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4.1 Principles of Heat Transfer, Temperature & Pressure
Alabama HVAC Contractor License Exam
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4 General Technical Knowledge & Thermodynamics
4.1 Principles of Heat Transfer, Temperature & Pressure 4.2 The Mechanical Refrigeration Cycle & Refrigerant Properties 4.3 Psychrometrics & Air Properties for Humid Climates 4.4 Basic Electricity, Ohm's Law & Electrical Power for HVAC
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12 Electrical Applications & Construction Documents
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4.1 Principles of Heat Transfer, Temperature & Pressure
Key Takeaways Explain This Topic AI
Sensible heat alters substance temperature without phase change (Q = m * c_p * delta T), whereas latent heat drives isothermal phase transitions including fusion (144 BTU/lb for water) and vaporization (970.3 BTU/lb for water).
Heat energy flows naturally from higher to lower thermal potentials via conduction (Fourier's law), convection (fluid movement across heat exchangers), and radiation (electromagnetic emission governed by Stefan-Boltzmann).
One British Thermal Unit (BTU) is the heat required to raise 1 lb of water by 1°F; 1 ton of refrigeration equals 12,000 BTU/hr (melting 2,000 lbs of ice at 32°F in 24 hours).
Gauge pressure (psig) indicates pressure relative to atmospheric baseline, whereas absolute pressure (psia) accounts for the total barometric column (P_psia = P_psig + 14.696 at sea level).
Deep evacuation targets levels below 500 microns where water boils at -12°F, verified by a standing vacuum decay test where pressure must stabilize below 1,000 microns without continuous rising.
Last updated: September 2026
4.1 Principles of Heat Transfer, Temperature & Pressure
Note
Thermodynamic Foundations for Alabama Contractors : The Alabama Heating and Air Conditioning Contractor Examination (administered by Prov) places heavy weight on foundational thermodynamic principles. Heating, ventilating, air conditioning, and refrigeration systems do not "create cold"; rather, they operate as thermodynamic machines designed to remove, transfer, and relocate heat from an enclosed space to an external sink using the laws of physics.
Every mechanical heating and cooling system operates according to fundamental laws of thermodynamics. Mastering heat transfer mechanisms, temperature measurements, absolute and gauge pressure conversions, and deep evacuation standards is essential for sizing, diagnosing, and installing HVAC equipment safely and in full compliance with technical standards.
The Nature of Heat: Sensible Heat vs. Latent Heat
Heat is a form of thermal energy associated with the kinetic motion of molecules within a substance. In HVAC engineering, heat is classified into two distinct operational categories based on whether thermal transfer produces a temperature change or a phase change. +---------------------------------------------------------------------------------------------------+
- | SENSIBLE HEAT vs. LATENT HEAT |
- +----------------------------------------------------+----------------------------------------------+
- | SENSIBLE HEAT | LATENT HEAT ("Hidden Heat") |
- | • Causes a measurable temperature change | • Causes a change of physical state (phase) |
- | • Measured with a standard dry-bulb thermometer | • Occurs at constant temperature & pressure |
- | • Formula: Q = m * c_p * delta T | • Formula: Q = m * h_latent |
- | • Example: Heating liquid water from 32°F to 212°F | • Example: Vaporizing water at 212°F to steam|
- +----------------------------------------------------+----------------------------------------------+
- 1. Sensible Heat
Sensible heat is thermal energy that, when absorbed or rejected by a substance, results in a change in temperature that can be directly sensed and measured using a standard thermometer. During a sensible heating or cooling process, the physical state (solid, liquid, or gas) remains unaltered. Sensible Heat Formula: Q s = m × c p × Δ T \text{Sensible Heat Formula: } Q_s = m \times c_p \times \Delta T Sensible Heat Formula: Q s = m × c p × Δ T
Where:
- Q s Q_s Q s = Sensible heat transferred in British Thermal Units (BTU)
- m m m = Mass of the substance in pounds (lbs)
c p c_p c p = Specific heat capacity of the substance in BTU / ( lb ⋅ ∘ F ) \text{BTU}/(\text{lb}\cdot^\circ\text{F}) BTU / ( lb ⋅ ∘ F )
- Δ T \Delta T Δ T = Temperature differential in degrees Fahrenheit ( T 2 − T 1 T_2 - T_1 T 2 − T 1 )
- 2. Latent Heat ("Hidden Heat")
Latent heat is thermal energy absorbed or released when a substance transitions from one physical state of matter to another without any change in temperature . Because a standard thermometer shows no temperature variation during a phase change, early thermodynamicists termed it "latent" (from the Latin for hidden).
