NATE Certification

Exam blueprint

Sourced from NATE Knowledge Areas of Technical Expertise (KATEs) — Core + AC Service + Heat Pump Service specialties

• Core: safety, tools, soft skills10%
• Core: basic electrical theory + meters10%
• Core: refrigeration cycle fundamentals10%
• AC Service: diagnostics + charging20%
• AC Service: airflow + indoor coil performance10%
• Heat Pump: reversing valve + defrost cycle15%
• Heat Pump: supplemental heat + dual-fuel control10%
• Heat Pump: cold-climate + variable-speed inverters10%
• Customer interaction + work documentation5%

1. 01NATE Core — universal HVACR knowledge

   ~90min

   The Core exam tests baseline knowledge every HVACR tech must have regardless of specialty: safety procedures, meter use, basic electrical theory, and refrigeration-cycle fundamentals. You take Core ONCE; it pairs with any specialty for 2 years.

   • Job-site safety + LOTO for service work

     NATE Core safety questions focus on what a tech does DAILY: lockout/tagout (LOTO) on disconnects before opening any panel, PPE selection (Class 0 rubber gloves for energized testing up to 1,000V, arc-rated faceshield + balaclava for breaker work), ladder selection (Type IA — 300 lb capacity — is the standard for HVAC service), and refrigerant handling safety (no open flame near a vented refrigerant line; refrigerant displaces oxygen in confined spaces). Roof-access work over 6 ft requires fall protection per OSHA 1926.501 — service ladders propped against rooftop units do NOT count as edge protection. The exam tests whether you know to PULL THE DISCONNECT and verify zero voltage with a meter set to AC volts BEFORE removing service panels — even on a cooling-only call where 'just the contactor needs replacing.'

     Reference: OSHA 1910.147 + 1926.501 LOTO + Fall Protection (general industry crossover)

   • Multimeter use — voltage, continuity, capacitance, microamps

     A residential service tech's meter sees four jobs daily. (1) AC VOLTAGE: verify supply at the disconnect (240 VAC ±10%) and at the contactor load side when called. (2) DC VOLTAGE: rare on legacy equipment; common on inverter-driven systems where the DC bus reads 300-400 VDC — KNOW that DC bus capacitors retain dangerous voltage AFTER power-off; bleed before service per the manufacturer. (3) CAPACITANCE: dual-run capacitors fail MORE THAN any other component on residential AC. Test in MICROFARADS (μF) with the cap discharged and out of circuit — a 45/5 dual cap that reads 35 μF / 4 μF is failed. (4) MICROAMPS DC: flame-rectification on gas-furnace flame sensors should read 2-6 μA DC; below ~0.5 μA the board shuts the burner off and locks out. Memorize ranges and what failure mode each indicates.

   • Refrigeration cycle review for service techs

     Core assumes you can identify the four states + four components in your sleep: low-pressure cool VAPOR enters the COMPRESSOR → high-pressure hot VAPOR. Hot vapor enters the CONDENSER → high-pressure warm LIQUID (rejected heat to outdoor air). Liquid passes the METERING DEVICE (TXV/EEV/orifice) → low-pressure cold MIST. Mist enters the EVAPORATOR → low-pressure cool VAPOR (absorbed heat from indoor air). Service-tech additions Core tests: SUPERHEAT is measured at the suction line near the evaporator outlet; SUBCOOLING at the liquid line near the condenser outlet. PRESSURE-TEMPERATURE charts are refrigerant-specific — R-410A and R-32 have very different P/T curves than legacy R-22. The questions reward fluency with the cycle + saturation concept rather than memorizing specific numbers.

   Practice questions (2)

   1. 1. A tech responds to a no-cool call. The condenser fan and compressor are both off; outdoor disconnect is in. Before removing the contactor cover to test for 24V coil voltage, the tech should:

      • A.Pull the disconnect and verify zero voltage at the contactor line side with a meter✓ correct
      • B.Tape the contactor face and shoot a thermal image to spot heat
      • C.Remove the cover quickly with insulated screwdriver to minimize exposure
      • D.Cycle the thermostat off and on to reset the unit

      NATE Core safety + LOTO doctrine: ALWAYS pull the disconnect and meter the line side to verify zero voltage before opening any energized component. The contactor LINE side is at 240 VAC any time the disconnect is in — it does not de-energize when the system is off. Thermal imaging through the cover (B) is a diagnostic add-on, not a substitute for LOTO. 'Quickly with insulated tools' (C) is a fatality vector — insulated tools do not eliminate arc-flash risk. Cycling the thermostat (D) energizes the contactor coil and risks closing the contacts unexpectedly while the tech's hand is inside.

