Temperature and Heat
Temperature measures intensity, heat measures quantity. Confusing the two is the root of many bad diagnoses.
What it is
Temperature
A measure of how intense the thermal energy is in a substance, read in degrees. It says nothing about how much energy is present.
Heat
A quantity of thermal energy, measured in BTU. One BTU raises one pound of water one degree Fahrenheit.
A lit match and a bathtub of warm water can be very different temperatures while holding very different amounts of heat. The match is hotter, the tub holds far more energy.
Why it matters
Equipment is rated in BTU per hour, a quantity of heat over time, but the instruments in your hand read temperature. To connect the two you also need to know how much air or water is moving. Temperature alone cannot tell you capacity.
Sensible cooling or heating in air = 1.08 x CFM x temperature differenceThe 1.08 factor bundles the density and specific heat of air at standard conditions.
Heat moved in water = 500 x GPM x temperature differenceScales you will use
| Scale | Water freezes | Water boils | Where you see it |
|---|---|---|---|
| Fahrenheit | 32 | 212 | Most North American field work |
| Celsius | 0 | 100 | Metric equipment and most manufacturer data outside North America |
| Rankine | 491.67 | 671.67 | Absolute scale used in engineering calculations |
| Kelvin | 273.15 | 373.15 | Absolute metric scale |
Absolute scales matter when you work with gas laws, because pressure and temperature relationships only behave predictably when temperature is measured from absolute zero.
Common mistakes
- Judging capacity from temperature split alone without checking airflow.
- Assuming a system is undersized when the real problem is duct leakage moving heat into an unconditioned space.
- Comparing readings taken with instruments that were never checked against each other.
Check your thermometers against each other
Where it is used
Load calculations, equipment selection, charging, airflow verification and combustion analysis all rest on the difference between temperature and heat quantity.
Practice & Mastery
- Temperature and Heat Troubleshooting Scenarios2
Practice is part of HVAC Pro — $5/month or $50/year.
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