This is the technical Q&A reference. If you want the broader interview prep guide (resume, what to wear, salary, follow-up), our HVAC interview tips post covers all of that. This page is the deep end of the pool: 17 specific HVAC questions, with the kind of answers that show a hiring manager you actually understand the work.
Useful whether you are a candidate practicing your answers, or a service manager building an interview rubric. The first batch is interpersonal; the rest is technical.
Interpersonal Questions
1. Tell Me About Yourself
Keep it under 90 seconds. Structure it around three beats: how you got into HVAC (school, apprenticeship, or career change), what kinds of work you have done since, and what you are looking for now. Avoid family backstory, do not bad-mouth previous employers, and do not volunteer that you were fired unless asked. The interviewer already has your resume; this question is mostly about how you communicate under mild pressure.
2. Turning an Unhappy Customer Around
Use STAR: situation, task, action, result. Pick a real example. Walk through the problem, what you said, what you did, and how the customer reacted. Hiring managers are listening for whether you stayed calm, took ownership, and followed through. "They were upset. I listened. I showed up the next day with the part. They left a five-star review" is a complete answer.
If you are entry-level and have no HVAC examples yet, pull from any service or customer-facing job. Same lesson, different industry.
3. Customer Refrigerant Concerns
This question comes up more often now because of the AIM Act refrigerant transition. Stay technical, stay neutral. Walk the customer through the actual change (R-410A is being phased out for new equipment, R-32 and R-454B are the replacements, both have a much lower global warming potential), explain the practical impact (their existing system can still be serviced for years, but if they replace it, the new system uses A2L refrigerant), and let them make their own decision. The right answer is informed and respectful, not opinionated.
Technical Questions
4. Walk Through the Refrigeration Cycle
Compressor compresses low-pressure superheated vapor to high-pressure superheated vapor. The condenser rejects heat to the outdoor air, condensing the refrigerant to a high-pressure subcooled liquid. The metering device (TXV or fixed orifice) drops the pressure. The evaporator absorbs heat as the refrigerant boils back to a low-pressure superheated vapor. Repeat. Be ready to label the four components and the two pressure changes on a whiteboard.
5. What Is Superheat?
Superheat is the difference between the actual temperature of the refrigerant vapor and its saturation temperature at the current pressure. To measure it: clamp a thermometer to the suction line near the compressor, take the suction pressure with your gauges, look up the saturation temperature for that pressure on a P-T chart for the refrigerant in the system, and subtract. Typical target on a fixed-orifice system is 8 to 15 F at design conditions, depending on the equipment. Too low: liquid floodback risk. Too high: capacity loss and overheated compressor.
6. What Is Subcooling?
Subcooling is how much the high-pressure liquid leaving the condenser has cooled below its saturation temperature. To measure it: clamp a thermometer to the liquid line at the condenser outlet, take the high-side pressure, look up the saturation temperature on a P-T chart, and subtract the line temperature from it. Typical target on a TXV system is 8 to 12 F. Subcooling is the primary charging method on TXV systems; superheat is the primary method on fixed-orifice systems.
7. What Is Ohm's Law?
V = I × R. Voltage equals current times resistance. In HVAC, you use it to confirm motor health and to size electrical components. "If a 240V blower motor is pulling 12 amps, the resistance is 20 ohms. If that same motor jumps to 18 amps, you are looking at a winding short or a bearing problem dragging the motor." Knowing the formula is table stakes; being able to apply it to a real diagnostic is what wins the question.
8. What Is the AIM Act?
The American Innovation and Manufacturing Act phased out R-410A for new HVAC equipment effective January 1, 2025. New residential and light commercial split systems and heat pumps use R-32 or R-454B, both A2L (mildly flammable) refrigerants with a much lower global warming potential than R-410A. The practical implications for techs: A2L training, A2L-rated tools (recovery machine, hoses, leak detector for newer refrigerants), spark-free brazing precautions, and ventilation requirements at the install site. Existing R-410A equipment can still be serviced; the change applies to new manufacture and install.
9. R-32 vs. R-454B
R-32 is a single-component refrigerant (pure difluoromethane) with a GWP of 675. R-454B is a blend of R-32 and R-1234yf with a GWP of 466. Both are A2L. Different manufacturers chose different refrigerants: Daikin pushed R-32 globally, while Carrier, Trane, Lennox, Goodman, and most other major US brands chose R-454B for the 2025 transition. Practical takeaway for service: you need P-T charts and gauge sets that cover both.
10. BTU, CAV, and AHU Defined
- BTU: British Thermal Unit. The amount of energy needed to raise one pound of water by 1 F. HVAC equipment is rated in BTU/hr (12,000 BTU/hr = 1 ton of cooling).
