In this article
How to use these answers
Technical rounds in any engineering field reward the same habit: state the definition, show the calculation, then name the assumption that could break it. The coding interview guide shows that routine for software; the quality assurance (QA) questions and solution architect questions are other examples of general technical-round method.
Each entry below gives the answer in a few sentences, one line on why interviewers ask it, and a link to the source. Where a number is computed, it comes from a short script.1 Reported practice differs between regions and editions of standards, so check the current edition before you quote a value in a real design.
Refrigeration and heat basics
How does the refrigeration cycle work?
A refrigeration cycle moves heat; it does not create cold. The compressor raises the refrigerant's pressure (MIT 2.972 report). The condenser rejects heat to a lower-temperature medium, usually outdoor air, so the refrigerant becomes a high-pressure liquid, and the expansion device drops its pressure. The evaporator then absorbs heat from the space as the refrigerant boils back to vapor (Grandy and Associates).
Why it is asked: it separates people who can trace the heat from people who can only name the parts. Read the full explanation: Grandy and Associates, the four-stage cycle.
What is superheat, and why does it matter?
Superheat is the number of degrees by which refrigerant vapor is hotter than its boiling point at that pressure. The vapor leaving the evaporator should be superheated, because this prevents liquid slugging, meaning liquid refrigerant flowing back into the compressor (HVAC Laboratory). A thermostatic or electronic expansion valve meters refrigerant flow to hold superheat in range. Quote the manufacturer's target rather than a remembered number.
Why it is asked: it tests whether you know the failure the control protects against. Read the full explanation: HVAC Laboratory.
What is the difference between sensible and latent heat?
Sensible heat is the heat that changes the temperature of the air, the part a thermometer shows. Latent heat is the heat tied to moisture, removed when water vapor condenses on the cooling coil. Total cooling load is the sum of sensible and latent cooling load (Captive-Aire). The latent load is about 0.69 × airflow in cubic feet per minute (cfm) × the change in grains of moisture per pound of dry air (same source), while the sensible load is 1.08 × cfm × the temperature change (New Jersey Institute of Technology (NJIT) lecture notes).
Why it is asked: coil selection depends on both loads, not temperature alone. Read the full explanation: Captive-Aire, navigating the psychrometric chart.
What is the sensible heat ratio?
The sensible heat ratio (SHR) is sensible heat divided by total heat, sensible plus latent; for a room it is called the room sensible heat ratio (HVAC School). With a sensible load of 1,800 British thermal units per hour (Btu/h) and a latent load of 700 Btu/h (illustrative), the total is 2,500 Btu/h and the ratio is 0.72.1 A lower ratio means a more humid load, so the coil must remove more moisture.
Why it is asked: it checks that you size for moisture as well as temperature. Read the full explanation: HVAC School, room sensible heat.
Knowing it is half the job. Saying it is the other half.
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Start Free Trial →Loads, sizing and airflow
What do Manual J, Manual S and Manual D cover?
Manual J assesses the heat loss and gains of a home; it does not tell you which equipment size to install. Manual S provides procedures for selecting and sizing residential heating and cooling equipment from the Manual J loads and the manufacturer's performance data, so the equipment meets the sensible and latent loads. Manual D designs the residential duct systems (Air Conditioning Contractors of America (ACCA) blog). All three are residential methods.
Text description of the diagram
- Manual J assesses the heat loss and gains of a home.
- Manual S selects equipment from those loads, and Manual D designs the duct systems.
Why it is asked: it checks that you know the order of the workflow and that a load is not an equipment size. Read the full explanation: ACCA, when to consider Manual J, S and D.
How do you calculate supply airflow from a sensible load?
Divide the room's sensible load by 1.08 times the temperature difference between return and supply air: , where is the sensible load (HVAC School). For 1,800 Btu/h and a 20°F difference the airflow is 83 cfm; for a 12°F difference it is 139 cfm.1 As the temperature difference between return and supply air decreases, the required airflow increases.
