Thermodynamics Basics for HVAC Reference

Why this matters

Thermodynamics is the rulebook for everything HVAC, refrigeration, and plumbing do with heat and energy. The First Law tells you heat doesn't disappear - it goes somewhere. The Second Law tells you heat naturally flows hot to cold, and going the other way (refrigeration) requires work input. Understanding these laws gives you a mental model that explains why every piece of HVAC equipment works the way it does and why certain diagnostic readings make sense.

The First Law - energy conservation

Energy is neither created nor destroyed; it only changes form.

Q = ΔU + W

Where Q is heat in, ΔU is internal energy change, W is work out.

Practical HVAC applications:

Air conditioning energy balance:

• Electrical input to compressor + fan motors = electrical power in
• Heat removed from indoor air = energy out indoor side
• Heat rejected to outdoor air = electrical power in + heat removed from indoor
• Total heat rejected by outdoor unit > heat absorbed indoors (by the amount of electrical input)

This is why outdoor units feel hot in summer - they reject indoor heat PLUS the work the compressor did to move it.

Heat pump in heating mode:

• Electrical input to compressor + fan motors = electrical power in
• Heat extracted from outdoor air = energy in from outdoors
• Heat delivered to indoor air = electrical input + heat extracted from outdoors

Coefficient of Performance (COP) = heat delivered / electrical input. Typically 3-4 for a modern heat pump at 47 °F outdoor - meaning 3-4 BTU of heat per 1 BTU electricity. The "extra" heat came from outdoor air (it doesn't violate the First Law because outdoor heat is the energy source).

The Second Law - direction of heat flow

Heat flows naturally from higher temperature to lower temperature; the reverse requires work input.

Practical implications:

Heat exchangers transfer heat only when there's a temperature difference. A condenser at 110 °F surrounded by 95 °F outdoor air rejects heat. A condenser at 95 °F in 110 °F outdoor air can't reject - system fails (overheat).

Refrigeration cycles use work to move heat against the natural gradient. The compressor's work is what allows heat to flow from a cold indoor space to a hot outdoor environment.

No process is 100% efficient. Some energy always ends up as low-grade heat (entropy increase). Even the best chiller has efficiency limits.

Key thermodynamic properties

Temperature - measure of average kinetic energy of molecules.

• °F: Fahrenheit, US standard
• °C: Celsius, metric standard, 0 = water freezing, 100 = water boiling at 1 atm
• °R: Rankine, °F + 460, absolute temperature
• K: Kelvin, °C + 273, absolute temperature

Pressure - force per unit area.

• psi: pounds per square inch
• psig: pressure relative to atmosphere (gauge pressure)
• psia: absolute pressure
• inches mercury (in. Hg): often for vacuum and low pressure
• inches water column (in. w.c.): HVAC duct static pressure, gas pressure
• bar / Pa / kPa: metric

Heat (Q) - energy transferred due to temperature difference.

• BTU (British Thermal Unit): heat required to raise 1 lb of water by 1 °F
• Joule (J): SI unit of energy; 1 BTU ≈ 1055 J
• Calorie: 1 cal = 4.184 J

Power - rate of energy transfer.

• Watt (W): 1 J/s
• BTU/hr: 1 W = 3.412 BTU/hr
• Ton of cooling: 12,000 BTU/hr = 3.517 kW (the energy to melt 1 ton of ice in 24 hours)
• Horsepower (HP): 1 HP = 746 W = 2545 BTU/hr

Specific heat (c) - heat required to raise 1 lb of substance by 1 °F.

• Water: 1.0 BTU/lb·°F (the reference)
• Air: 0.24 BTU/lb·°F (sensible only)
• Steel: 0.11 BTU/lb·°F
• Copper: 0.092 BTU/lb·°F

Sensible heat - heat that changes temperature (no phase change).

• Q = m × c × ΔT
• Example: 1 lb of air heated 10 °F = 0.24 × 10 = 2.4 BTU

Latent heat - heat that changes phase (no temperature change).

• Water freezing/melting: 144 BTU/lb at 32 °F
• Water vaporizing/condensing: 970 BTU/lb at 212 °F
• The huge latent heat of water is why steam systems carry so much energy per pound and why dehumidification consumes so much energy.

Enthalpy (h) - total heat content (internal energy + flow work). Used in psychrometrics for HVAC calculations.

Sensible vs latent in HVAC

When AC cools air from 80 °F to 55 °F with humidity removal:

• Sensible cooling: temperature dropped 25 °F (sensible heat removed)
• Latent cooling: water vapor in air condensed on the cold coil (latent heat removed, becomes condensate)

Total cooling = sensible + latent.

In humid climates (Houston, Miami), latent fraction can be 30-40% of total cooling load. In dry climates (Phoenix, Denver), latent fraction is near zero.

This is why:

• Oversized AC in humid climate fails comfort: short cycles don't remove enough latent moisture
• Variable-speed equipment in humid climates wins on comfort: longer run times = better moisture removal
• Hot-humid climates need different equipment design than hot-dry

Saturation, dew point, and humidity

Saturation - air can hold a maximum amount of water vapor at any given temperature. Higher temp = higher max moisture capacity.

Relative humidity (RH) - current moisture / max moisture at current temperature × 100%.

Dew point - the temperature to which air must cool for moisture to start condensing. Air at 80 °F / 60% RH has a dew point of about 65 °F.

Wet-bulb - temperature read on a thermometer with a wet wick (cooled by evaporation). Tells you how much moisture is in the air. In dry climates, wet-bulb is much lower than dry-bulb; in humid climates, the two converge.

These concepts drive HVAC design and diagnostics - see Psychrometrics Fundamentals.

States of matter and phase changes

Most HVAC and plumbing work involves substances in three states:

Solid - fixed shape and volume. Pipe materials, structural elements.

Liquid - fixed volume, takes container shape. Water in supply pipes, condensate, refrigerant in liquid line.

Gas / vapor - variable volume. Air, refrigerant in suction line, natural gas, steam.

Phase changes:

• Melting (solid → liquid) and freezing (liquid → solid) at melting point
• Vaporization (liquid → gas) and condensation (gas → liquid) at boiling point (varies with pressure)
• Sublimation (solid → gas directly, e.g., dry ice)

Phase change consumes / releases LATENT heat without temperature change. This is the trick refrigeration uses - refrigerant boils in the evaporator (absorbing heat without rising in temperature) and condenses in the condenser (rejecting heat without dropping in temperature).

Pressure-temperature relationship in refrigeration

When liquid and vapor coexist in a sealed container, pressure and temperature are locked together at the saturation curve. Knowing one tells you the other.

References

• ASHRAE Handbook - Fundamentals (thermodynamic properties)
• Howell & Buckius, Fundamentals of Engineering Thermodynamics
• Cengel & Boles, Thermodynamics: An Engineering Approach
• "How an AC Works" by Bill Spohn (engineering reference)
• NATE technical study materials (HVAC-applied thermodynamics)