Latent Heat of Fusion : The thermal energy required to change a substance between a solid and a liquid at its melting/freezing temperature. For water at 32 ∘ F 32^\circ\text{F} 3 2 ∘ F , the latent heat of fusion is 144 BTU per pound 144\text{ BTU per pound} 144 BTU per pound .
Latent Heat of Vaporization / Condensation : The thermal energy required to change a substance between a liquid and a vapor at its boiling/condensing saturation point. For water at standard atmospheric pressure ( 212 ∘ F 212^\circ\text{F} 21 2 ∘ F at 14.696 psia 14.696\text{ psia} 14.696 psia ), the latent heat of vaporization is 970.3 BTU per pound 970.3\text{ BTU per pound} 970.3 BTU per pound .
Specific Heat Capacity ( c p c_p c p )
Specific heat capacity is the amount of thermal energy (in BTUs) required to raise the temperature of one pound of a substance by one degree Fahrenheit ( 1 lb by 1 ∘ F 1\text{ lb by } 1^\circ\text{F} 1 lb by 1 ∘ F ).
Substance Physical State Specific Heat Capacity ( c p c_p c p )
Water (Liquid) Liquid ( 32 ∘ F 32^\circ\text{F} 3 2 ∘ F to 212 ∘ F 212^\circ\text{F} 21 2 ∘ F ) 1.00 BTU / ( lb ⋅ ∘ F ) 1.00\text{ BTU}/(\text{lb}\cdot^\circ\text{F}) 1.00 BTU / ( lb ⋅ ∘ F ) (Standard Reference)
Ice (Solid Water) Solid (below 32 ∘ F 32^\circ\text{F} 3 2 ∘ F ) 0.50 BTU / ( lb ⋅ ∘ F ) 0.50\text{ BTU}/(\text{lb}\cdot^\circ\text{F}) 0.50 BTU / ( lb ⋅ ∘ F )
Steam (Water Vapor) Gas (at/above 212 ∘ F 212^\circ\text{F} 21 2 ∘ F ) 0.48 BTU / ( lb ⋅ ∘ F ) 0.48\text{ BTU}/(\text{lb}\cdot^\circ\text{F}) 0.48 BTU / ( lb ⋅ ∘ F ) (approx. 0.50)
Standard Dry Air Gas ( 70 ∘ F 70^\circ\text{F} 7 0 ∘ F , sea level) 0.24 BTU / ( lb ⋅ ∘ F ) 0.24\text{ BTU}/(\text{lb}\cdot^\circ\text{F}) 0.24 BTU / ( lb ⋅ ∘ F )
Copper Solid (lineset / coil tubing) 0.092 BTU / ( lb ⋅ ∘ F ) 0.092\text{ BTU}/(\text{lb}\cdot^\circ\text{F}) 0.092 BTU / ( lb ⋅ ∘ F )
Aluminum Solid (coil fins / casing) 0.215 BTU / ( lb ⋅ ∘ F ) 0.215\text{ BTU}/(\text{lb}\cdot^\circ\text{F}) 0.215 BTU / ( lb ⋅ ∘ F )
Liquid R-410A Liquid ( 80 ∘ F 80^\circ\text{F} 8 0 ∘ F ) 0.42 BTU / ( lb ⋅ ∘ F ) 0.42\text{ BTU}/(\text{lb}\cdot^\circ\text{F}) 0.42 BTU / ( lb ⋅ ∘ F )
Comprehensive Thermal Energy Calculation
To illustrate the combined interaction of sensible and latent heat, consider the total thermal energy required to transform 1 lb 1\text{ lb} 1 lb of ice at 0 ∘ F 0^\circ\text{F} 0 ∘ F completely into superheated steam at 240 ∘ F 240^\circ\text{F} 24 0 ∘ F at standard atmospheric pressure:
Sensible Heating of Ice ( 0 ∘ F 0^\circ\text{F} 0 ∘ F to 32 ∘ F 32^\circ\text{F} 3 2 ∘ F ): Q 1 = 1 lb × 0.50 BTU / ( lb ⋅ ∘ F ) × ( 32 − 0 ) ∘ F = 16.0 BTU Q_1 = 1\text{ lb} \times 0.50\text{ BTU}/(\text{lb}\cdot^\circ\text{F}) \times (32 - 0)^\circ\text{F} = 16.0\text{ BTU} Q 1 = 1 lb × 0.50 BTU / ( lb ⋅ ∘ F ) × ( 32 − 0 ) ∘ F = 16.0 BTU