Why it is asked: it is the basic duct and fan sizing step, and the units are easy to get wrong. Read the full explanation: HVAC School, room sensible heat.
Where does the 1.08 factor come from, and when does it fail?
The factor is 60 minutes per hour × 0.075 pounds per cubic foot × 0.24 Btu per pound per °F, which equals 1.08 (HVAC School). The air density of 0.075 is standard air, so at high altitude or unusual temperatures the factor changes and a density-corrected value is needed (Captive-Aire). Say the assumption aloud instead of treating 1.08 as a constant.
Why it is asked: it separates memorized formulas from understood ones. Read the full explanation: HVAC School, room sensible heat.
How do you size the cooling coil for a given airflow?
Multiply airflow by 1.08 and by the air temperature drop across the coil. The 1.08 factor is explained in the HVAC School article. For 2,000 cfm and a 15°F drop, that is 32,400 Btu/h of sensible cooling.1 This counts sensible heat only; add the latent load to get total capacity, and correct for density if the air is not standard.
Why it is asked: it tests whether you finish the calculation in the right units. Read the full explanation: HVAC School, room sensible heat.
Why is oversizing equipment a problem?
Larger equipment is not a safety margin. The purpose of Manual S is to select equipment that meets both the sensible and the latent loads of the house (ACCA blog). In practice an oversized unit satisfies the thermostat quickly and cycles on and off, which gives less run time to remove moisture and to mix the air. A strong answer names run time and humidity control, then ties the fix back to accurate loads.
Why it is asked: the pull to add margin is strong, and interviewers want to see you resist it with reasons. Read the full explanation: ACCA, when to consider Manual J, S and D.
Ventilation and comfort
How do you calculate the breathing-zone outdoor airflow in ASHRAE 62.1?
Under the Ventilation Rate Procedure, breathing-zone outdoor airflow is : a people rate times the zone population plus an area rate times the floor area, as written in an addendum to the 2016 edition of the standard from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) (ASHRAE, July 2019). The rates depend on space type, occupancy level and floor area, and the procedure may be used for any zone or system.
Why it is asked: many candidates remember only a per-person number. Read the full explanation: ASHRAE addendum to Standard 62.1-2016.
What outdoor air does a 5,000 square foot office need?
One worked example uses 5 cfm per person and 0.06 cfm per square foot for a typical office, with 25 occupants: 125 cfm for people plus 300 cfm for area gives 425 cfm minimum outdoor air (Envigilance).1 The rates in that example come from a secondary source, so check them against the space-type table of the current standard before relying on them in a design.
Why it is asked: it shows whether you can apply the formula with units and a sanity check. Read the full explanation: Envigilance, ASHRAE 62.1 example.
What does zone air distribution effectiveness do to the result?
The zone outdoor airflow is , where is the zone air distribution effectiveness; if is below 1.0, the required outdoor airflow increases (HeatWise; Envigilance). With an Ez of 0.8 (illustrative), the 425 cfm breathing-zone airflow becomes 531 cfm.1 The reason: when supply air mixes poorly with the occupied zone, more outdoor air is needed to deliver the same amount to people.
Why it is asked: it checks that you carry the calculation past the first step. Read the full explanation: HeatWise, ASHRAE 62.1.
What is the difference between ASHRAE Standard 55 and Standard 62.1?
Standard 55 is about thermal comfort: it addresses temperature, thermal radiation, humidity, air speed, clothing and activity level, aiming for satisfactory conditions for a substantial majority, more than 80 percent, of occupants (ASHRAE fact sheet). It does not address air quality (ASHRAE Standard 55-2023 scope). Standard 62.1 sets ventilation rates, the air quality side.
Why it is asked: confusing comfort with air quality is a common error. Read the full explanation: ASHRAE Standard 55 fact sheet.
Efficiency ratings
What is the difference between SEER2, EER2 and HSPF2?
