HVAC technician interview questions and answers
HVAC interviews check how you diagnose a fault step by step and how you deal with the customer while you do it. Expect questions on airflow, refrigerant, safety and your certifications.
The most common question in the videos we reviewed: What is HVAC, and how is it different from a basic air conditioner.
Which questions come up most?
We read the transcripts of the 4 most watched interview videos for this job and counted how many of them say each question gets asked. The table shows the top ones. The answers below are ours, written to show the shape of a strong reply.
| Question | Videos that mention it (of 4) |
|---|---|
| What is HVAC, and how is it different from a basic air conditioner? | 4 |
| What is the difference between a fan coil unit and an air handling unit? | 2 |
| What is CFM and why does it matter? | 2 |
| What safety devices does an HVAC system have, and what do they do? | 2 |
| What do dampers do, and what is a fire damper? | 2 |
| What are the most common HVAC faults and how do you find them? | 2 |
| How does the refrigeration cycle work, and what is refrigerant for? | 2 |
| How does a chilled water system work? | 2 |
| What are superheat and subcooling, and why do they matter? | 1 |
| When would you use a two way valve and a three way valve? | 1 |
The questions, with strong answers
What is HVAC, and how is it different from a basic air conditioner?
What they check: Whether you know the whole system: heating, ventilation and cooling, and can explain it in plain words.
A strong answer: HVAC means heating, ventilation and air conditioning. It controls temperature, fresh air, humidity and air quality, often across a whole building. A basic air conditioner mainly cools one space. An HVAC system adds heating, fresh air, filtration and ducted distribution, run by a thermostat or building controls. A furnace or heat pump heats, a refrigerant circuit cools, and fans and ducts move the air.
A weak answer: Saying HVAC is just a bigger air conditioner. It leaves out heating, ventilation and air quality, so it looks like shallow knowledge.
What is the difference between a fan coil unit and an air handling unit?
What they check: Whether you know the equipment you will service and how size and use differ.
A strong answer: Both use a coil and fan to condition air with chilled or hot water. A fan coil unit is small and serves one room or zone, often above the ceiling or on the wall. An air handling unit is larger, sits in a plant room, moves much more air through ducts, and has filters, coils, fans and sometimes fresh air dampers and heat recovery. Give the airflow difference.
A weak answer: Saying they are the same thing with different names. It shows you have not worked with both.
What is CFM and why does it matter?
What they check: Whether you understand airflow and how it affects comfort, efficiency and coil performance.
A strong answer: CFM is cubic feet per minute, the volume of air moved. Too little airflow means poor cooling or heating, frozen coils and high running cost. Too much causes noise and poor humidity control. Airflow is set by fan speed, duct size, filters and dampers. You measure it with an airflow hood or by duct velocity and area, then adjust to the design figure.
A weak answer: Saying it is how strong the air feels. It misses that it is a measured volume used to balance a system.
What safety devices does an HVAC system have, and what do they do?
What they check: Whether you know the controls that protect people and equipment and never bypass them.
A strong answer: Common ones are high pressure and low pressure switches, a pressure relief valve, water flow switches, freeze protection, and overloads on motors. They trip the system before a fault becomes damage or injury. Never bypass a safety. If one trips, find the cause first, for example dirty coil, low airflow or no water flow. Lock out and tag out before working inside any unit.
A weak answer: Saying you would bridge a tripping switch to keep the unit running. That is dangerous and shows poor judgement.
What do dampers do, and what is a fire damper?
What they check: Whether you understand air control in ducts and life safety devices in a building.
A strong answer: Dampers control or stop airflow in ducts. Volume dampers balance flow, and motorised ones open or close under controls, for example to set the fresh air share. A fire damper closes when heat melts a link, so fire and smoke do not spread through the duct to another room. It needs access for inspection, and must be tested and left free to close.
A weak answer: Saying a damper just makes the air quieter. It misses control and fire safety.
What are the most common HVAC faults and how do you find them?
What they check: Whether you follow a clear checking order instead of swapping parts at random.