Latent Heat of Fusion (Melting ice at 32 ∘ F 32^\circ\text{F} 3 2 ∘ F to liquid water at 32 ∘ F 32^\circ\text{F} 3 2 ∘ F ): Q 2 = 1 lb × 144.0 BTU/lb = 144.0 BTU Q_2 = 1\text{ lb} \times 144.0\text{ BTU/lb} = 144.0\text{ BTU} Q 2 = 1 lb × 144.0 BTU/lb = 144.0 BTU
Sensible Heating of Water ( 32 ∘ F 32^\circ\text{F} 3 2 ∘ F to 212 ∘ F 212^\circ\text{F} 21 2 ∘ F ): Q 3 = 1 lb × 1.00 BTU / ( lb ⋅ ∘ F ) × ( 212 − 32 ) ∘ F = 180.0 BTU Q_3 = 1\text{ lb} \times 1.00\text{ BTU}/(\text{lb}\cdot^\circ\text{F}) \times (212 - 32)^\circ\text{F} = 180.0\text{ BTU} Q 3 = 1 lb × 1.00 BTU / ( lb ⋅ ∘ F ) × ( 212 − 32 ) ∘ F = 180.0 BTU
Latent Heat of Vaporization (Boiling water at 212 ∘ F 212^\circ\text{F} 21 2 ∘ F to steam at 212 ∘ F 212^\circ\text{F} 21 2 ∘ F ): Q 4 = 1 lb × 970.3 BTU/lb = 970.3 BTU Q_4 = 1\text{ lb} \times 970.3\text{ BTU/lb} = 970.3\text{ BTU} Q 4 = 1 lb × 970.3 BTU/lb = 970.3 BTU
Sensible Heating of Steam ( 212 ∘ F 212^\circ\text{F} 21 2 ∘ F to 240 ∘ F 240^\circ\text{F} 24 0 ∘ F ): Q 5 = 1 lb × 0.48 BTU / ( lb ⋅ ∘ F ) × ( 240 − 212 ) ∘ F = 13.44 BTU Q_5 = 1\text{ lb} \times 0.48\text{ BTU}/(\text{lb}\cdot^\circ\text{F}) \times (240 - 212)^\circ\text{F} = 13.44\text{ BTU} Q 5 = 1 lb × 0.48 BTU / ( lb ⋅ ∘ F ) × ( 240 − 212 ) ∘ F = 13.44 BTU Total Energy Q total = 16.0 + 144.0 + 180.0 + 970.3 + 13.44 = 1 , 323.74 BTU \text{Total Energy } Q_{\text{total}} = 16.0 + 144.0 + 180.0 + 970.3 + 13.44 = 1,323.74\text{ BTU} Total Energy Q total = 16.0 + 144.0 + 180.0 + 970.3 + 13.44 = 1 , 323.74 BTU
Notice that the latent heat of vaporization ( 970.3 BTU 970.3\text{ BTU} 970.3 BTU ) represents over 73% of the total thermal energy involved in the entire process. This massive energy absorption during vaporization forms the thermodynamic foundation for mechanical refrigeration.
- British Thermal Units (BTU), Enthalpy & Refrigeration Tonnage
- The British Thermal Unit (BTU)
The British Thermal Unit (BTU) is the standard imperial unit of thermal energy in North American HVAC engineering: 1 BTU = Thermal energy required to raise the temperature of 1 pound of pure liquid water by 1 ∘ F \text{1 BTU } = \text{Thermal energy required to raise the temperature of 1 pound of pure liquid water by 1}^\circ\text{F} 1 BTU = Thermal energy required to raise the temperature of 1 pound of pure liquid water by 1 ∘ F
Specifically, this standard is calibrated between 59 ∘ F 59^\circ\text{F} 5 9 ∘ F and 60 ∘ F 60^\circ\text{F} 6 0 ∘ F at a barometric pressure of 14.696 psia 14.696\text{ psia} 14.696 psia .