The seasonal energy efficiency ratio 2 (SEER2) is an average of cooling efficiency over a whole cooling season, with temperatures from 65°F to 104°F. The energy efficiency ratio 2 (EER2) measures efficiency at peak cooling need: 95°F outdoors, 80°F indoors and 50% humidity (Trane). The heating seasonal performance factor 2 (HSPF2) rates heat pump heating, and SEER2 is shown in Btu per watt-hour (U.S. Department of Energy (DOE), Federal Energy Management Program).
Why it is asked: a higher seasonal number does not mean better performance at peak. Read the full explanation: Trane, HVAC efficiency metrics.
Are SEER and SEER2 numbers interchangeable?
No. The 2017 DOE rule established SEER2, EER2 and HSPF2, which generally have different numerical values than the old metrics, and it changed the external static pressure conditions used in testing (DOE, January 2017). A 2025 DOE rule maintains these metrics in its appendix M1, and it also adds an appendix M2 with different integrated metrics that include off-mode power (Federal Register, January 2025). One manufacturer page gives 13.4 SEER2 as roughly 14 SEER (Trane).
Why it is asked: comparing old and new labels directly is a frequent mistake. Read the full explanation: Federal Register, DOE test procedure.
How much energy does a higher SEER2 save?
In the DOE table for northern states, a SEER2 of 13.4 uses 4,581 kWh a year and a SEER2 of 15.2 uses 4,038 kWh (DOE Federal Energy Management Program). The difference is 543 kWh a year, about 12 percent of the lower-rated unit's use.1 Savings depend on climate and run hours, so name the region when you quote them.
Why it is asked: it tests whether you can turn a rating into a cost argument. Read the full explanation: DOE Federal Energy Management Program.
Commercial and building systems
Roles in mechanical, electrical and plumbing (MEP) design lean on commercial practice. The entries below give the concepts without numbers; take values from the current standard or the equipment data.
What is the difference between dew point and wet-bulb temperature?
Dew point is the temperature at which air at its current moisture content becomes saturated, so moisture begins to condense. Wet-bulb temperature is what a thermometer with a wetted wick reads, cooled by evaporation. Dry-bulb is at least the wet-bulb, which is at least the dew point, and all three coincide only in saturated air. To remove moisture, a cooling coil surface must be colder than the dew point of the air passing over it. These are standard psychrometric definitions; confirm them in the psychrometrics chapter of the ASHRAE Handbook (Fundamentals).
Why it is asked: it checks that you can read a psychrometric chart. Read the full explanation: Captive-Aire, navigating the psychrometric chart.
Why is the industry moving to R-454B and R-32?
Regulation under the American Innovation and Manufacturing (AIM) Act phases down hydrofluorocarbon refrigerants with high global warming potential, and the Environmental Protection Agency (EPA) technology-transitions rule sets restrictions by product type and date; the EPA page gives 1 January 2025 as the compliance date for newly manufactured residential and light-commercial air-conditioning systems (EPA, Technology Transitions). Lower-warming replacements such as R-454B and R-32 are generally classed as mildly flammable, A2L-class refrigerants, which changes handling, leak detection and codes. Check the current EPA rule before you quote a date for another product type.
Why it is asked: it tests awareness of the current refrigerant change and its safety consequences. Read the full explanation: EPA, Technology Transitions.
What does ASHRAE Standard 90.1 cover?
ASHRAE Standard 90.1 sets minimum energy-efficiency requirements for most commercial buildings, covering the envelope, lighting, mechanical systems and power. Many energy codes reference it, so a design engineer checks equipment efficiency and controls against the edition the local code adopts. It is the commercial counterpart to the residential rules above.
Why it is asked: commercial design is judged against an energy code, not only against load calculations. Read the full explanation: ASHRAE, Standard 90.1.
Follow-up questions
What are the common interview questions for an HVAC engineer?
The nineteen above cover the recurring themes: the refrigeration cycle, sensible and latent heat, load and airflow calculation, ventilation and comfort standards, psychrometrics, refrigerants, energy standards and efficiency ratings. Expect to be asked to work one calculation by hand. Interviewers in other fields use the same structure, as in the networking interview questions.
