Key Takeaways
- Heat is Energy: In HVAC, we don’t add cold – we remove heat, which is measured in British Thermal Units (BTUs)
- Sensible Heat: Temperature changes you can measure with a thermometer, like cooling a beverage in your fridge
- Latent Heat: Hidden heat that changes a substance’s state (solid/liquid/vapor) without changing temperature
- Refrigeration Cycle: Both sensible and latent heat exchanges occur as refrigerant absorbs heat and changes state
- Managing Comfort: Understanding heat transfer is essential for controlling temperature, humidity, air quality, and circulation
Understanding Heat: The Foundation of HVAC
In the refrigeration and air conditioning world, we don’t add cold – we remove heat! From childhood, we’re taught that heat equals hot, but the terms “hot” and “cold” are strictly for describing comfort. Heat is energy, of which there are many different types. Mechanical energy can be expressed as horsepower, electrical energy as watts, and thermal energy as British Thermal Units (BTUs).
The Birth of the BTU
Where did the term British Thermal Unit come from? It all started with a gentleman from England named Thomas Tregold. He was a railroad engineer who dabbled in heating and ventilation in the mid-1800s, during which time he wrote a book called “The Warming and Ventilating of Public Buildings”. He writes:
> “In order to compare the effects of different kinds of fuel, some convenient measure of effect should be adopted: not only for the purpose of lessening the trouble of calculation, but also to render it more clear and intelligible. I shall, therefore, without regarding the measures of effect employed by others, adopt one of my own, which I have found useful in this and other inquiries of a similar nature. I take as the measure of the effect of a fuel, the quantity, in pounds avoirdupois, which will raise the temperature of a cubic foot of water one degree of Fahrenheit’s scale.”
…hence the birth of the British Thermal Unit! “A cubic foot of water” later became one pound of water, and the modern definition of one BTU became “the amount of energy needed to change the temperature of one pound of water one degree Fahrenheit”.
Now that we understand how heat is quantified, let’s discuss the finer points of two distinguishable types of heat that are crucial for understanding how refrigerant works in your systems.
Sensible Heat: The Heat You Can Feel
Sensible heat is simply heat that can be measured with a thermometer. A thermometer reacts to the intensity of BTUs. It can also be described as a change in the temperature of a substance with no change in its state.
For example, let’s say you’ve brought home your favorite carbonated adult beverage. It’s at room temperature (70°F) and you like it frosty, so you put it in your fridge which is much cooler than the room (35°F). Your beverage gives up its heat to the air inside the fridge and therefore drops in temperature. According to Newtonian physics, high energy will tend to move towards low energy. Basically, heat moves from high heat to low heat. In air conditioning, the temperature difference across the evaporator is also a sensible heat change.
Latent Heat: The Hidden Energy
The word “latent” is Latin for “hidden”. Latent heat is hidden heat and therefore cannot be measured with a thermometer, but make no mistake – it still exists. The best example of latent heat is boiling water. As water is heated to 212°F, the temperature remains constant but the state of the water (solid/liquid/vapor) changes.
Try This Experiment
Fill a pot with distilled water and bring it to a boil. The temperature will remain at 212°F (at sea level) until all of the water has vaporized. You can do the same experiment with crushed ice and distilled water. The temperature will remain at 32°F until all of the ice has melted and only liquid water remains. The temperature of the liquid will then begin to rise until it equals the room temperature. This is a great way to test your digital thermometer for calibration and make the appropriate adjustments.
Application in Air Conditioning
In the process of conditioning air, we’re controlling four things: temperature, humidity, indoor air quality, and circulation. The temperature change is the removal of sensible heat, while the humidity change is the removal of latent heat. The size of an air conditioner depends upon the amount of sensible and latent heat gained inside a conditioned space – something that’s critical when calculating accurate heat loads for equipment sizing.
Heat Exchange in the Refrigeration Cycle
Within the refrigeration cycle, there are both sensible and latent heat changes. For example, the refrigerant enters the evaporator as low temperature, low pressure boiling liquid. As the liquid travels through the evaporator, it absorbs heat from the air around it, causing the boiling liquid to vaporize. The temperature of the liquid will remain the same until ALL of the liquid has vaporized – that’s latent heat exchange.
After all the liquid has boiled off and only vapor remains, the temperature will begin to rise. The state of the vapor won’t change but the temperature will – that’s sensible heat exchange. Both are examples of BTUs being added to the refrigerant.
Understanding these heat transfer principles becomes even more important when working with advanced components like bi-flow TXVs in heat pumps, where refrigerant flow reverses seasonally.
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The Bottom Line
It is essential, as HVACR technicians, that we have a complete understanding of heat. Managing comfort is all about managing heat! Whether you’re troubleshooting a system that’s not removing enough sensible heat (temperature) or dealing with humidity issues from inadequate latent heat removal, these fundamental concepts guide every decision we make in the field.
Check out the link to my YouTube channel for more tips, tricks, and troubleshooting videos and check out The HVAC Know It All podcast here or on your favourite podcast app. Happy HVACing…