   2. 2. A 45/5 μF dual-run capacitor on a 3-ton AC reads 36 μF on the FAN terminal and 4.7 μF on the HERM terminal at the meter. Diagnosis?

      • A.Capacitor is good — both readings are within 10%
      • B.FAN side has failed (significantly low μF); HERM side is acceptable✓ correct
      • C.HERM side has failed; FAN side is acceptable
      • D.Both sides have failed

      Dual-run cap tolerance is typically ±6% of rated value. 45 μF × 0.94 = 42.3 μF minimum acceptable for the FAN side; 36 μF is well below that → failed FAN side (condenser fan motor will run weakly or not at all). 5 μF × 0.94 = 4.7 μF minimum for HERM; 4.7 μF is right at the edge but technically acceptable per most OEM specs. Replacement should be a 45/5 like-for-like, NOT a 45 + 5 single. 'Within 10%' (A) is too generous; cap tolerance is tighter. 'HERM failed' (C) inverts the readings.

2. 02AC Service Specialty — diagnostic, charging, airflow

   ~120min

   The Air Conditioning Service specialty tests the field diagnostic skills of a working residential/light-commercial cooling tech: subcooling vs. superheat charging, airflow setup, and the most common no-cool / partial-cool failure modes.

   • Charging by subcooling vs. superheat — pick the right method

     First diagnostic decision before adding/removing refrigerant: identify the metering device. TXV / EEV systems: charge by SUBCOOLING measured at the liquid line near the condenser outlet, target typically 8-12°F (verify against the manufacturer's charging chart on the unit). The TXV maintains a roughly constant superheat at the evaporator, so superheat is NOT a useful charge indicator on a TXV system. FIXED-ORIFICE / piston systems: charge by SUPERHEAT measured at the suction line near the evaporator, target depends on outdoor dry-bulb + indoor wet-bulb (consult the SUPERHEAT CHART on the unit, typically 8-25°F across a normal operating envelope). Adding refrigerant to a TXV system raises subcooling (it stacks more liquid in the condenser) without changing superheat much; adding refrigerant to a fixed-orifice system lowers superheat. Charging the wrong way leads to chronic over- or under-charge that looks 'fine on the gauges' to a careless tech.

   • No-cool diagnostic decision tree

     Working diagnostic order on a no-cool call: (1) THERMOSTAT calling? Verify Y at thermostat → Y at air handler. If no Y, the call is a thermostat / wiring problem, not an AC problem. (2) 24V at the contactor coil at the outdoor unit? If yes but contactor not pulling in: failed contactor coil OR open low-voltage wiring outdoors (rodent damage is shockingly common). (3) Contactor pulled in but compressor/fan not running? Check capacitor (μF), check incoming 240 VAC at line side, check compressor/fan windings ohms (open winding = burned-out motor). (4) Compressor + fan both running but no cooling at the registers? Refrigerant issue (low charge, restricted metering device, dirty condenser/evaporator, failed reversing valve on a heat pump stuck in heat). (5) Some cooling but not enough? Dirty filter + dirty evaporator + low blower speed account for the majority of 'weak cooling' calls.

   • Airflow setup — 400 CFM/ton baseline + measurement

     Industry baseline for cooling airflow is 400 CFM per ton of nominal capacity (3-ton system → 1,200 CFM target). Range: 350-450 CFM/ton depending on humidity (lower CFM = more dehumidification + lower SHR; higher CFM = more sensible cooling). Verify in the field by: (1) MEASURING TOTAL EXTERNAL STATIC PRESSURE (TESP) with a manometer across the air handler — typical residential design is 0.5 in. WC; many real systems run at 0.8-1.2 in. WC, indicating undersized ducts or restrictive filters. (2) Reading the BLOWER PERFORMANCE TABLE on the air handler door for the speed tap selected and the measured TESP — the table cross-references CFM. (3) Verifying the EVAPORATOR TEMPERATURE SPLIT (return-air dry bulb minus supply-air dry bulb) — typical 16-22°F at design conditions; over 22°F = airflow too low (or low charge); under 16°F = airflow too high (or high charge). NATE tests the connection between TESP, blower speed tap, and CFM directly.

