HVAC · Lesson 2 of 13Beginner9 min read

Heat, Humidity and Comfort

Before the machines, the physics. This lesson covers the three ways heat moves, the units HVAC uses to measure it, and why the water in the air matters as much as the temperature.

Three ways heat moves

Conduction is heat moving through matter that stays put, by contact. Heat conducts through a wall from the warm side to the cold side, and through the glass of a window.
Convection is heat carried by a moving fluid, such as air or water. Warm air rising from a radiator is natural convection. A furnace blower pushing warm air through ducts is forced convection.
Radiation is heat carried by infrared light and other electromagnetic waves, which need no air at all. Sunshine through a window heats the floor, and you feel chilly sitting next to a cold window even when the air is warm, because your body radiates heat toward it.

A fireplace uses all three: most of the warmth reaches you by radiation from the flames, some heat conducts into the floor, and convection carries hot air up the chimney.

The three ways heat moves, all in one room. Source: OpenStax College Physics 2e, 14.4.

Measuring heat: BTUs, watts and tons

In the United States, heating and cooling are measured in British thermal units. One BTU is the heat needed to raise the temperature of one pound of water by 1°F, about the energy released by burning a match. Equipment is rated by how fast it moves heat, in BTU per hour (BTU/h). A furnace label might say 60,000 BTU/h.

Cooling capacity is often given in tons. One ton of cooling is 12,000 BTU per hour, or about 3.5 kilowatts. The number comes from ice: melting one ton (2,000 pounds) of ice absorbs about 287,000 BTU, and spreading that over 24 hours gives about 12,000 BTU per hour. A “3-ton” air conditioner can remove about 36,000 BTU of heat every hour.

1 ton of cooling = 12,000 BTU/h ≈ 3.52 kW
1 kW ≈ 3,412 BTU/h 1 BTU ≈ 1,055 joules
Melting ice: 334 kJ/kg ≈ 144 BTU per pound; 2,000 lb × 144 ÷ 24 h ≈ 12,000 BTU/h

Units you’ll meet in this course
Unit	What it measures	Example
BTU	An amount of heat	Warming 1 pound of water by one Fahrenheit degree
BTU/h	A rate of heat flow (power)	A furnace’s output; a wall’s heat loss
ton	Cooling capacity: 12,000 BTU/h	A 3-ton air conditioner
watt (W), kilowatt (kW)	Power, including electricity	1 kW ≈ 3,412 BTU/h
cfm	Airflow, in cubic feet per minute	About 400 to 500 cfm per ton of cooling (DOE)
relative humidity (%)	How close the air is to holding all the water vapor it can	EPA’s ideal indoor range: 30 to 50 percent
dew point	The temperature at which air would start to drop water	Why a cold glass “sweats”
R-value	How well a material resists heat flow	A 3.5-inch fiberglass batt is about R-11

Sensible heat and latent heat

Heat that changes an object’s temperature is called sensible heat, because you can sense it with a thermometer. Warming air or water follows a simple rule: heat = mass × specific heat × temperature change.

Heat that changes a substance’s phase, from solid to liquid or liquid to vapor, is called latent heat. While water boils, its temperature doesn’t rise; the energy goes into pulling the molecules apart. The same energy comes back out when the vapor condenses. The amounts are large: at body temperature it takes 2,430 kilojoules to evaporate one kilogram of water (about 1,045 BTU per pound), enough to heat almost six kilograms of water from freezing to boiling.

An air conditioner handles both. It removes sensible heat when it cools the air, and latent heat when water vapor condenses on its cold coil. In a humid climate, a big share of an air conditioner’s work goes to drying the air.

sensible heat: Q = m c ΔT latent heat: Q = m L
water: c = 4,186 J/(kg·°C); L_f = 334 kJ/kg; L_v = 2,256 kJ/kg at 100 °C, 2,430 kJ/kg at 37 °C

Relative humidity and dew point

Air can hold only so much water vapor, and warm air can hold much more than cold air. Relative humidity compares the water vapor in the air with the most the air could hold at its temperature. At 100 percent, the air is saturated.

