NATE / HVAC Trade Certification: Fundamentals (Core)

HVAC Fundamentals: Heat Transfer, Pressure & Temperature, Compressors, Safety, Efficiency Ratings

Audience: working professional / trade certification candidate

HVAC fundamentals cover the physics of heat movement, pressure-temperature relationships, compressor operation, jobsite safety protocols, and system efficiency ratings — the science layer that makes every diagnostic and installation decision make sense.

Key Points

• Heat always moves from warmer to cooler — refrigeration exploits this by creating an artificially cold surface.
• Sensible heat changes temperature; latent heat changes state — both measurable in BTUs.
• 1 BTU = heat required to raise 1 lb of water 1°F.
• PSIG + 14.7 = PSIA; PSIA − 14.7 = PSIG. Standard atmospheric pressure = 14.696 psi.
• Saturation temperature = temperature at which refrigerant changes state at a given pressure.
• Compression ratio = absolute discharge pressure ÷ absolute suction pressure.
• High compression ratio → high discharge temperature → oil breakdown risk above ~300°F.
• Scroll compressor: most common in residential; no valves; continuous compression; most efficient.
• Reciprocating compressor: piston-driven; serviceable valve plates; most common in commercial/light commercial.
• Rotary compressor: high-side shell (hot to touch); accumulator required; spring-loaded vane.
• SEER: cooling efficiency over a season; higher = more efficient. Minimum federal standard has increased over time.
• AFUE: furnace fuel-to-heat efficiency; 80% AFUE = 20% of fuel wasted. High-efficiency ≥ 90% AFUE.
• COP: heat pump efficiency = heat output (watts) ÷ electrical input (watts). COP > 1 is possible because heat is moved, not generated.
• Lock Out Tag Out (LOTO): mandatory before servicing energized equipment — keep the only key with you.
• Never mix oxygen with oil — risk of explosive chemical reaction.
• Refrigerant cylinders: store upright; move with protective cap; larger cylinders use carts.

Why It Matters: Fundamentals questions appear on every NATE exam because a technician who doesn't understand why pressure and temperature are related will misread a manifold gauge every time. Compressor type knowledge determines service approach — you can't change valves on a hermetic scroll. Safety rules exist because the consequences of ignoring them are permanent. Efficiency ratings appear on every equipment proposal and permit application.

Terms To Remember

BTU (British Thermal Unit)

Heat energy to raise 1 lb of water 1°F.

Sensible Heat

Heat that changes temperature; measurable with a thermometer.

Latent Heat

Heat that changes state without changing temperature (melting, boiling, condensing).

Latent Heat of Vaporization

Heat absorbed when liquid becomes vapor.

Latent Heat of Condensation

Heat released when vapor becomes liquid.

Latent Heat of Fusion

Heat absorbed or released during melting/freezing.

Specific Heat

BTUs required to raise 1 lb of a substance 1°F; water = 1 BTU/lb·°F.

Conduction

Heat transfer through direct contact between materials; occurs in solids.

Convection

Heat transfer through fluid movement (liquid or gas).

Radiation

Heat transfer via electromagnetic waves; requires no medium (works in vacuum).

PSIG

Pounds per square inch gauge — pressure relative to atmospheric.

PSIA

Pounds per square inch absolute — PSIG + 14.7.

Saturation Temperature

Temperature at which refrigerant changes state at a given pressure.

Absolute Zero

−460°F (−273°C); lowest possible temperature; no heat energy remains.

Compression Ratio

Absolute discharge pressure ÷ absolute suction pressure.

SEER

Seasonal Energy Efficiency Ratio; cooling efficiency over a full season.

EER

Energy Efficiency Ratio; cooling efficiency at a single operating condition (peak).

AFUE

Annual Fuel Utilization Efficiency; percentage of fuel converted to usable heat in furnaces.

COP

Coefficient of Performance; heat output ÷ electrical input; heat pump efficiency metric.

HSPF

Heating Seasonal Performance Factor; heat pump heating efficiency over a full season.

Hermetic Compressor

Welded steel shell; motor and compressor sealed together; not field-serviceable.

Semi-Hermetic Compressor

Bolted assembly; field-serviceable valve plates and motor; used in commercial.

Clearance Volume

Space at top of reciprocating compressor piston stroke; compressed gas must re-expand.

