Here's a detailed explanation of common HVAC interview questions, covering both conceptual and practical aspects, especially useful for HVAC engineers, technicians, and freshers:
🔹 1. Explain the working principle of a chiller.
✅ Answer:
A chiller removes heat from a liquid (usually water or a glycol solution), which is then circulated through air handlers or fan coil units to cool a space.
🔄 Working Principle (Vapor Compression Cycle):
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Evaporator: Warm water from the building transfers heat to the refrigerant → refrigerant evaporates.
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Compressor: Compresses low-pressure vapor into high-pressure vapor.
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Condenser: Releases heat to the environment → refrigerant condenses into liquid.
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Expansion Valve: Reduces refrigerant pressure → cools it before entering evaporator again.
💡 Chillers can be air-cooled (release heat to air) or water-cooled (release heat to a cooling tower).
🔹 2. How do you perform a cooling load calculation?
✅ Answer:
Cooling load is the amount of heat energy that must be removed from a space to maintain the desired temperature and humidity.
Basic Steps:
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Collect building data (location, orientation, dimensions).
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Calculate:
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Transmission load (walls, roof, windows)
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Internal load (people, equipment, lighting)
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Ventilation load (fresh air requirement)
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Solar gain (windows, glazing type)
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Use Manual J (residential) or Manual N (commercial) or software like HAP/TRACE.
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Total the BTUs or Tons required.
💡 1 Ton = 12,000 BTU/hr
🔹 3. What is the function of an expansion valve?
✅ Answer:
An expansion valve regulates the flow of refrigerant into the evaporator by reducing its pressure and temperature.
Functions:
Drops the refrigerant pressure suddenly → refrigerant becomes cold.
Controls the superheat to prevent liquid refrigerant from entering the compressor.Maintains efficient heat absorption in the evaporator.
🔧 Types: Thermal Expansion Valve (TXV), Electronic Expansion Valve (EEV), Capillary Tube.
🔹 4. What are VRV/VRF systems?
✅ Answer:
VRV (Variable Refrigerant Volume) and VRF (Variable Refrigerant Flow) refer to advanced HVAC systems that allow individual control of multiple indoor units using a single outdoor unit.
Key Features:
Uses inverter-driven compressors to vary refrigerant flow.
Energy-efficient as only needed rooms are cooled/heated.Zoning and flexibility — ideal for hotels, offices, and multi-story buildings.
Can provide simultaneous heating and cooling.
- 💡 VRV is Daikin’s trademark. VRF is the generic term.
🔹 5. How do you balance an HVAC system?
✅ Answer:
Balancing an HVAC system ensures that each zone receives the correct airflow (CFM) for proper comfort and system efficiency.
Steps:
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Measure airflow at each diffuser/grille using anemometers or flow hoods.
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Compare with design CFM values.
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Adjust damper positions or fan speed to increase/decrease airflow.
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Ensure even temperature distribution and no hot/cold spots.
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Record the final values in a balancing report.
📘 Part of TAB: Testing, Adjusting, and Balancing.
🔹 6. What is the difference between EER and SEER?
| Feature | EER (Energy Efficiency Ratio) | SEER (Seasonal Energy Efficiency Ratio) |
|---|---|---|
| Definition | Ratio of cooling output to power input at a fixed condition | Ratio over a range of seasonal temperatures |
| Condition | Fixed (usually 95°F outdoor) | Varying seasonal conditions |
| Use | Spot efficiency | Overall seasonal efficiency |
| Climate | Better for hot climates | Better for mild/variable climates |
💡 Higher EER/SEER = more energy-efficient system.
🔹 7. Explain the importance of a thermostat in an HVAC system.
✅ Answer:
A thermostat is a control device that monitors room temperature and signals the HVAC system to turn on/off to maintain the setpoint.
Importance:
Ensures thermal comfort.
Reduces energy consumption by preventing overcooling/heating.Allows programmable schedules for efficiency.
Used in zoning systems for room-wise control.
📱 Modern thermostats (smart thermostats) can be controlled via apps and use AI for energy savings.