   • Leak detection + repair workflow

     When refrigerant charge is consistently low: confirm with EVACUATION-AND-WEIGH-IN of factory charge, NOT by topping off. Leak detection methods, in increasing sensitivity: (1) SOAP BUBBLES — finds gross leaks at flare/braze joints; useless on micro-leaks. (2) ELECTRONIC LEAK DETECTOR (heated-diode or infrared) — sensitivity to ~0.1 oz/yr; reliable for service work; calibrate before each call. (3) UV DYE — injected into the system, found with UV lamp + yellow lens after a few days of operation; great for slow leaks on coils. (4) NITROGEN PRESSURE TEST at 250-450 psi (per equipment rating) with a soap solution at every joint — definitive for braze/flare leaks. After repair: full system EVACUATION to 500 microns or below, decay test (rises to no more than 1,000 microns in 10 minutes confirms leak-free + dry), then weigh in the manufacturer-specified factory charge.

   Practice questions (2)

   1. 1. A 4-ton AC with a TXV indoors reads: liquid line 105°F, ambient 95°F. Liquid-line saturation temperature for R-410A at the measured high-side pressure is approximately 124°F. OEM charging chart calls for 8-12°F subcooling. What is the subcooling and what does it indicate?

      • A.Subcooling 19°F — overcharged✓ correct
      • B.Subcooling 19°F — undercharged
      • C.Subcooling 6°F — overcharged
      • D.Subcooling 6°F — undercharged

      Subcooling = saturation temp − measured liquid line temp = 124 − 105 = 19°F. On a TXV system, target subcooling per OEM is 8-12°F. 19°F is well above target, meaning liquid is stacking in the condenser before the metering device → OVERCHARGED. High subcooling ALWAYS = overcharge on a TXV (more refrigerant in the high side = more time in the condenser = colder liquid line). 'Undercharged' (B) inverts the relationship. Options C and D miscalculate the math.

   2. 2. A 3-ton residential system measures TESP 1.0 in. WC at the air handler. The blower-table for the selected speed tap shows 850 CFM at 1.0 in. WC. Per industry baseline, this airflow is:

      • A.High — needs to be reduced to 1,050 CFM/ton
      • B.Acceptable — within 350-450 CFM/ton
      • C.Low — needs more airflow (target ~1,200 CFM)✓ correct
      • D.Irrelevant — TESP doesn't affect cooling

      3-ton baseline: 3 × 400 = 1,200 CFM target (range 1,050-1,350). Measured 850 CFM is well below the floor → airflow is LOW. Cause is likely the high TESP (1.0 in. WC vs. 0.5 design) — restrictive ducts or filter dropping the blower's effective output. Fix is to reduce restriction (clean filter, larger return, replace pleated 1 in. with 4 in. media) BEFORE jumping to a higher speed tap, which raises noise and load. (B) misreads the math; baseline is per-ton, not per-system. (D) ignores that TESP DIRECTLY drives blower output. (A) inverts the diagnosis.

3. 03Heat Pump Service Specialty — reversing valve, defrost, dual-fuel

   ~120min

   Heat Pump Service tests what makes a heat pump different from a straight AC: reversing valve operation, defrost cycle logic, supplemental heat staging, and the modern variable-speed inverter platform.

   • Reversing valve — function, energization, failure modes

     The 4-way reversing valve is the single component that makes a heat pump a heat pump. In COOL mode, it routes compressor discharge to the OUTDOOR coil (condenser) and suction return from the INDOOR coil (evaporator) — same as a straight AC. In HEAT mode, the valve is energized (or de-energized — see below) to swap the flows: compressor discharge to INDOOR coil (now the condenser, rejecting heat to the house) and suction return from OUTDOOR coil (now the evaporator, absorbing heat from outdoor air). ENERGIZATION CONVENTION VARIES: 'O' systems energize the reversing valve in COOL mode (most American OEMs — Carrier, Trane, Rheem); 'B' systems energize in HEAT mode (some legacy + Goodman/Amana). Failure modes: (1) STUCK — valve refuses to shift, system runs in only one mode regardless of call. Tap the valve body firmly while shifting; if it shifts, the slide is sticking — replace. (2) INTERNAL LEAK — valve shifts but leaks high-pressure discharge into the suction side, system has weak capacity in BOTH modes. Detect with a temperature gradient across the valve body in operation.

   • Defrost cycle — initiation, termination, demand vs. timed

     When a heat pump runs in heating, the OUTDOOR coil is operating below freezing under most conditions, and atmospheric moisture forms FROST on the coil. Frost insulates the coil → capacity drops → indoor comfort suffers. Defrost reverses the cycle to melt the frost. INITIATION: legacy units use TIMED defrost (every 30/60/90 min if conditions warrant) checked against an outdoor coil thermistor (frost requires coil temp below ~30°F + active heat call). Modern units use DEMAND defrost — outdoor coil thermistor + ambient thermistor + delta-T calculation; defrosts only when frost is actually building. TERMINATION: when outdoor coil rises to ~50-65°F per the defrost board's thermistor, OR after a maximum time elapsed (typically 10 minutes). During defrost: reversing valve switches to COOL mode (heat is being absorbed from indoor coil to dump on the frosted outdoor coil), outdoor fan is OFF (so defrost heat doesn't blow away), and SUPPLEMENTAL HEAT energizes to keep supply air from feeling cold. The supply-air discharge MUST stay above ~80-85°F during defrost for occupant comfort — that is what the supplemental electric strips or gas furnace are doing.

