How Heat Moves
Explain conduction, convection and radiation, and use BTUs, tons, sensible heat and latent heat to describe what an HVAC system does.
Table of contents
Air conditioners do not make cold. Furnaces and heat pumps do not make comfort out of nothing. Every one of these machines moves heat from a place where it is not wanted to a place where it is, or makes heat and delivers it. If you understand how heat moves and how it is measured, equipment ratings and service readings start to make sense.
What You Will Be Able to Do
- Name the three ways heat moves and give a building example of each.
- Tell heat apart from temperature.
- Convert between BTU/h and tons of cooling.
- Explain sensible heat and latent heat, and why humidity matters to a cooling system.
- Estimate how much heat an air stream carries from its airflow and temperature change.
Heat and Temperature Are Different
Temperature tells you how hot something is. Heat is the amount of energy in it. A bathtub of warm water holds far more heat than a cup of boiling water, even though the cup is at a higher temperature.
Heat always flows from a warmer place to a cooler one. It never flows the other way on its own. There is no such thing as "cold" moving into a room. When a room cools, heat is leaving it. This one rule explains the whole trade. To cool a house, you need something colder than the room air to absorb the heat, and then something hotter than the outdoor air to give it away.
The Three Ways Heat Moves
| Method | How it works | Building example |
|---|---|---|
| Conduction | Heat passes through a solid by direct contact | Heat passing through a wall, a window frame or a metal duct |
| Convection | Moving air or liquid carries heat with it | A blower pushing air across a coil |
| Radiation | Heat travels as energy waves and needs no air or contact | Sun through a window, or a radiant floor warming your feet |
Most HVAC equipment depends on forced convection, which means a fan or a pump moves the air or water on purpose. Conduction moves the heat through the coil metal in the middle of that process. Radiation shows up mainly as a load the building has to deal with, such as a west-facing window on a summer afternoon.
Insulation slows conduction. Air sealing slows unwanted convection. Reflective surfaces and shades cut radiation. That is why two houses of the same size can need very different equipment.
Measuring Heat: BTUs and Tons
A British Thermal Unit (BTU) is the amount of heat it takes to raise one pound of water by one degree Fahrenheit. A kitchen match, burned all the way down, gives off about one BTU.
Equipment capacity is a rate, the number of BTUs moved each hour, so it is written BTU/h. A furnace with an output of 60,000 BTU/h delivers 60,000 BTUs into the house each hour it runs.
Cooling capacity is often given in tons. One ton of cooling equals 12,000 BTU/h. The name comes from the heat needed to melt a ton of ice in 24 hours, which is 288,000 BTUs, or 12,000 each hour. Most homes use somewhere between 1.5 and 5 tons.
Worked example: a condensing unit is rated at 36,000 BTU/h. Divide by 12,000 and you get 3 tons. Going the other way, a 2.5-ton system moves 2.5 x 12,000 = 30,000 BTU/h.
Sensible Heat and Latent Heat
These two terms come up constantly, and they describe different jobs.
Sensible heat changes the temperature of something. You can sense it with a thermometer. Warming air from 70°F to 95°F adds sensible heat.
Latent heat is absorbed or released when a substance changes state, such as liquid to vapor, while its temperature stays the same. Water at 212°F turning into steam at 212°F is taking in latent heat. A thermometer shows no change, but a lot of energy is moving. It takes about 970 BTUs to boil one pound of water at 212°F, compared with just 1 BTU to warm that pound by one degree.
That huge difference is why refrigeration systems work. A refrigerant soaks up far more heat by boiling than it could by warming up.
Latent heat also matters in the air itself. When an air conditioner pulls moisture out of air, the water vapor condenses on the coil and gives up its latent heat. The thermometer does not show that work, but the compressor still has to do it. So a cooling system's total load has two parts:
- Sensible load: lowering the air temperature.
- Latent load: removing moisture from the air.
A home can feel clammy at 74°F if the system handled the sensible load but not the latent load. Humid climates demand more latent capacity than dry ones.
How Much Heat an Air Stream Carries
You can estimate sensible heat in moving air with a standard field formula:
BTU/h = 1.08 x CFM x temperature difference
CFM means cubic feet of air per minute. The 1.08 is a constant for standard air, and it changes a little at high elevation.
Worked example: air moves across a coil at 1,000 CFM and drops 20°F. The sensible heat removed is 1.08 x 1,000 x 20 = 21,600 BTU/h. That counts only the temperature change. Any moisture the coil removed is extra latent work on top of that number.
Common Mistakes
| Mistake | What it costs |
|---|---|
| Treating tons as weight | A "3-ton" unit is a heat-moving rate, so sizing talk goes wrong from the start |
| Reading only temperature | You miss the latent load and cannot explain a sticky house |
| Thinking cold moves in | You look for the wrong cause when a space will not cool |
| Using BTU and BTU/h as if they match | One is an amount and the other is a rate, so load math comes out wrong |
Check Your Understanding
A homeowner says cold air is leaking in through a closed window on a winter night. What is really happening? Heat is leaving the room. It moves through the glass and frame by conduction and through gaps by convection, because heat flows from warm to cool.
A cooling load is listed as 48,000 BTU/h. How many tons is that? 48,000 divided by 12,000 is 4 tons.
A dehumidifying coil removes a lot of water but the supply air temperature barely moves. Is the coil doing any work? Yes. It is removing latent heat. A thermometer shows sensible change only.
Air flows at 800 CFM and rises 50°F across a heat exchanger. About how much sensible heat is it picking up? 1.08 x 800 x 50 = 43,200 BTU/h.
Key Takeaways
- Heat flows from warm to cool on its own. Cooling means removing heat.
- Conduction, convection and radiation are the three ways heat moves.
- One BTU raises one pound of water one degree Fahrenheit, and one ton equals 12,000 BTU/h.
- Sensible heat changes temperature, and latent heat changes state.
- A cooling system handles both temperature and moisture, so judge it on both.