A strong answer: Start with the easy ones: dirty filters, dirty coils, weak airflow, blocked drains, a failed thermostat, a worn blower motor or bearings, leaks and tripped safeties. Follow a method. Ask the customer, check power and controls, then airflow, then refrigerant side readings, then electrical parts. Fix the cause, run it again, and record what you found and changed.
How does the refrigeration cycle work, and what is refrigerant for?
What they check: Whether you know the four main parts and why heat moves from inside to outside.
A strong answer: Refrigerant carries heat. In the evaporator it boils at low pressure and absorbs heat from the air. The compressor raises its pressure and temperature. In the condenser it gives that heat away and turns back to liquid. The expansion device drops the pressure, and the cycle repeats. Use only the refrigerant the system is rated for, and recover it, never vent it.
How does a chilled water system work?
What they check: Whether you can trace the loop from chiller to coils and back for larger buildings.
A strong answer: A chiller cools water in its evaporator. Pumps send the cold water along the supply pipe to air handlers and fan coils, where the coil takes heat from the air. The warmer water returns to the chiller. The chiller rejects heat through a condenser, often with a cooling tower. Valves and pumps control flow, and a pressurisation unit keeps the loop full.
What are superheat and subcooling, and why do they matter?
What they check: Whether you can check refrigerant charge with measurements, not guesses.
A strong answer: Superheat is how far the suction vapour is above its boiling point. Subcooling is how far the liquid leaving the condenser is below its condensing point. Both come from pressure and temperature readings. They show if the charge is right and the metering device is working. Low superheat risks liquid in the compressor. High superheat suggests a low charge or restriction.
When would you use a two way valve and a three way valve?
What they check: Whether you understand control valve choices tied to pump type in water systems.
A strong answer: A two way valve closes to stop flow through a coil, so flow in the loop changes. It suits systems with variable speed pumps. A three way valve sends water through the coil or bypasses it, so total flow stays roughly constant. It suits constant speed pumps. Mention that the right choice depends on the design, and the controls must match.
How do you size a system or calculate the load for a space?
What they check: Whether you know that sizing comes from a load calculation, not rules of thumb alone.
A strong answer: Load depends on floor area, building envelope, windows and sun, insulation, people, lighting, equipment and fresh air. A proper calculation follows a recognised method, often with software. Oversizing causes short cycling and poor humidity control. Undersizing means the space never reaches the target. Check duct size and airflow match the chosen unit, and confirm with the designer.
How would you explain a repair to a customer who is not technical?
What they check: Whether you can communicate clearly, listen, and build trust.
A strong answer: Listen first and ask questions about what they noticed. Then explain in everyday words, with a simple comparison and by showing the part. Say what is wrong, what it will cost, the options, and what happens if they wait. Give simple care advice, like changing filters on time. Leave contact details and write up the work in plain language.
Why do HVAC pipes and ducts need insulation?
What they check: Whether you know insulation stops condensation, heat gain and wasted energy.
A strong answer: Cold pipes and ducts in warm air collect condensation, which drips and damages ceilings. Insulation keeps surfaces above the dew point, cuts heat gain or loss, and saves energy. It must have a vapour barrier on cold lines, with no gaps at joints and supports. Ducts in unconditioned spaces need it too. Check it after any repair and replace damaged sections.
How do you test a new chilled water pipe system?
What they check: Whether you know pipework must be proven tight before it is hidden or put into use.
A strong answer: Fill the pipework with water, vent the air, and raise the pressure to the test figure in the specification, then hold it for the required time and watch for a drop. Check all joints and supports. For very large pipes or welds, other checks such as dye penetrant or air tests may be used. Record results and get them signed off.
What to know about the hiring process
- Interviews for HVAC technicians usually combine technical questions with safety and customer communication questions.
- Refrigerant handling usually needs a recognised certificate in many countries, so expect to be asked what you hold.
- Some employers set a practical task such as fault finding, or ask you to read a wiring or pipe diagram.
- Electrical knowledge is often tested alongside mechanical and refrigeration knowledge.
Practise these out loud
On Just One Interview, you type the role and the skills you want tested, and an AI interviewer asks you the questions out loud. You get a report with a score for each answer and a count of your filler words. It runs in the browser on your phone, and the free plan includes 20 minutes of practice a month.