   • Supplemental heat staging + dual-fuel control

     Heat pumps lose capacity as outdoor temperature drops. The BALANCE POINT is the outdoor temperature where heat-pump capacity equals the home's heat loss; below that, supplemental heat must make up the gap. ELECTRIC SUPPLEMENTAL: 5-15 kW of resistance strips installed in the air handler, staged in 5 kW increments, called by either (a) the thermostat detecting outdoor temp below the configured 'balance point' setpoint, OR (b) the indoor stat sensing temp falling more than ~2°F below setpoint. EMERGENCY HEAT (E call) uses ONLY the strips and locks out the heat pump — used when the outdoor unit has failed. DUAL-FUEL control combines a heat pump (efficient, low-cost above ~35°F) with a GAS FURNACE for supplemental heat (cheaper than resistance below the gas-vs-electric crossover, typically 32-40°F). The thermostat or outdoor sensor energizes the furnace and DEACTIVATES the heat pump below the crossover — running both simultaneously is harmful (high return-air temp into the heat pump's indoor coil pegs the high-pressure switch). NATE tests both the staging logic and the wiring (W2 vs. AUX vs. E terminals).

   • Cold-climate + variable-speed inverter heat pumps

     Modern cold-climate heat pumps (Mitsubishi Hyper-Heat, Carrier Greenspeed, Trane XV20i, Daikin Aurora) maintain 100% capacity to 5°F outdoor, 70%+ to -13°F. Two enabling technologies: (1) VARIABLE-SPEED INVERTER COMPRESSOR — DC bus voltage modulates compressor speed continuously (10-130%) instead of single-stage on/off, allowing the system to OVER-PRODUCE in cold weather rather than under-produce. (2) ENHANCED VAPOR INJECTION (EVI) — a second metering device feeds intermediate-pressure refrigerant vapor into the compressor mid-stroke, increasing mass flow at low ambient. Service implications: DC bus capacitors retain dangerous voltage for several minutes post-power-off — bleed per OEM. Diagnostic interface is a service ANALYZER or a smartphone app, not gauges-and-temps; the system reports faults as numeric codes that decode to specific component failures. Charging is by FACTORY WEIGH-IN of refrigerant; subcooling/superheat targets vary continuously with compressor speed and are not field-chargeable by gauges. Treat as a sealed appliance — diagnose by code, replace components as units.

   Practice questions (3)

   1. 1. A heat pump in heat mode produces only 'cool' (~50°F) supply air. Outdoor unit is running. Diagnosis?

      • A.Reversing valve stuck in cool position OR not energized for heat call✓ correct
      • B.Indoor blower failed
      • C.Supplemental heat is overactive
      • D.Refrigerant overcharge

      If the outdoor unit is running but supply air is cool, the system is actually IN COOL MODE despite the heat call. On an 'O' system (American OEMs), the reversing valve is energized to be in COOL — failure to energize on the heat call leaves the valve in heat OR vice versa depending on convention. Test: with the system in heat call, measure 24V at the reversing valve solenoid and verify the valve body shows the expected hot/cold split (discharge HOT to INDOOR coil = correct heat; HOT to outdoor = stuck in cool). Indoor blower failure (B) would produce no airflow at all. Supplemental heat (C) would HEAT the air, not cool it. Overcharge (D) reduces capacity but doesn't reverse direction.

   2. 2. A dual-fuel system has the gas furnace and heat pump configured to operate together below the balance point. Risk?

      • A.No risk — running both maximizes capacity
      • B.Heat pump high-pressure switch trips because return-air temp into the indoor coil exceeds ~80°F✓ correct
      • C.Gas furnace heat exchanger cracks from low return-air temp
      • D.Refrigerant freezes in the outdoor coil

      Dual-fuel control DEACTIVATES the heat pump below the crossover point — only one heat source operates at a time. Running both simultaneously sends 90-110°F supply air from the furnace BACK as return air into the heat pump's indoor coil (which is now operating as a CONDENSER in heat mode). Condenser inlet at 100°F drives saturation pressure and discharge pressure beyond the high-pressure switch (typically 590-650 psig on R-410A), tripping the unit on lockout. Gas furnaces don't fail from high return temp (C inverts cause/effect); refrigerant doesn't freeze in this fault (D is unrelated).