Lock Out Tag Out (LOTO)

Safety procedure: lock and tag energy sources before servicing; keep only key with you.

Crankcase Heater

Prevents refrigerant from migrating into compressor oil during off cycle.

Step Process Formula

Two Core Calculations: Pressure Conversion (PSIG ↔ PSIA ↔ Saturation Temp) + Compression Ratio — Side-by-Side

Temperature Conversion Formulas

F To C: °C = (°F − 32) ÷ 1.8

C To F: °F = (1.8 × °C) + 32

Part A Pressure Conversion

Part A — Pressure Conversion & Saturation Temperature Lookup

Pressure Temperature Relationship: Pressure and temperature have a DIRECT relationship in a closed refrigerant system — they rise and fall together. This is why a PT chart works: fix the pressure, you know the saturation temperature and vice versa.

Formulas

Psig To Psia: PSIA = PSIG + 14.7

Psia To Psig: PSIG = PSIA − 14.7

Standard Atmosphere: 14.696 psi (≈ 14.7 psi); 29.92 in. Hg on barometer

Worked Examples

Scenario 1

Scenario: Manifold gauge reads 69 PSIG on the low side (R-22). What is the saturation temperature?

Steps

• Step 1 — Convert to PSIA: 69 + 14.7 = 83.7 PSIA.
• Step 2 — Look up R-22 PT chart: 83.7 PSIA corresponds to a saturation temperature of approximately 40°F.
• Step 3 — This 40°F is the evaporator temperature — the refrigerant is boiling at 40°F inside the coil.

Answer: Saturation temperature ≈ 40°F (evaporator temperature)

Scenario 2

Scenario: High-side gauge reads 260 PSIG (R-22). What is the condensing temperature?

Steps

• Step 1 — Convert to PSIA: 260 + 14.7 = 274.7 PSIA.
• Step 2 — Look up R-22 PT chart: 274.7 PSIA ≈ 120°F saturation temperature.
• Step 3 — This 120°F is the condensing temperature — refrigerant is condensing at 120°F in the condenser.

Answer: Condensing temperature ≈ 120°F

Scenario 3

Scenario: R-22 boils at atmospheric pressure (0 PSIG / 14.7 PSIA). What is its boiling point?

Steps

• From PT chart: R-22 at 14.7 PSIA = −41°F.
• This is why refrigerant causes frostbite on contact — it instantly boils and absorbs heat from skin.

Answer: R-22 boiling point at atmospheric pressure = −41°F

Part B Compression Ratio

Part B — Compression Ratio Calculation & Diagnosis

Critical Rule: Always convert PSIG to PSIA (add 14.7) before calculating. Never divide gauge pressures directly.

Formula: Compression Ratio = Absolute Discharge Pressure (PSIA) ÷ Absolute Suction Pressure (PSIA)

Discharge Temp Reference

Item 1

Compression Ratio: 3.3 : 1

Approx Discharge Temp: ~175°F

Item 2

Compression Ratio: 6 : 1

Approx Discharge Temp: ~220°F

Item 3

Compression Ratio: 11 : 1

Approx Discharge Temp: ~280°F

Item 4

Max Oil Temp: ~300°F

Note: Above 300°F oil begins to break down — compressor damage risk

Worked Examples

Scenario 1

Scenario: Suction gauge = 69 PSIG. Discharge gauge = 278 PSIG. What is the compression ratio?

Steps

• Step 1 — Convert suction: 69 + 14.7 = 83.7 PSIA.
• Step 2 — Convert discharge: 278 + 14.7 = 292.7 PSIA.
• Step 3 — Compression ratio = 292.7 ÷ 83.7 = 3.5 : 1.
• Step 4 — Diagnosis: 3.5 : 1 is a healthy ratio for a standard A/C application. Discharge temp ~175–180°F — well within oil stability range.

Answer: Compression ratio = 3.5 : 1 — normal

Scenario 2

Scenario: Blast freezer: evaporator pressure = 15 PSIA (already absolute). Condensing pressure = 278 PSIG. What is the compression ratio?

Steps

• Step 1 — Suction already in PSIA: 15 PSIA.
• Step 2 — Convert discharge: 278 + 14.7 = 292.7 PSIA.
• Step 3 — Compression ratio = 292.7 ÷ 15 = 19.5 : 1.
• Step 4 — Diagnosis: This is extremely high — discharge temp approaches or exceeds 300°F. Oil breakdown likely. Low-temperature applications require two-stage compression or special oil.