Refrigeration Tonnage: Derivation and Values
Cooling capacity in commercial and residential air conditioning is rated in tons of refrigeration : 1 Ton of Refrigeration = 12 , 000 BTU/hr = 200 BTU/min \text{1 Ton of Refrigeration} = 12,000\text{ BTU/hr} = 200\text{ BTU/min} 1 Ton of Refrigeration = 12 , 000 BTU/hr = 200 BTU/min
Important
Historical Origin of the Ton : The term originated during the commercial ice harvesting era. One ton of refrigeration is defined as the latent cooling capacity provided by melting one standard short ton (2,000 pounds) of pure ice at 32 ∘ F 32^\circ\text{F} 3 2 ∘ F into water at 32 ∘ F 32^\circ\text{F} 3 2 ∘ F over a 24-hour period : Cooling Rate = 2 , 000 lbs of ice × 144 BTU/lb (Latent Heat of Fusion) 24 hours = 288 , 000 BTU 24 hours = 12 , 000 BTU/hr \text{Cooling Rate} = \frac{2,000\text{ lbs of ice} \times 144\text{ BTU/lb (Latent Heat of Fusion)}}{24\text{ hours}} = \frac{288,000\text{ BTU}}{24\text{ hours}} = 12,000\text{ BTU/hr} Cooling Rate = 24 hours 2 , 000 lbs of ice × 144 BTU/lb (Latent Heat of Fusion) = 24 hours 288 , 000 BTU = 12 , 000 BTU/hr
Enthalpy ( h h h )
Enthalpy, denoted by the symbol h h h , represents the total thermodynamic heat content of a substance (both sensible and latent) above an arbitrary base reference temperature (typically − 40 ∘ F -40^\circ\text{F} − 4 0 ∘ F for refrigerants or 0 ∘ F 0^\circ\text{F} 0 ∘ F for dry air). Enthalpy is measured in BTU per pound of substance ( BTU/lb \text{BTU/lb} BTU/lb ).
Mathematically, enthalpy is defined as the sum of internal thermal energy ( u u u ) plus the flow work of pressure and volume ( P v Pv P v ): h = u + P v h = u + Pv h = u + P v
In refrigeration diagnostics and psychrometrics, enthalpy differences ( Δ h \Delta h Δ h ) quantify the exact amount of heat absorbed across an evaporator or rejected through a condenser.
The Three Fundamental Modes of Heat Transfer
Under the Second Law of Thermodynamics , thermal energy flows spontaneously in only one direction: from a substance of higher temperature to a substance of lower temperature . Heat transfer across HVAC systems occurs via three distinct physical mechanisms: +---------------------------------------------------------------------------------------------------+
- | THE THREE MODES OF HEAT TRANSFER |
- +------------------------------------+----------------------------------+--------------------------+
- | CONDUCTION | CONVECTION | RADIATION |
- | • Direct molecular contact | • Fluid transport (air/water) | • Electromagnetic waves |
- | • Governed by Fourier's Law | • Natural vs. Forced convection | • Stefan-Boltzmann Law |
- | • Fin-and-tube coil conduction | • Blowers moving air across coil | • Solar gain, IR heaters |
- +------------------------------------+----------------------------------+--------------------------+
- 1. Conduction
Conduction is the transfer of heat between two bodies in physical contact, or across adjacent parts of the same body, through the kinetic vibration and collision of neighboring molecules and free electrons. There is no gross movement of the material itself.
Conduction is governed by Fourier's Law of Heat Conduction : Q t = k × A × ( T hot − T cold ) L = A × Δ T R \frac{Q}{t} = \frac{k \times A \times (T_{\text{hot}} - T_{\text{cold}})}{L} = \frac{A \times \Delta T}{R} t Q = L k × A × ( T hot − T cold ) = R A × Δ T
Where:
k k k = Thermal conductivity of the material in BTU / ( hr ⋅ ft ⋅ ∘ F ) \text{BTU}/(\text{hr}\cdot\text{ft}\cdot^\circ\text{F}) BTU / ( hr ⋅ ft ⋅ ∘ F )
- A A A = Surface area perpendicular to heat flow in ft 2 \text{ft}^2 ft 2
- L L L = Thickness of material path in ft \text{ft} ft
- R R R = Thermal resistance of the material ( R = L / k R = L / k R = L / k )
- U U U = Overall heat transfer coefficient ( U = 1 / R total U = 1 / R_{\text{total}} U = 1/ R total )
In HVAC equipment, heat transfers by conduction through copper lineset walls, aluminum coil fins, brazed plate chiller plates, and building insulation envelopes.
