, labs, hospitals | simple; wastes fan energy at part load; reheat for zone control |
| Variable air volume (VAV) | constant supply temperature, flow varies by zone | offices, commercial | saves fan energy; needs minimum flow for ventilation |
| Fan coil unit with chilled and hot water | water brings heat or cold to a coil in each room | hotels, apartments, offices | little duct space; condensate drains and filters need service |
| Radiant heating and cooling | warm or cool water in floors, ceilings or panels | quiet or high-end spaces | comfort at lower air temperature; needs ventilation and dehumidification |
| Split and VRF heat pump | refrigerant to indoor units; variable-speed compressors | small offices, houses, retrofit | efficient; refrigerant pipe length and safety limits apply |
| Packaged rooftop unit | everything in one box: coil, fan, compressor, gas heat | shops, schools, warehouses | cheap and common; limited efficiency and life |
| Dedicated outdoor air system (DOAS) | treats the ventilation air alone, with heat recovery | with radiant or fan-coil systems | decouples ventilation and humidity from temperature control |
Worked example: winter heating of an office
Worked example: the heating load of the 100 m² office
The example office of this guide has 40 m² of wall (U = 0.321 W/m²·K, from Heat Transfer), 12 m² of double glazing (U = 2.8) and 55 L/s of outdoor air. Heat it to 21 °C when it is −5 °C outside (ΔT = 26 K); assume the roof and floor are adiabatic.
- Walls: 0.321 × 40 × 26 = 334 W.
- Windows: 2.8 × 12 × 26 = 874 W: 12 m² of glass loses more than 40 m² of insulated wall.
- Ventilation: 55 L/s × 1.2 × 1.006 × 26 = 1 726 W.
- Total = 2.93 kW, or 29 W/m² of floor. The ventilation air is 59 percent of it: this is why heat recovery pays (see Ventilation and Indoor Air Quality).
- In summer the same office needs 7.27 kW of cooling (Heating and Cooling Load Calculations).
Numbers worth knowing
| Quantity | Value |
|---|---|
| Sensible heat of air | Q (W) = 1.2 × flow (L/s) × ΔT (K) |
| Heat carried by water | Q (kW) = 4.186 × flow (kg/s) × ΔT (K); about 1.16 W per (L/h·K) |
| One ton of refrigeration | 3.517 kW = 12 000 BTU/h |
| Supply air per kW of sensible load | about 75 L/s per kW at an 11 K supply difference (the rule of thumb of 400 cfm per ton is 54 L/s per kW) |
| Cooling load of an office | 80 to 120 W/m² of floor; heating 40 to 80 W/m² depending on insulation |
| Outdoor air | 2.5 L/s per person plus 0.3 L/s per m² (offices, ASHRAE 62.1 style) |
| People | sensible 75 W, latent 55 W (seated, office) |
Related
- Next: Moist Air and the Psychrometric Chart.
- Psychrometric Calculator and the Heat Transfer guide for the envelope.
- Power converter for tons, BTU/h and kW.
- HVAC and Psychrometrics: the whole guide.
FAQ
What is the difference between sensible and latent heat?
Sensible heat changes temperature; latent heat changes the amount of water vapor. A cooling coil removes both, and a space with many people or wet processes has a large latent load.
Why is 21 °C comfortable in winter and 25 °C in summer?
Because people wear more clothing in winter. Comfort depends on the operative temperature and clothing, not a single number.
Is relative humidity or humidity ratio better?
Relative humidity tells you about comfort and mold; humidity ratio tells you how much water is in the air, so it is what you use for calculations (it does not change when air is heated or cooled).
What does a ton of cooling mean?
The cooling effect of melting one US ton of ice in 24 hours: 12 000 BTU/h or 3.517 kW.
References
- ASHRAE, Handbook – Fundamentals (thermal comfort, psychrometrics, load calculations); ASHRAE Standard 55 (thermal environmental conditions) and 62.1 (ventilation).
- ISO 7730 (ergonomics of the thermal environment) and EN 16798-1 (indoor environmental input parameters).
- McQuiston, F. C., Parker, J. D., Spitler, J. D., Heating, Ventilating, and Air Conditioning: Analysis and Design, Wiley.
Last reviewed: October 2026. Comfort ranges and loads are typical.