   3. 3. A defrost cycle initiates on a heat-pump unit. What does the OUTDOOR fan do during defrost, and why?

      • A.Runs at high speed to blow off the frost
      • B.Runs at low speed to supplement defrost heat
      • C.Stops, so defrost heat stays in the outdoor coil✓ correct
      • D.Reverses direction to push warm air outward

      During defrost the reversing valve switches to COOL mode — heat is now being absorbed FROM the indoor coil and DUMPED on the outdoor coil to melt frost. If the outdoor fan kept running, it would blow that defrost heat away into the atmosphere, defeating the cycle. The fan stops; the coil heats from the inside out and frost melts. The fan restarts AFTER the defrost board determines termination conditions are met (coil temp ~50-65°F or max-time expired). 'High speed to blow off frost' (A) is a common misconception — frost is ICE, not snow; airflow doesn't remove it.

4. 04Customer interaction + work documentation

   ~30min

   NATE Core includes ~5% questions on the customer-facing side: explaining diagnostic findings without jargon, written invoices that protect both tech and homeowner, and warranty-claim documentation that OEMs accept.

   • Explain the fault in homeowner language

     The skill: translate a diagnostic finding into a sentence the homeowner understands and can repeat to a spouse. 'Failed dual-run capacitor' becomes 'The starter battery for the outdoor unit's motors gave out; without it the motors can't get going. We replace it with a new $80 part and you're back in business.' Avoid jargon ('TXV', 'subcooling', 'pressure drop') unless the homeowner asks for it. ALWAYS show the homeowner the failed part — visual proof reduces 'do I really need this?' resistance. ALWAYS quote the repair cost BEFORE starting work, even on a $80 cap; surprise bills generate complaints regardless of value. NATE rewards the soft-skill question by recognizing that the customer interaction is part of the technical scope, not separate from it.

   • Warranty claim documentation

     OEM warranties (Carrier, Trane, Rheem, etc.) almost always require: (1) PROOF OF INSTALLATION DATE — registered in the OEM portal within 60-90 days of installation, with serial number. (2) PROOF OF FAILED PART — serial of the failed component AND the replacement, plus a brief failure description. (3) PROOF OF LICENSED INSTALLER — your contractor license number; some OEMs require a NATE-certified tech for the labor portion. (4) DATE OF FAILURE + DATE OF REPAIR. Without these, OEM denies the warranty claim and the contractor eats the part cost. Modern dispatch software (ServiceTitan, Housecall Pro) auto-collects these fields and generates the warranty submission; legacy paper invoices need a dedicated 'Warranty info' section. NATE tests recognition that warranty paperwork is part of the JOB, not paperwork after the job.

   Practice questions (1)

   1. 1. A homeowner calls about a 4-year-old Carrier system that just stopped cooling. The compressor is failed. The contractor wants to claim warranty on the replacement compressor. What is most likely required by Carrier?

      • A.Just a credit-card receipt for the labor
      • B.Original installation registered with Carrier within 90 days, plus failed-part serial and replacement-part serial✓ correct
      • C.A photograph of the failed compressor
      • D.A signed homeowner statement

      OEM compressor warranties (typically 10 years residential) require: (a) the system was REGISTERED with the OEM within 60-90 days of original install (otherwise the warranty defaults to 5 years parts only with no compressor coverage); (b) the failed-part serial AND replacement-part serial documented; (c) a licensed/certified installer doing the work. Receipt for labor (A) doesn't substantiate the failure or original registration. A photo (C) helps but is not the OEM-required evidence. Homeowner statements (D) are not part of the warranty submission.

External resources

• Official
  NATE Candidate Handbook + KATEs (Knowledge Areas) ↗

  NATE's official handbook with the published Knowledge Areas of Technical Expertise (KATEs) for Core and every specialty exam. Pull the KATEs for the specialty you're sitting and use them as your study outline.

• Official
  NATE Practice Tests + Pre-Test ↗

  Official NATE practice tests for Core and major specialties. The format matches the actual exam and is the closest you can get to the real difficulty without sitting it.

• Third-party
  ACCA — Quality Installation + Quality Maintenance Standards ↗

  ACCA Standards 5 (Quality Installation) and 4 (Quality Maintenance) describe the field procedures every NATE-certified service tech is expected to follow — airflow setup, charge verification, refrigerant management. They are the practical reference behind many NATE Service exam questions.