Cool a parcel of air without adding or removing water, and its relative humidity climbs, because the same vapor is now closer to the limit. The temperature where it reaches 100 percent is the dew point. Cool any surface below the air’s dew point and water condenses on it. That’s why a glass of ice water sweats on a humid day, why windows fog up in winter, and why a cold-water pipe can drip in a damp basement.

OpenStax works an example: air at 25°C holding 9.40 grams of water vapor per cubic meter has a relative humidity of about 41 percent, and its dew point is 10°C. Anything in that room colder than 10°C collects water.

The dew point comes from the Magnus formula with the constants of Alduchov and Eskridge (Journal of Applied Meteorology, 1996): a = 17.625 and b = 243.04 °C. It matches the saturation vapor densities in OpenStax Table 13.5 to within half a percent from 0 to 40 °C. Drag on the chart, or use the sliders, to change the air.

Why an air conditioner dries the air

The indoor coil of an air conditioner runs colder than the dew point of the air blowing across it. Water vapor condenses on the fins, giving up its latent heat, and drips into a drain pan. From there a drain line carries it outdoors, to a floor drain or to a small pump. On a muggy day a home air conditioner can pull gallons of water out of the air.

The air leaving the coil is cool and close to saturated. As it mixes with the room and warms up, its relative humidity drops, so the room ends up both cooler and drier. EPA recommends using air conditioners or dehumidifiers when needed to keep indoor humidity in check, and keeping the drip pans clean and the drain lines flowing.

Comfort is more than temperature

Your body sheds heat by all three methods, plus the evaporation of sweat. When the air is humid, sweat evaporates more slowly, so the same temperature feels hotter. Moving air helps sweat evaporate: the Department of Energy says ceiling fans let you raise the thermostat setting about 4 degrees with no loss of comfort. Cold windows and walls pull heat from you by radiation, which is one reason a well-insulated room feels warmer at the same air temperature.

WhyWhy condensation matters to HVAC

Condensation is useful on the indoor coil, where a drain is waiting for it. Anywhere else it can cause trouble: cold ducts sweating in a hot attic, water dripping from uninsulated cold pipes, and damp spots that grow mold. EPA suggests covering cold surfaces such as cold-water pipes with insulation, and acting quickly when you see condensation on windows, walls or pipes.

Check yourself

Tap an answer to check it, or open “Show the answer.”

One ton of air-conditioning capacity equals:
Show the answer
12,000 BTU per hour. 12,000 BTU per hour, about 3.5 kilowatts: the rate that melts a ton of ice in a day.
What is the dew point?
Show the answer
The temperature at which the air, cooled without adding or removing water, reaches 100 percent relative humidity. At the dew point the relative humidity reaches 100 percent. Any surface colder than that collects water.
Why does a glass of ice water sweat on a humid day?
Show the answer
The glass’s surface is colder than the air’s dew point, so vapor condenses on it. The cold surface cools the air touching it below its dew point, so water vapor condenses there.
Which kind of heat changes a substance’s phase without changing its temperature?
Show the answer
Latent heat. Latent heat. An air conditioner removes latent heat when water vapor condenses on its coil.
What indoor relative humidity does EPA call ideal?
Show the answer
30 to 50 percent. EPA suggests below 60 percent, ideally 30 to 50 percent.

Go deeper

OpenStax College Physics 2e, 13.6 Humidity, Evaporation, and Boiling Humidity, evaporation and boiling, with worked examples.
OpenStax College Physics 2e, 14.3 Phase Change and Latent Heat Phase change and latent heat, in depth.
Science Lab: Gas in a Box See how temperature is the motion of molecules.
Science Lab: The Laws of Thermodynamics The laws of thermodynamics, with simulations.

Sources

I finished this lessonSaved on this device only.

A free course from Teddy Rook Book. It explains how HVAC systems work in general and follows U.S. sources; your equipment’s manuals and the local code are the final word, along with your gas utility for anything involving gas.