- CAV: Constant Air Volume. A duct system or unit that delivers a fixed airflow regardless of zone load. Older commercial systems are mostly CAV; modern systems often use VAV (Variable Air Volume) for energy efficiency.
- AHU: Air Handling Unit. The cabinet that contains the blower, heating and cooling coils, filters, and dampers. The indoor side of a split system.
11. Walk Through a No-Cool Call
This is your chance to show systematic troubleshooting. Work simple to complex.
- Confirm the call (thermostat set to cool, set point below room temp).
- Check the breaker and indoor disconnect.
- Check the air filter; a packed filter can ice the coil.
- Verify thermostat is calling for cool with a meter at the Y terminal.
- At the outdoor unit: check disconnect, capacitor, contactor, and visually inspect the coil.
- If the unit runs but is not cooling: gauges, suction and discharge pressures, superheat or subcooling, then look for the right diagnostic from there (low refrigerant, dirty coil, blocked TXV, weak compressor).
The right answer is not memorizing one fix; it is showing you have a process.
12. Testing a Capacitor
Kill power, pull the disconnect, and discharge the capacitor across the terminals with a properly insulated resistor. Set your multimeter to capacitance (microfarads). Touch the leads to each pair of terminals on a dual-run capacitor (HERM to C, FAN to C) and read the values. Compare against the rated MFD on the label, with a tolerance of plus or minus 6 percent. A reading outside that range, a swollen top, or a burnt smell means the cap is done. Bonus points if you mention discharging the cap before testing; that is a safety filter most interviewers run.
13. TXV vs. Fixed Orifice
A fixed orifice (also called a piston) is a single fixed-size hole that the refrigerant has to flow through. Cheap, no moving parts, only well-charged at one design condition. A TXV (Thermostatic Expansion Valve) modulates flow based on suction-line temperature, keeping superheat constant across a range of conditions. A TXV system holds capacity better at off-design conditions and is more efficient, which is why most modern equipment uses TXVs. Trade-off: more parts to fail.
14. Checking for a Refrigerant Leak
- Soap bubble test for accessible joints (cheap, slow).
- Electronic leak detector (heated diode or infrared) for general searching.
- UV dye injected into the system, then UV light a few cycles later. Best for hard-to-find slow leaks.
- Nitrogen pressure test with the system isolated. Bring it up to working pressure and watch the gauges; a drop over time means a leak. Standard procedure on new installs and after major repairs.
For A2L refrigerants (R-32 and R-454B), make sure your detector is rated for the refrigerant. Older R-410A leak detectors do not always read A2Ls reliably.
15. Diagnosing Short Cycling
Short cycling is the system kicking on and off in rapid succession. Common causes:
- Oversized equipment relative to the load (Manual J was wrong or skipped).
- Dirty filter or restricted airflow tripping the high-limit switch.
- Low refrigerant charge cycling on the low-pressure cutout.
- Bad thermostat anticipator or location (in direct sun, near a supply register).
- Failed pressure switch, contactor, or float switch.
Diagnose with airflow check, gauges, and a meter at the safety switches. Order matters: check the cheap stuff first.
16. Evacuate and Charge a System
After brazing in the new equipment: pressure test with nitrogen at the manufacturer’s spec, hold and watch for drop. Triple evacuate to 500 microns or below with a high-quality vacuum pump and a micron gauge. Verify the vacuum holds (rise should be slow and stop below ambient pressure). Weigh in the factory charge per the nameplate, accounting for line set length. Confirm with superheat or subcooling per the manufacturer’s commissioning procedure. Skipping the micron gauge is the single most common reason new installs fail in the first year.
17. Caring for Your Tools and Truck
Wipe down gauges and thermometers at the end of every shift. Send manifolds in for calibration on the manufacturer’s schedule. Keep your truck inventoried with a checklist on your phone, and re-stock at the supply house weekly so you do not lose a job because you ran out of capacitors. "My truck inventory list lives on my phone" is a one-line answer that signals you take the job seriously.
For Hiring Managers
If you are on the other side of the desk, the goal is a balanced rubric:
- Two interpersonal questions to evaluate communication and customer focus.
- Five to seven technical questions covering refrigeration cycle, electrical, refrigerant, and modern equipment.
- One scenario walkthrough that mirrors a real call your shop runs.
- One question about how the candidate handles after-hours, on-call, and overtime.
If your interview process includes a hands-on skills demo (recommended for journeyman roles), give the candidate a multimeter and a defective capacitor and let them work through the diagnosis. Watching someone troubleshoot is faster than asking them to describe troubleshooting.
The Bottom Line
Strong technical answers come from preparation. Use this list as practice flashcards: read each question, talk through your answer out loud, and refine it until it sounds natural. Pair it with the broader prep in our HVAC interview tips guide (resume, dress, salary, follow-up).
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