2. Convection
Convection is the transfer of thermal energy from one location to another through the actual physical motion of a fluid (liquid or gas). Convection combines microscopic conduction at the fluid-surface boundary layer with bulk fluid displacement. Convective Heat Transfer: Q c = h c × A × ( T s − T ∞ ) \text{Convective Heat Transfer: } Q_c = h_c \times A \times (T_s - T_\infty) Convective Heat Transfer: Q c = h c × A × ( T s − T ∞ )
Where h c h_c h c is the convective heat transfer film coefficient, A A A is heat transfer surface area, and ( T s − T ∞ ) (T_s - T_\infty) ( T s − T ∞ ) is the temperature difference between the surface and fluid stream.
Natural (Free) Convection : Fluid motion is driven purely by natural buoyancy forces caused by thermal density differences (e.g., warm air rising off an electric baseboard heater or cold air descending from a ceiling-mounted gravity coil).
Forced Convection : Fluid motion is driven mechanically by external pressure differentials generated by mechanical equipment—such as furnace blowers forcing air across heat exchangers, condenser fans forcing ambient air through finned coils, or chilled water circulating pumps moving water through air handling units. Forced convection dramatically increases the film coefficient h c h_c h c , allowing compact coils to transfer massive heat loads.
3. Radiation
Radiation is the transfer of thermal energy via electromagnetic waves (principally within the infrared spectrum) that requires no physical contact and no intervening physical medium . Radiant energy travels through a complete vacuum at the speed of light ( 186 , 000 miles/second 186,000\text{ miles/second} 186 , 000 miles/second ).
Governed by the Stefan-Boltzmann Law of Thermal Radiation : E = ϵ × σ × A × T 4 E = \epsilon \times \sigma \times A \times T^4 E = ϵ × σ × A × T 4
Where ϵ \epsilon ϵ is emissivity of the surface, σ \sigma σ is the Stefan-Boltzmann constant, and T T T is the absolute thermodynamic temperature in degrees Rankine ( ∘ R ^\circ\text{R} ∘ R ). Because radiant heat transfer increases with the fourth power of absolute temperature , radiation dominates at high temperatures:
HVAC Applications : Direct gas-fired infrared tube heaters in commercial warehouses, outdoor patio radiant heaters, solar radiation loads on residential building roofs, and radiant floor hydronic heating systems.
Comparison of Heat Transfer Modes in HVAC Systems
Mode Physical Mechanism Intervening Medium Required? Core HVAC System Example
Conduction Molecular vibration & electron transfer Yes (Solid or stationary fluid) Heat passing through copper tube wall and aluminum fins
Convection Bulk macroscopic fluid flow (liquids/gases) Yes (Liquid or gas fluid) Forced air moving across an evaporator coil via supply blower
Radiation Electromagnetic infrared wave emission No (Transfers through vacuum) Solar radiation striking roof deck; infrared tube heater warming floor
Temperature Measurement & Thermodynamic Absolute Scales
Temperature measures the average molecular kinetic energy (intensity of heat) of a substance, independent of the substance's mass. In contrast, total thermal energy (BTU) depends on both temperature and mass. Absolute Zero ──────────── Freeze Point ──────────────── Boil Point
- 459.67°F 32°F 212°F (Fahrenheit)
- 273.15°C 0°C 100°C (Celsius)
- 0°R 491.67°R 671.67°R (Rankine)
- 0 K 273.15 K 373.15 K (Kelvin)
- Relative vs. Absolute Temperature Scales
Fahrenheit Scale ( ∘ F ^\circ\text{F} ∘ F ) : Imperial relative scale where pure water freezes at 32 ∘ F 32^\circ\text{F} 3 2 ∘ F and boils at 212 ∘ F 212^\circ\text{F} 21 2 ∘ F under standard sea level pressure ( 180 ∘ F 180^\circ\text{F} 18 0 ∘ F span).
Celsius Scale ( ∘ C ^\circ\text{C} ∘ C ) : Metric relative scale where water freezes at 0 ∘ C 0^\circ\text{C} 0 ∘ C and boils at 100 ∘ C 100^\circ\text{C} 10 0 ∘ C ( 100 ∘ C 100^\circ\text{C} 10 0 ∘ C span).
