Sensible Heat

Sensible heat is the amount of heat, (molecular motion), you can measure with a thermometer. It’s the amount of heat you add or remove to a substance that changes a substance temperature but does not change the state of a substance.

Latent Heat

Latent heat is heat added or removed that will change the state of a substance but does not affect the temperature of the substance. Water to steam is a change of state.

Example: We’ll look at a pot of water on a stove and see what happens. Using distilled water will give more accurate numbers

• The pot of water heats up from 40°F to 90°F. The water is getting nice and warm, but there’s no change of state (boiling/steam). So this is only sensible heat added
• As the pot of water continues to accept heat from the burners, the pot eventually reaches 212°F, the boiling point. There’s a change of state happening as the liquid water changes to steam. Even with heat still being applied to the pot of water, the pot will remain at 212°F. This heat energy being applied is the latent heat causing the change in state from liquid to vapor steam

What this means on a real service call

Sensible heat is the part you can track with a thermometer. When air warms from 70 degrees to 75 degrees without changing state, that is sensible heat at work. Latent heat is the hidden part. When water evaporates, or when moisture in the air condenses on a cold coil, a large amount of heat moves without the thermometer showing the full change.

That is why a cooling system does two jobs at once: it cools the air, which is sensible, and it removes moisture, which is latent. On a humid day, a system can run for a long time and still feel clammy if it is not removing enough moisture, even though the air temperature has dropped. On a dry day, the same system may drop the temperature quickly and cycle off, because there is little moisture to remove.

Numbers and measurements a technician actually sees

Watch both dry-bulb and wet-bulb temperatures, or dry-bulb temperature and relative humidity, when you suspect a latent problem. A dry-bulb reading alone will not tell you how much moisture is in the air. For example, air at 75 degrees Fahrenheit and 50 percent relative humidity feels very different from air at 75 degrees and 80 percent relative humidity, even though a simple thermometer reads the same.

On an evaporator coil, condensation is latent heat being removed. Water dripping from a condensate drain in summer is normal, and a dry drain on a very humid day is a clue worth checking. A frozen coil, a blocked drain, or low airflow can all change how much latent heat the coil can remove. Also remember the familiar change-of-state points for water at normal atmospheric pressure: it freezes and melts around 32 degrees Fahrenheit, and boils around 212 degrees Fahrenheit. Refrigerants change state at very different temperatures and pressures, so do not apply water’s numbers to a refrigerant circuit.

A common mistake

A common mistake is to assume that if the temperature has not changed, nothing has happened. Boiling water stays around 212 degrees Fahrenheit while it turns to steam, even though a lot of heat is being added. The same idea applies to an evaporator: refrigerant can absorb a large amount of heat while it changes from liquid to vapor without a large temperature change in the refrigerant itself. Judging a coil only by a single temperature reading can therefore hide a real capacity problem.

A practical checking step

If a home feels humid even though the air conditioner is cooling, check in this order: first, the filter and airflow, because low airflow hurts both sensible and latent capacity. Second, the condensate drain, to confirm water is actually leaving the system. Third, the return and supply temperatures, and the indoor humidity if you have a hygrometer. Fourth, whether the system is oversized and short-cycling, which can cool the air quickly but shut off before it has time to remove moisture. Do not open a refrigerant circuit or an electrical panel unless you are qualified and the power is safely isolated, and do not add refrigerant to fix a humidity problem without finding the real cause.

A useful habit is to record dry-bulb temperature, humidity, and supply and return temperatures on the same visit. Together they tell you whether the complaint is mostly sensible, mostly latent, or both. That habit will save you from replacing parts for a problem that was really airflow or humidity.

Related posts

Start with About heat for the basic idea of heat as molecular motion, then see Types of Heat Transfer for how that heat actually moves from one place to another.