C-20 Warm-Air Heating, Ventilating and Air-Conditioning exam questions, with answers
10 practice questions for the CSLB Warm-Air Heating, Ventilating and Air-Conditioning trade exam, each with the answer, a plain explanation and the source it rests on.
These are not real exam questions. CSLB does not release its test questions or answer keys, and nobody else has them. We write our own questions from the C-20 outline and the codes, standards and references CSLB lists for the exam. Each answer names its source so you can check it.
The C-20 exam has 4 sections. The questions below are grouped by section, with the share of the exam each one carries. Read the question, pick an answer, then open “Show the answer”. The other questions of the free test, scored section by section, are in the free 25-question C-20 test.
Planning and Estimating · 22% of the exam
HVAC system design and evaluation; load and psychrometric calculations; duct system design and layout; job cost estimation.
1. A customer insists on an air conditioner one size larger than the load calculation calls for. Besides short-cycling, what other problems does DOE's right-sizing fact sheet tie to oversized systems?
- More blower fan power, and more duct leakage at higher duct pressure
- Longer run times that dry the air too much and raise the electric bill
- Less blower fan power, but louder ducts from the higher pressures
- Lower summer peak demand, but faster wear on the compressor
Show the answer
Answer: A. The DOE fact sheet says oversized systems also use more fan power for the blower and often show more duct leakage because the ducts run at higher pressures. Oversized units run shorter cycles, not longer ones, and the fact sheet says they make the summer peak demand worse, not better.
Source: DOE/NREL Right-Size Heating and Cooling Equipment fact sheet, p. 1
2. A kitchen-and-family-room zone has a sensible cooling load of 16,200 Btu/h. The room is held at 75°F and the supply air leaves the registers at 55°F. Using cfm = sensible Btu/h ÷ (1.08 × ΔT), how much supply air does the zone need?
- 875 cfm
- 750 cfm
- 810 cfm
- 180 cfm
Show the answer
Answer: B. ΔT is 75 − 55 = 20°F, so cfm = 16,200 ÷ (1.08 × 20) = 16,200 ÷ 21.6 = 750 cfm. Leaving out the 1.08 gives 810 cfm, multiplying by 1.08 instead of dividing gives about 875 cfm, and using the 4.5 total-heat factor gives 180 cfm.
Source: LibreTexts Coalinga HVAC 7.06 Conversions and Formulas: airflow (CFM) calculation
3. A mechanical room is at 80°F dry bulb and 50% relative humidity; the chart gives a wet bulb of 67°F and a dew point of 59°F. The insulated part of a suction line has a jacket surface at 62°F, and a bare section of the same line is at 45°F. Where will water condense?
- On both surfaces, since both are colder than the 80°F dry-bulb room air
- Only on the bare 45°F section, which is below the 59°F dew point
- On both surfaces, since both are below the 67°F wet bulb
- On neither surface, since air at 50% RH is only half saturated
Show the answer
Answer: B. Water condenses on a surface at or below the air's dew point, here 59°F. The 45°F bare copper is below it and will sweat; the 62°F jacket is above it and stays dry. The wet bulb and dry bulb are not the condensation limit, and 50% RH air still condenses on a surface colder than its dew point.
Source: UNL G1849, Air Temperature (dew point) and Table I (80°F, 50% RH)
Fabrication, Installation, and Startup · 38% of the exam
Duct system fabrication; equipment installation; electrical and mechanical components; plumbing and refrigeration components; air distribution and duct systems; HVAC system startup.
4. A fiberglass duct board plenum will connect to the sheet metal outlet of an air handler, and that joint will see more than 1/2 in w.g. (125 Pa). Which closure does NAIMA's AH105 guide call the only acceptable system for this joint?
- Flexible duct foil tape (UL 181B-FX)
- Glass fabric embedded in mastic (UL 181A-M)
- Heat-activated aluminum tape (UL 181A-H)
- Pressure-sensitive aluminum tape (UL 181A-P)
Show the answer
Answer: B. NAIMA says glass fabric and mastic is the only acceptable closure for connections to equipment or sheet metal ducts under pressures over 1/2 in w.g., and for some fire-rated assemblies. Pressure-sensitive tape bonds to metal only where system pressure permits, heat-activated tape is not recommended on sheet metal, and 181B tapes are for flexible duct.
5. You are choosing where to set a Rheem RP14AY heat pump outdoor unit beside a house. Which placement follows Rheem's location guidance?
- Beside the main bedroom window, on a pad kept separate from the house
- On a pad tied to the foundation, right under the eave for rain shelter
- On a pad not tied to the house, clear of the roof drip edge and bedrooms
- Under the valley where two rooflines meet, on a pad tied to the slab
Show the answer
Answer: C. Rheem says to keep the unit away from the roof drip edge, away from living and sleeping spaces, and on a pad that is not connected to the structure so noise is not carried into the house. Water pouring off a roofline junction can damage the fan and coil, and a pad tied to the foundation or slab transmits noise.