Rankine Scale ( ∘ R ^\circ\text{R} ∘ R ) : Imperial absolute temperature scale. Absolute zero is defined as 0 ∘ R 0^\circ\text{R} 0 ∘ R —the theoretical point where all molecular kinetic motion ceases completely. ∘ R = ∘ F + 459.67 ^\circ\text{R} = ^\circ\text{F} + 459.67 ∘ R = ∘ F + 459.67
Kelvin Scale ( K K K ) : Metric absolute temperature scale: K = ∘ C + 273.15 K = ^\circ\text{C} + 273.15 K = ∘ C + 273.15
Scale Conversions ∘ F = ( ∘ C × 1.8 ) + 32 = ( ∘ C × 9 5 ) + 32 ^\circ\text{F} = (^\circ\text{C} \times 1.8) + 32 = \left(^\circ\text{C} \times \frac{9}{5}\right) + 32 ∘ F = ( ∘ C × 1.8 ) + 32 = ( ∘ C × 5 9 ) + 32 ∘ C = ∘ F − 32 1.8 = ( ∘ F − 32 ) × 5 9 ^\circ\text{C} = \frac{^\circ\text{F} - 32}{1.8} = (^\circ\text{F} - 32) \times \frac{5}{9} ∘ C = 1.8 ∘ F − 32 = ( ∘ F − 32 ) × 9 5
Pressure Scales: Atmospheric, Gauge (psig) & Absolute (psia)
Pressure is defined as a perpendicular force exerted per unit area: Pressure ( P ) = Force ( F ) Area ( A ) = Pounds (lbs) Square Inches (in. 2 ) = psi \text{Pressure } (P) = \frac{\text{Force } (F)}{\text{Area } (A)} = \frac{\text{Pounds (lbs)}}{\text{Square Inches (in.}^2\text{)}} = \text{psi} Pressure ( P ) = Area ( A ) Force ( F ) = Square Inches (in. 2 ) Pounds (lbs) = psi
Atmospheric Pressure at Sea Level
Earth's atmosphere exerts downward force due to the gravitational weight of the air column extending upward into space. At sea level at 59 ∘ F 59^\circ\text{F} 5 9 ∘ F , standard atmospheric pressure is: Standard Atmospheric Pressure = 14.696 psia ≈ 14.7 psia = 29.92 in. Hg = 760 mm Hg = 407 in. w.c. = 101.325 kPa \text{Standard Atmospheric Pressure} = 14.696\text{ psia} \approx 14.7\text{ psia} = 29.92\text{ in. Hg} = 760\text{ mm Hg} = 407\text{ in. w.c. = } 101.325\text{ kPa} Standard Atmospheric Pressure = 14.696 psia ≈ 14.7 psia = 29.92 in. Hg = 760 mm Hg = 407 in. w.c. = 101.325 kPa
Gauge Pressure (psig) vs. Absolute Pressure (psia)
Gauge Pressure (psig) : Pressure read on standard Bourdon tube service gauges. A gauge is calibrated to read 0 psig 0\text{ psig} 0 psig when exposed to open atmospheric air at sea level. It measures pressure relative to local ambient atmosphere.
Absolute Pressure (psia) : Total pressure measured relative to a perfect, complete vacuum ( 0 psia 0\text{ psia} 0 psia ). Absolute pressure accounts for the atmospheric air column pressing down on the system. P absolute ( psia ) = P gauge ( psig ) + P atmospheric P_{\text{absolute}} (\text{psia}) = P_{\text{gauge}} (\text{psig}) + P_{\text{atmospheric}} P absolute ( psia ) = P gauge ( psig ) + P atmospheric P psia = P psig + 14.7 (at sea level) P_{\text{psia}} = P_{\text{psig}} + 14.7\text{ (at sea level)} P psia = P psig + 14.7 (at sea level) P psig = P psia − 14.7 (at sea level) P_{\text{psig}} = P_{\text{psia}} - 14.7\text{ (at sea level)} P psig = P psia − 14.7 (at sea level)
Diagnostic Example : If an R-410A suction gauge reads 118 psig 118\text{ psig} 118 psig at sea level, the true thermodynamic absolute pressure is 118 + 14.7 = 132.7 psia 118 + 14.7 = 132.7\text{ psia} 118 + 14.7 = 132.7 psia .
