HVAC and Psychrometrics
Comfort, the psychrometric chart and its processes, heating and cooling loads, ventilation, ducts and fans, chillers and cooling towers, hydronic systems and controls, with worked numbers.
Heating, ventilating and air conditioning keeps people comfortable and healthy, and keeps equipment and processes within limits, and it is one of the largest uses of energy in any building. The subject rests on one set of ideas: air is a mixture of dry air and water vapor, so every process has a sensible part that changes the temperature and a latent part that changes the humidity; and every system is a chain of heat balances, from the room through the air, the water and the refrigerant to the outdoors. This guide covers comfort and system types, the psychrometric chart and its processes, heating and cooling loads, ventilation and air quality, ducts and fans, chillers and cooling towers, hydronic systems, and controls, energy and troubleshooting. One 100 m² office, with its loads and equipment, runs through every page, and every number is computed. The thermodynamics behind it is in Thermodynamics and Steam.
Start here: what do you need?
| I need to… | Go to |
|---|---|
| understand comfort, system types and the basic numbers | HVAC Basics and Thermal Comfort |
| find dew point, wet bulb, enthalpy or read the chart | Moist Air and the Psychrometric Chart |
| work out a coil, humidifier, mixing box or evaporative cooler | Psychrometric Processes |
| calculate a heating or cooling load and the supply airflow | Heating and Cooling Load Calculations |
| choose ventilation rates, check CO₂, size heat recovery | Ventilation and Indoor Air Quality |
| size ducts, pick a fan or understand fan laws | Ducts, Fans and Air Distribution |
| size a chiller loop and cooling tower | Coils, Chillers and Cooling Towers |
| size radiators, pipes, pumps and expansion vessels | Hydronic Heating and Cooling |
| understand controls, estimate annual energy or diagnose a fault | Controls, Energy and Troubleshooting |
A suggested order
Start with HVAC Basics and Thermal Comfort and Moist Air and the Psychrometric Chart. Psychrometric Processes and Heating and Cooling Load Calculations use them to size the air side, and Ventilation and Indoor Air Quality and Ducts, Fans and Air Distribution move the air. Coils, Chillers and Cooling Towers and Hydronic Heating and Cooling cover the water side, and Controls, Energy and Troubleshooting ties it together.
- 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.
- Moist Air and the Psychrometric Chart: Humidity ratio, relative humidity, dew point, wet bulb and enthalpy, how to read the chart and how to calculate every property.
- Psychrometric Processes: Sensible heating and cooling, cooling and dehumidifying coils, humidifying, mixing air streams and evaporative cooling, with worked examples.
- Heating and Cooling Load Calculations: Heat loss, solar and internal gains, ventilation load and the sensible and latent split, worked for a 100 square meter office.
- Ventilation and Indoor Air Quality: Ventilation rates, carbon dioxide as an indicator, air changes, infiltration, filters and heat recovery.
- Ducts, Fans and Air Distribution: Duct sizing, friction and fitting losses, fan curves and fan laws, diffusers and balancing.
- Coils, Chillers and Cooling Towers: Chilled-water flow, chiller COP, condenser water, cooling tower range and approach, and water use.
- Hydronic Heating and Cooling: Water-based heating and cooling, flow and temperature difference, pipe sizing, emitters, expansion vessels and pumps.
- Controls, Energy and Troubleshooting: Thermostats and control loops, economisers, variable air volume, balance point and degree days, and a table of common problems.
One office, from load to plant
Worked example: the same office on every page
- The air (Moist Air and the Psychrometric Chart): room air at 25 °C and 50 percent has a humidity ratio of 9.88 g/kg, a dew point of 13.9 °C and an enthalpy of 50.3 kJ/kg.
- The load (Heating and Cooling Load Calculations): 7.27 kW of cooling (6.21 kW sensible, 1.06 kW latent) needs 425 L/s of supply air at 13 °C.
- The ventilation (Ventilation and Indoor Air Quality): 55 L/s of outdoor air holds CO₂ near 1 360 ppm.
- The ducts and fan (Ducts, Fans and Air Distribution): about 0.5 m³/s at 250 Pa and 200 W.
- The plant (Coils, Chillers and Cooling Towers and Hydronic Heating and Cooling): about 0.3 kg/s of chilled water to the coil, or 20 kW of radiators on a 70/50 °C circuit in winter.
Quick reference: the equations
| What you need | Equation |
|---|---|
| Humidity ratio | W = 0.622 pw ÷ (p − pw) |
| Enthalpy of moist air | h = 1.006 T + W (2 501 + 1.86 T) |
| Sensible heat of air | Q (W) = 1.2 × flow (L/s) × ΔT (K) |
| Latent heat of air | Q = ṁ ΔW × 2 501 kJ/kg |
| Heat in water | Q (kW) = 4.186 × flow (kg/s) × ΔT (K) |
| Mixing | Wm and hm are mass-weighted averages |
| Room CO₂ | C = Cout + G ÷ Q (steady state) |
| Duct friction | ΔP ÷ L = f ÷ D × ρ v² ÷ 2 |
| Fan laws | Q ∝ N, ΔP ∝ N², P ∝ N³ |
| Chiller | W = Q ÷ COP; heat to tower = Q + W |
| Annual heating | E = UA × HDD × 24 |
Related guides and tools
- Psychrometric Calculator: every moist-air property from any two values.
- Pipe Flow and Pressure Drop Calculator and Pump Power and Sizing Calculator.
- Thermodynamics and Steam, Heat Transfer and Fluid Mechanics and Pipe Flow.
- The Power, Flow and Temperature converters, and the Downloads library.
Safety first
- Refrigerants can be flammable, toxic or asphyxiating, and are regulated; only qualified people should open a refrigerant circuit.
- Legionella grows in warm stored water, cooling towers and humidifiers. Keep water moving, treated and clean, and follow the local code.
- Combustion appliances make carbon monoxide: keep flues, air supply and alarms working, and never block a ventilation opening.
- Electrical, rotating and hot parts: isolate and lock out before opening a unit, and never bypass a safety control.
- Pressure and heights: hot water, steam and refrigerant systems are pressure equipment; roof units and ducts mean working at height.
- The numbers here are for study and preliminary design. Real projects need the local codes, a competent designer and commissioning.
FAQ
Where do I start with HVAC design?
Read HVAC Basics and Thermal Comfort, then Load Calculations. The load sets the air, water and plant sizes.
Why does humidity matter so much?
It sets the latent load (a large share of the cooling load in humid climates), affects comfort and mold, and is why a cooling coil has to be colder than the room needs for temperature alone.
Can I use a rule of thumb for equipment size?
For a first estimate, yes (see Load Calculations). Always calculate before ordering equipment.
Where are hydraulics and pneumatics?
They are the last guide in the Thermal and Fluids section (the actuator side is in the Motion Control guide).
References
- ASHRAE, Handbook (Fundamentals, HVAC Systems and Equipment, HVAC Applications, Refrigeration), and ASHRAE Standards 55, 62.1 and 90.1.
- CIBSE Guides A, B, C and H; McQuiston, F. C., Parker, J. D., Spitler, J. D., Heating, Ventilating, and Air Conditioning: Analysis and Design, Wiley.
- Kreith, F., Manglik, R., Bohn, M., Principles of Heat Transfer; Çengel, Y. A. and Boles, M. A., Thermodynamics: An Engineering Approach.
- EN 16798, EN 12831, EN 12828, ISO 7730, ISO 52016.
Last reviewed: November 2026. Values are typical; check the standard, the manufacturer’s data and the code that applies to your project.