Answer: Compression ratio ≈ 19.5 : 1 — dangerously high; two-stage compression recommended

Heat Transfer Reference

Heat Transfer — Modes & Types

Modes

Item 1

Mode: Conduction

Definition: Heat transfer through direct contact; moves through solids.

Hvac Example: Heat moving through the metal wall of a heat exchanger.

Item 2

Mode: Convection

Definition: Heat transfer by fluid movement (liquid or gas).

Hvac Example: Air flowing over evaporator coil; warm air rising and cool air falling.

Item 3

Mode: Radiation

Definition: Heat transfer via electromagnetic waves; no medium required.

Hvac Example: Heat from the sun through a window warming a floor.

Sensible Vs Latent

Sensible Heat

Definition: Heat that changes the TEMPERATURE of a substance; measurable with a thermometer.

Formula: BTU = Weight (lbs) × Specific Heat × ΔT

Hvac Example: Heating air from 55°F to 75°F — temperature rises, no state change.

Latent Heat

Definition: Heat that changes the STATE of a substance without changing its temperature.

Hvac Example: Refrigerant boiling in evaporator at constant 40°F — absorbs large amount of heat with no temperature change.

Types

• Latent heat of vaporization — absorbed when liquid boils to vapor (evaporator).
• Latent heat of condensation — released when vapor condenses to liquid (condenser).
• Latent heat of fusion — absorbed/released during melting/freezing.

Compressor Types

Compressor Types — Key Distinctions for Exam

Types

Reciprocating

Type: Reciprocating

Operation: Piston moves up and down; suction and discharge reed valves control flow; three cycles: suction, compression, discharge.

Key Features

• Clearance volume at top of stroke — re-expansion reduces volumetric efficiency.
• Compression ratio directly affects discharge temperature.
• Semi-hermetic version: field-serviceable valve plates, pistons, and motor.
• Hermetic version: welded shell, not field-serviceable.

Application: Commercial refrigeration, light commercial A/C.

Normal Valve Position: Backseated (fully back) for normal operation.

Scroll

Type: Scroll

Operation: Orbiting scroll moves against fixed scroll creating shrinking pockets that continuously compress refrigerant; discharged at center.

Key Features

• No suction or discharge valves — continuous compression.
• No clearance volume — higher volumetric efficiency than reciprocating.
• Fewer moving parts — more reliable, quieter.
• Most common in modern residential split systems.

Application: Residential and light commercial A/C and heat pumps.

Note: Cannot be field-serviced — replace as a unit.

Rotary

Type: Rotary

Operation: Roller or piston rotates inside cylinder; spring-loaded vane separates suction and discharge gas.

Key Features

• High-side shell — discharge vapor fills the shell, making it HOT to the touch.
• Accumulator required on suction line to prevent liquid slugging.
• Compact and efficient at low compression ratios.

Application: Window A/C units, small residential systems.

Screw

Type: Screw

Operation: Two asymmetric meshing rotors compress refrigerant between rotor lobes; timing gears synchronize rotation.

Key Features

• Continuous compression — no pulsing.
• Cooling jackets manage compression heat.
• Shaft seals required.
• Variable frequency drive (VFD) models available for capacity modulation.

Application: Large commercial and industrial refrigeration.

Centrifugal

Type: Centrifugal

Operation: High-RPM impeller uses centrifugal force to accelerate refrigerant vapor, converting velocity to pressure.

Key Features

• Oil-free versions available (magnetic bearings) — run noisier.
• VFD motor adjusts speed to match heat load.
• Not effective at high compression ratios — suited for high-volume, low-pressure-difference applications.

Application: Large industrial water chillers.

Note: Turbo-style centrifugal compressors use VFD for variable capacity.

Hermetic Vs Semi Hermetic

Hermetic: Welded steel shell; motor cooled by suction vapors passing across it; not field-serviceable; most residential compressors.

Semi Hermetic: Bolted assembly; field-serviceable valve plates, motor, pistons; air-cooled, water-cooled, or suction-gas-cooled models; lubricated by oil pump driven by crankshaft.

Key Safety Note: Suction-cooled hermetic compressors must never be operated in a vacuum — motor windings overheat without vapor flow.