Atmospheric Variations with Altitude
As elevation increases, the height and density of the atmospheric air column decrease, lowering barometric pressure:
Sea Level: 14.7 psia 14.7\text{ psia} 14.7 psia ( 29.92 in. Hg 29.92\text{ in. Hg} 29.92 in. Hg )
Birmingham, AL (~ 600 ft 600\text{ ft} 600 ft ): 14.4 psia 14.4\text{ psia} 14.4 psia ( 29.3 in. Hg 29.3\text{ in. Hg} 29.3 in. Hg )
Huntsville / Sand Mountain, AL (~ 1 , 200 − 1 , 800 ft 1,200 - 1,800\text{ ft} 1 , 200 − 1 , 800 ft ): 13.8 − 14.1 psia 13.8 - 14.1\text{ psia} 13.8 − 14.1 psia ( 28.1 − 28.7 in. Hg 28.1 - 28.7\text{ in. Hg} 28.1 − 28.7 in. Hg )
At higher altitudes, reduced atmospheric pressure lowers the boiling point of liquids, decreases air density (reducing mass airflow across fans and blowers), and elevates the compression ratio of refrigeration compressors.
Inches of Water Column (in. w.c.)
For measuring low-pressure fluids—such as ductwork static pressure and fuel gas manifold pressure—HVAC technicians utilize inches of water column (in. w.c.) : 1.0 psi = 27.7 in. w.c. 1.0\text{ psi} = 27.7\text{ in. w.c.} 1.0 psi = 27.7 in. w.c.
Residential Natural Gas Appliance Manifold Pressure: 3.5 in. w.c. 3.5\text{ in. w.c.} 3.5 in. w.c. (approx. 0.126 psi 0.126\text{ psi} 0.126 psi )
Residential LP / Propane Gas Manifold Pressure: 11.0 in. w.c. 11.0\text{ in. w.c.} 11.0 in. w.c. (approx. 0.397 psi 0.397\text{ psi} 0.397 psi )
Standard Residential Duct Total External Static Pressure (TESP): 0.50 in. w.c. 0.50\text{ in. w.c.} 0.50 in. w.c. (approx. 0.018 psi 0.018\text{ psi} 0.018 psi )
Vacuum Measurement, Micron Scales & Deep Evacuation Standards
Pressures below atmospheric pressure are termed vacuum . HVAC technicians measure vacuum using two distinct scales: Atmosphere (Sea Level) ──────────────────────────────────────────────── Deep Evacuation Goal
- 0 in. Hg Vac 14.7 in. Hg Vac 29.92 in. Hg Vac
- 760,000 Microns 380,000 Microns 0 Microns < 500 Microns
- Coarse Vacuum: Inches of Mercury Vacuum (in. Hg vac)
Standard mechanical manifold compound gauges read vacuum in inches of mercury below atmosphere ( 0 to 30 in. Hg vac 0\text{ to }30\text{ in. Hg vac} 0 to 30 in. Hg vac ). However, a mechanical Bourdon tube gauge lacks the precision required to verify moisture removal. The final fraction of an inch of mercury ( 29.0 to 29.92 in. Hg 29.0\text{ to }29.92\text{ in. Hg} 29.0 to 29.92 in. Hg ) encompasses the entire critical evacuation range.
Precision Vacuum: The Micron Scale ( μ m Hg \mu\text{m Hg} μ m Hg )
A micron is a metric unit of linear measurement equal to one-millionth of a meter ( 1 / 1 , 000 of a millimeter 1/1,000\text{ of a millimeter} 1/1 , 000 of a millimeter ) of mercury column height. 1.0 inch of Mercury (in. Hg) = 25 , 400 microns 1.0\text{ inch of Mercury (in. Hg)} = 25,400\text{ microns} 1.0 inch of Mercury (in. Hg) = 25 , 400 microns Standard Sea Level Atmosphere = 29.92 in. Hg = 760 mm Hg = 760 , 000 microns \text{Standard Sea Level Atmosphere} = 29.92\text{ in. Hg} = 760\text{ mm Hg} = 760,000\text{ microns} Standard Sea Level Atmosphere = 29.92 in. Hg = 760 mm Hg = 760 , 000 microns
The Thermodynamic Purpose of Deep Evacuation
Deep evacuation with a two-stage rotary vane vacuum pump achieves two essential objectives:
Removal of Non-Condensable Gases : Air and nitrogen cannot condense in the condenser. If left in a system, they accumulate in the top of the condenser, causing excessive discharge head pressure, elevated compression ratios, and overheating of compressor motor windings.
