Florida’s got a lot of great qualities: some pretty neat wildlife, beaches all around, and theme parks. On the flip side, Florida also has hurricanes and a lot of humidity. As much of a pain as it might be for residents and contractors alike, humidity is one of my favorite topics. 

We see relative humidity (RH) levels in the 70–80% range consistently on extremely hot, muggy Florida days. However, the cooler nights feel a lot less muggy but often have RH values approaching 100%. What gives? It doesn’t feel more humid, but the RH is higher—why?

It’s the same reason why the supply relative humidity can be near 100%, even though the HVAC system is supposed to dehumidify the air. In this tech tip, we’ll talk a bit about how the same relative humidity level can feel so much different at different temperatures, and we’ll answer the age-old question: 

If the HVAC unit is supposed to remove moisture from the air, how come the relative humidity of the air in the space is <60% and the supply RH is 85–100%?

The Two Types of Humidity

We measure relative humidity (RH) in the work we do, as do meteorologists and weather reporters. RH is the ratio of the moisture in the air to the amount of moisture the air can hold. It is NOT the total amount of moisture in the air. 

The total amount of moisture in the air is absolute humidity, and we measure it in grains. We can see how many grains there are if we plot the temperature and relative humidity on a psychrometric chart. (Use the dry-bulb temperature horizontally and RH vertically; the grains of moisture will be visible on the right-hand side.)

So, why don’t we measure the true amount of moisture as it is instead of as a ratio to the current temperature? 

Absolute humidity may tell us precisely how much water vapor is in the air, but it doesn’t provide the critical context that RH does. RH lets us know how easy or difficult it is to sweat (which is why humid heat is so much more dangerous than dry heat). It also gives us an idea of how close we are to the dew point, which is when water vapor condenses and becomes a liquid. When our ducts and vents sweat, those surfaces are below the dew point. 

That doesn’t even scratch the surface of the role of RH in IAQ. (The chart below speaks for itself; it’s precisely why we aim for an indoor RH of 30–60%.)

So, why does the air in the supply airstream often have an RH of around 100%?

Air is Squishy!

I like to think of air as a sponge, and the water is the humidity. If you drop a sponge in a bucket, it’ll be dripping when you take it out. If you put it in a sink and run the water, that water will run right off and not be absorbed. That’s because the sponge is holding all the water it possibly can. That’s essentially what it’s like when the air is at 100% RH, also known as the dew point.

Think of that sponge as our air mass.

If you squeeze the sponge, you’ll wring the water right out because the sponge can’t hold as much water when it’s smaller. The same is true of colder air; hotter air can hold more moisture than cold air. 

I’ll also bring out the old coffee analogy: a sugar packet will mix into a cup of hot coffee or tea quite easily, but a lot of that sugar will sink to the bottom and not dissolve in a cup of iced coffee or tea.

“Wringing Out” the Air

When air moves over the evaporator coil, heat moves out of it, which brings down the temperature. As the temperature goes down, the air can’t hold as much water, so water vapor condenses onto the coil and drains out. 

That “sponge” gets smaller, and it stays small as it moves into the supply plenum; it may be carrying a lot less total moisture than it was before, but it’s probably holding as much water vapor as it can possibly hold. That’s why the RH is usually close to 100%. (It won’t be exactly 100% due to bypass air that doesn’t make good contact with the coil.)

Magical Reheat

So, how do we go from 100% RH to <60% without a dehumidifier? 

The air reheats when it moves through the ductwork and mixes with the air in the rest of the space. Reheating is like letting a sponge expand back to its original size after squeezing it; it’ll have a lot less moisture. The air is at 100% RH (or close to it) when it goes into the supply plenum, but it will absorb heat and be able to hold more moisture by the time it mixes with the air in the space. As a result, the RH will drop significantly, all without taking any more humidity out of the air.

Dehumidifiers apply the same principle; they consist of an evaporator and condensing coil all in one box. The evaporator takes the moisture out of the air while cooling it, and the condensing coil provides that reheat effect. That’s how dehumidifiers can help us control latent BTUs but at the cost of some sensible BTU gains.

Back to the supply RH, there will be dehumidification whenever the coil is cold enough for moisture to condense on its surface. You’ll just see high relative humidity in the supply because the supply air is cooler than the return air. A high supply RH reading should not be a sign that the coil isn’t dehumidifying as it should. (On that note, aim for long runtimes and slower fan speeds for the best possible dehumidification.)

Conclusion

All that said, RH can be confusing when you consider how the same amount of moisture can translate to very different RH values depending on the temperature. Just remember that the temperature tells us how much moisture the air can hold before we start seeing wet surfaces everywhere—hot air holds more, and cold air holds less. 

For that reason, relative humidity is useful for our diagnostic purposes because it can tell us a lot more about the air as it relates to the safety of the air around us than absolute humidity can. Absolute humidity won’t let us know how close we are to the dew point or our relative risk level for IAQ threats like bacterial and fungal growth.

But if you’re ever curious about the absolute humidity, you’re more than welcome to plot the dry-bulb temperature and relative humidity on a psychrometric chart to see the exact amount of moisture in the air. It might not be that useful for our purposes, but it’s still neat.