 

Efficiency Ratings

Efficiency Ratings — Definitions & Relationships

Ratings

Item 1

Rating: SEER (Seasonal Energy Efficiency Ratio)

Applies To: Air conditioners and heat pumps (cooling mode)

Definition: Total cooling BTUs delivered in a season ÷ total watt-hours of electricity consumed.

Higher Is: More efficient — lower operating cost.

Typical Range: Minimum federal standard ~14–16 SEER (varies by region/year); high-efficiency 20–26 SEER+.

Limitation: Measured over a range of conditions — more realistic than EER for annual cost comparison.

Item 2

Rating: EER (Energy Efficiency Ratio)

Applies To: Air conditioners (single operating condition)

Definition: BTU/hr of cooling ÷ watts of electricity at a specific test condition (95°F outdoor, 80°F/67°F WB indoor).

Higher Is: More efficient.

Vs Seer: EER is a snapshot; SEER is seasonal. EER is more relevant for hot climates where peak conditions dominate.

Item 3

Rating: AFUE (Annual Fuel Utilization Efficiency)

Applies To: Gas and oil furnaces

Definition: Percentage of fuel energy converted to usable heat over a heating season.

Example: 80% AFUE → 80 cents of every $1 of gas becomes heat; 20 cents lost through flue.

High Efficiency: ≥90% AFUE; uses secondary heat exchanger to extract latent heat from flue gases (condensing furnace).

Minimum Standard: 80% AFUE for most regions; 90%+ required in northern U.S. climate zones.

Item 4

Rating: COP (Coefficient of Performance)

Applies To: Heat pumps (heating or cooling mode)

Definition: Heat output (watts) ÷ electrical input (watts); dimensionless ratio.

Formula: COP = Heat Output (W) ÷ Electrical Input (W)

Why Cop Exceeds 1: Heat pumps MOVE heat rather than generate it — 1 watt of electricity can move 2–4 watts of heat energy. COP of 3 = 300% efficiency in conventional terms.

Typical Range: COP 2.0–4.0 for air-source heat pumps depending on outdoor temperature.

Note: As outdoor temperature drops, heat pump COP decreases — balance point where supplemental heat is needed.

Item 5

Rating: HSPF (Heating Seasonal Performance Factor)

Applies To: Heat pumps (heating mode seasonal)

Definition: Total heating BTUs delivered in a season ÷ total watt-hours consumed.

Higher Is: More efficient. Minimum federal standard ~8.8 HSPF2 (updated metric).

 

Efficiency Rating Comparison Table

Item 1

Rating: SEER

Applies To: A/C + heat pump cooling

Metric: Seasonal; BTU ÷ Wh

Higher Better: Yes

Item 2

Rating: EER

Applies To: A/C single condition

Metric: Point-in-time; BTU/hr ÷ W

Higher Better: Yes

Item 3

Rating: AFUE

Applies To: Gas/oil furnace

Metric: % fuel to heat

Higher Better: Yes

Item 4

Rating: COP

Applies To: Heat pump (any mode)

Metric: Output W ÷ Input W

Higher Better: Yes

Item 5

Rating: HSPF

Applies To: Heat pump heating

Metric: Seasonal; BTU ÷ Wh

Higher Better: Yes

Safety Reference

HVAC Jobsite Safety — Exam-Tested Rules

Electrical Safety

• Electrical shock occurs when you become PART of a circuit — current passes through body.
• Shock severity determined by: voltage, current magnitude, and path of current through body.
• Protective measures: insulated boots, avoid standing in water, use grounding wires.
• LOTO (Lock Out Tag Out): required whenever equipment is being serviced with risk of energization.
• LOTO procedure: apply lock, attach detailed tag explaining why lock is in place, KEEP THE ONLY KEY WITH YOU.
• Never rely on another person to keep equipment de-energized — your lock, your key, your responsibility.

Refrigerant Cylinder Safety

• Store cylinders in UPRIGHT position — keeps vapor (not liquid) in contact with relief valve.
• Move large cylinders only with protective cap in place; use cylinder carts — never roll or drag.
• Cylinder pressure increases as temperature increases (direct relationship).
• Never apply direct flame to a refrigerant cylinder to increase pressure.
• To increase cylinder pressure safely during vapor charging: place cylinder in 90°F water.