Dehydration (Moisture Removal) : Moisture inside a refrigeration system reacts with synthetic polyolester (POE) and polyvinyl ether (PVE) compressor lubricants to form hydrofluoric acid and organic sludge via hydrolysis, destroying motor insulation and plating copper onto bearing surfaces. Deep vacuum drastically lowers the boiling point of water. At room temperature ( 70 ∘ F 70^\circ\text{F} 7 0 ∘ F ), water boils at 0.363 psia 0.363\text{ psia} 0.363 psia ( 18 , 700 microns 18,700\text{ microns} 18 , 700 microns ). When pulled below 500 microns 500\text{ microns} 500 microns ( 0.0097 psia 0.0097\text{ psia} 0.0097 psia ), water boils at − 12 ∘ F -12^\circ\text{F} − 1 2 ∘ F , vaporizing all liquid water into steam, which is swept out through the vacuum pump exhaust.
Deep Evacuation Protocol & The Standing Vacuum Decay Test
Under EPA Section 608 and manufacturer installation standards, deep evacuation must follow a rigorous protocol:
Target Evacuation Level : Pull system vacuum down to below 500 microns (ideally below 300 microns 300\text{ microns} 300 microns ) using large-diameter vacuum-rated hoses and valve core removal tools.
The Standing Vacuum Decay Test : Once the system reaches ≤ 500 microns \le 500\text{ microns} ≤ 500 microns , isolate the vacuum pump from the system by closing the vacuum core tools or manifold isolation valves. Turn off the vacuum pump. Monitor the micron gauge for at least 10 to 15 minutes :
Tight and Dry System (PASS) : The micron level rises slightly (due to refrigerant outgassing from oil) and stabilizes below 1 , 000 microns 1,000\text{ microns} 1 , 000 microns , holding steady. The system is dehydrated, leak-free, and ready for refrigerant charging.
Moisture / Outgassing Present (FAIL - Dehydrate) : The vacuum rises rapidly from 500 microns and levels off between 1 , 500 and 3 , 000 microns 1,500\text{ and }3,000\text{ microns} 1 , 500 and 3 , 000 microns , holding steady. This plateau indicates liquid water is still boiling off inside the piping. Break the vacuum with dry nitrogen to 5 psig 5\text{ psig} 5 psig , sweep the lines, and re-evacuate.
Active Leak Present (FAIL - Leak) : The vacuum rises steadily, continuously, and without leveling off, ascending all the way to atmospheric pressure ( 760 , 000 microns 760,000\text{ microns} 760 , 000 microns ). Pressurize the system with dry nitrogen (and trace refrigerant if permissible) to 150 − 300 psig 150 - 300\text{ psig} 150 − 300 psig and perform bubble or electronic leak detection.
Saturation Temperature & Pressure (The P-T Relationship)
A substance is at its saturation state when liquid and vapor phases coexist in thermodynamic equilibrium at a specific temperature and pressure.
Saturation Pressure : The pressure at which a liquid boils into vapor, or a vapor condenses into liquid, at a given temperature.
Saturation Temperature : The boiling point / condensing point of a fluid at a given pressure. Pressure Increases ⟶ Saturation Temperature Increases \text{Pressure Increases } \longrightarrow \text{ Saturation Temperature Increases} Pressure Increases ⟶ Saturation Temperature Increases Pressure Decreases ⟶ Saturation Temperature Decreases \text{Pressure Decreases } \longrightarrow \text{ Saturation Temperature Decreases} Pressure Decreases ⟶ Saturation Temperature Decreases
In an open pot on an Alabama stove at sea level ( 14.7 psia 14.7\text{ psia} 14.7 psia ), water boils at 212 ∘ F 212^\circ\text{F} 21 2 ∘ F . In a pressurized commercial boiler operating at 100 psig 100\text{ psig} 100 psig ( 114.7 psia 114.7\text{ psia} 114.7 psia ), water cannot boil until it reaches 338 ∘ F 338^\circ\text{F} 33 8 ∘ F . Conversely, in an air conditioning evaporator operating with R-410A at 118 psig 118\text{ psig} 118 psig , the refrigerant boils at exactly 40 ∘ F 40^\circ\text{F} 4 0 ∘ F , absorbing heat from 75 ∘ F 75^\circ\text{F} 7 5 ∘ F indoor return air. Controlling pressure is the sole means by which an HVAC system controls the temperature at which heat is absorbed and rejected.