How do I answer a question I have not met, such as one on chillers or controls?
Reason from the heat balance: say where heat enters, where it leaves and what the equipment moves in between, then state your assumptions. Say plainly which parts you know from experience and which you would confirm in manufacturer data. This page does not cover chillers, variable air volume systems, duct sizing methods or building-automation sequences, so prepare those from the equipment documentation and design references you have used.
Do I need to memorize constants like 1.08?
Know where it comes from, 60 × 0.075 × 0.24, and which assumption it carries, standard air density. If you can rebuild the constant, you can correct it for altitude and show you understand the units.
How do I prepare for the technical round?
Practice saying each calculation aloud with its units, and rehearse two or three worked examples. The coding interview guide describes a think-aloud method that carries over; the full stack developer and JavaScript lists show the same general technical-round habit.
- A refrigeration cycle moves heat; superheat at the evaporator outlet protects the compressor.
- Supply airflow is the sensible load divided by 1.08 times the temperature difference, and 1.08 assumes standard air.
- Manual J gives loads, Manual S selects equipment, and Manual D designs ducts.
- Ventilation uses people plus area, then divides by zone air distribution effectiveness; comfort and air quality are separate standards.
- SEER2 is a seasonal average, EER2 is a peak point, and SEER and SEER2 numbers are not interchangeable.
Sources
- ASHRAE Addendum s to Standard 62.1-2016 (Ventilation Rate Procedure text) — ASHRAE, 26 Jul 2019
- ASHRAE Standard 55-2023 fact sheet: Thermal Environmental Conditions for Human Occupancy — ASHRAE (year-level date), 1 Jan 2025
- ASHRAE Standard 55-2023 standard page (scope statement) — ANSI Webstore (year-level date), 1 Jan 2023
- When To Consider Manual J, S, and D — ACCA HVAC Tech Tips blog (undated; read 2026-10-26), 26 Oct 2026
- Room Sensible Heat and CFM: Advanced Psychrometrics Part 1 — HVAC School (read 2026-10-26), 26 Oct 2026
- Psychrometrics lecture notes (PhEn 602, Spring 2009) — New Jersey Institute of Technology (year-level date), 1 Jan 2009
- Navigating the psychrometric chart — Captive-Aire (undated; read 2026-10-26), 26 Oct 2026
- ASHRAE 62.1 ventilation worked example — Envigilance (read 2026-10-26), 26 Oct 2026
- ASHRAE 62.1 explained (Voz and zone distribution effectiveness) — HeatWise (read 2026-10-26), 26 Oct 2026
- Energy Conservation Program: Test Procedure for Central Air Conditioners and Heat Pumps (final rule) — U.S. Department of Energy via the Federal Register, 7 Jan 2025
- Energy Conservation Program: Test Procedures for Central Air Conditioners and Heat Pumps (2017 rule) — U.S. Department of Energy via govinfo, 5 Jan 2017
- Purchasing Energy-Efficient Residential Central Air Conditioners — U.S. Department of Energy, Federal Energy Management Program (read 2026-10-26), 26 Oct 2026
- HVAC efficiency metrics — Trane (read 2026-10-26), 26 Oct 2026
- Compression refrigeration system report (MIT 2.972) — MIT (undated; read 2026-10-26), 26 Oct 2026
- The HVAC refrigeration cycle — Grandy and Associates (read 2026-10-26), 26 Oct 2026
- The cycle of refrigeration, from compressor to condenser — HVAC Laboratory (read 2026-10-26), 26 Oct 2026
- Technology Transitions (AIM Act, HFC restrictions by sector) — U.S. Environmental Protection Agency (read 2026-10-26), 26 Oct 2026
- ANSI/ASHRAE/IES Standard 90.1 (energy standard for buildings except low-rise residential) — ASHRAE (read 2026-10-26), 26 Oct 2026