6. Before the first firing of a new Lennox ML180UHVK furnace, you check the field-installed gas piping connections for leaks. Which method do Lennox's instructions call for?
- A 10-psi air test with the furnace's gas valve still connected
- An electronic refrigerant leak detector set to its highest sensitivity
- Household dish soap and water, left on the joints to show later leaks
- A commercial leak-detecting solution, then rinse the piping off well
Show the answer
Answer: D. Lennox says to use a commercially available solution made for leak detection, never an open flame, and to rinse the piping afterward because some soaps corrode certain metals. Lennox also says the furnace must be isolated by closing its manual shut-off valve during any gas supply test at 1/2 psig or more, and a refrigerant detector is not the tool it names.
Source: Lennox ML180UHVK Installation Instructions, Gas Piping: Leak Check
Troubleshooting, Repair, and Maintenance · 20% of the exam
HVAC system diagnostic tests and evaluation; HVAC system maintenance and repair.
7. A family with an old gas furnace and gas water heater reports headaches, dizziness and flu-like symptoms all winter that clear up when they leave home. Based on EPA's list of carbon monoxide sources, what should you check first?
- Mold growing on a dirty filter and being blown into rooms
- A refrigerant leak at the indoor coil giving off toxic vapor
- Back-drafting and a blocked, disconnected or leaking flue
- Very dry air from the furnace drying out the family's sinuses
Show the answer
Answer: C. EPA lists back-drafting from furnaces and gas water heaters, and flues that are blocked, disconnected, leaking or the wrong size, as carbon monoxide sources, and notes CO can cause flu-like symptoms that clear up after leaving home. That pattern calls for a combustion and venting check first; the other causes do not match it and do not rule out CO.
Source: EPA, Carbon Monoxide's Impact on Indoor Air Quality: Sources and Health Effects
8. On a fall check-up of a customer's gas furnace, which group of heating items does the ENERGY STAR maintenance checklist name?
- Refrigerant level, evaporator coil cleaning and the condensate trap
- Flue damper position, humidifier pad and duct leakage at the plenum
- Thermostat batteries, the outdoor coil and the outdoor fan blade
- Gas connections, gas pressure, burner combustion and the heat exchanger
Show the answer
Answer: D. The ENERGY STAR checklist's heating-specific item is to check all gas or oil connections, gas pressure, burner combustion and the heat exchanger, because bad gas connections are a fire hazard and a dirty burner or cracked heat exchanger makes the burner run improperly. Refrigerant level and coil cleaning are its cooling items, and the other groups are not on its heating list.
Safety · 20% of the exam
Personal protective equipment (PPE); job site safety requirements (Cal/OSHA); handling hazardous materials.
9. A technician who wears prescription glasses will spend the day brazing and grinding on a rooftop unit. Which eye protection meets Cal/OSHA §3382(c) for a worker who needs vision correction?
- Contact lenses worn under a pair of plain safety glasses
- His everyday prescription glasses with clip-on side shields
- Plain safety glasses worn in place of his prescription glasses
- Safety goggles designed to fit over his prescription glasses
Show the answer
Answer: D. §3382(c) allows three options: safety spectacles with corrective lenses, safety goggles made to fit over glasses, or goggles with corrective lenses mounted behind the protective lenses. Everyday glasses are not safety spectacles, contact lenses are prohibited around harmful light flashes unless special medically approved procedures exist, and plain safety glasses leave him without his vision correction.
Source: Cal/OSHA Title 8 §3382(c), Cal/OSHA Title 8 §3382 Eye and Face Protection
10. On a furnace change-out in an older home, the old supply plenum has white, unlabeled wrap at its seams that may contain asbestos, and the plenum must come out. What does EPA's asbestos guidance call for?
- Treat it as asbestos; trained, accredited asbestos professionals remove it
- Vacuum up the loose bits with a shop vacuum first, then pull the plenum out
- Cut off a sample yourself and send it to a lab before deciding what to do
- Wet it down well, cut it out yourself, and bag the pieces in heavy plastic
Show the answer
Answer: A. EPA says that if in doubt, treat the material as asbestos; when work will disturb it, removal must be done by a trained and accredited asbestos professional. EPA also says taking samples yourself is not recommended and warns not to dust, sweep or vacuum debris that may contain asbestos.
Where to go from here
How the license works, from experience to fees: how to get a C-20 license. Everything about the course: the C-20 exam page.
Questions from our free C-20 test, written from CSLB License Examination Study Guide, Warm-Air Heating, Ventilating and Air-Conditioning (C-20), form 13E-20 (01-2026), for examinations scheduled on or after April 1, 2026. CSLB notes that the percentages are approximate. Reviewed 14 October 2026. Study guidance only; we are not affiliated with CSLB or PSI.