Chemical And Combustion Safety

• NEVER mix oxygen and oil — chemical reaction can cause explosion.
• Three common gases requiring pressure reducing regulators: nitrogen, oxygen, acetylene.
• Torch safety: always keep fire extinguisher nearby; use fire shield when soldering near combustibles.
• Hot pipes and motors can cause burns — allow cool-down time before handling.
• Cold refrigerant causes frostbite — wear gloves; R-22 boils at −41°F at atmospheric pressure.
• Lock out equipment before using torches near electrical components.

General Jobsite

• Cold environments: wear warm clothing and waterproof boots in cold/wet conditions.
• Carbon monoxide: colorless, odorless — always install CO detector near combustion appliances.
• Combustion requires three elements: fuel, oxygen, heat (fire triangle) — remove any one to extinguish.
• Acid cleaners for condensers: wear gloves, goggles, ensure ventilation, follow dilution instructions.

Common Confusions

• Students confuse PSIG with PSIA in compression ratio calculations because manifold gauges read PSIG — always add 14.7 to BOTH pressures before dividing; dividing gauge pressures directly gives a wrong ratio that makes every system look more efficient than it is.
• Students confuse sensible heat with latent heat because both are measured in BTUs — sensible heat changes temperature (thermometer moves); latent heat changes state (thermometer stays still while phase change occurs); an evaporator coil removes BOTH simultaneously.
• Students confuse SEER with EER because both measure A/C efficiency — EER is measured at one specific hot-day condition; SEER is a seasonal average across varying conditions; SEER is more useful for annual operating cost; EER is more relevant for peak-load climates.
• Students confuse hermetic and semi-hermetic compressors because both seal the motor inside — hermetic has a WELDED steel shell and cannot be opened; semi-hermetic has BOLTED access covers and can be serviced in the field for valve, piston, and motor repairs.

Quick Questions

A suction gauge reads 58 PSIG and the discharge gauge reads 210 PSIG. What is the compression ratio?

Question: A suction gauge reads 58 PSIG and the discharge gauge reads 210 PSIG. What is the compression ratio?

Correct Answer: Convert to PSIA: suction = 58 + 14.7 = 72.7 PSIA; discharge = 210 + 14.7 = 224.7 PSIA. Compression ratio = 224.7 ÷ 72.7 = 3.09 : 1. This is a healthy ratio for a standard air conditioning application — discharge temperature approximately 170–175°F, well within safe oil operating range.

A gas furnace has an AFUE rating of 80%. If the homeowner spends $1,200 per year on gas, how much is wasted?

Question: A gas furnace has an AFUE rating of 80%. If the homeowner spends $1,200 per year on gas, how much is wasted?

Correct Answer: 80% AFUE means 20% of fuel is lost through the flue. $1,200 × 0.20 = $240 wasted per year. A 96% AFUE condensing furnace would waste only 4% ($48/year) — saving $192 annually in this example.

Why must refrigerant cylinders always be stored upright?

Question: Why must refrigerant cylinders always be stored upright?

Correct Answer: Storing cylinders upright ensures that only vapor — not liquid — is in contact with the pressure relief valve. If a cylinder is tipped on its side, liquid refrigerant contacts the relief valve; if the valve opens (as it should in an overpressure event), liquid refrigerant releases instead of vapor, creating a much larger and more dangerous release.

Exam Answer Frame

Style: 5-mark

Question: Explain the difference between sensible and latent heat, and describe where each occurs in the basic refrigeration cycle.

Model Answer: Sensible heat is heat that changes the temperature of a substance without changing its state — measurable with a thermometer. Latent heat is heat that changes the state of a substance (solid to liquid, liquid to vapor) without changing its temperature. In the refrigeration cycle: (1) Evaporator — the refrigerant absorbs LATENT heat from the indoor air as it changes state from liquid to vapor at a constant saturation temperature (e.g., 40°F). The indoor air simultaneously loses both latent heat (dehumidification — moisture condenses on the coil) and sensible heat (air temperature drops). (2) Condenser — the refrigerant releases LATENT heat to the outdoor air as it changes state from vapor to liquid at the condensing temperature. (3) Suction line — the refrigerant absorbs SENSIBLE heat (superheat) as vapor temperature rises above saturation before reaching the compressor. (4) Liquid line — the refrigerant releases SENSIBLE heat (subcooling) as liquid temperature drops below saturation after the condenser. Understanding both heat types explains why an A/C system both cools air AND removes humidity simultaneously.