What Are Sensible and Latent Heat – And Why Do They Matter for HVAC?
So you're looking at your air conditioner's specs. You see that you've got a 3-ton system, and you know that the tonnage is an expression of your AC's power. But what is it that your AC actually does when it exerts all that power - and how effective is it?
Well, your air conditioner's job is to remove heat from inside your home. That's what it's doing with all that power! However, to ensure that you stay comfortable, you need it to remove both types of heat that accumulate indoors: sensible and latent heat.
This is where things get complicated. Your air conditioner is built to remove both, but different equipment does so with varying levels of effectiveness.
Sensible heat is what registers on your thermostat. It reflects a temperature change.
Let's start with the most straightforward type of heat: sensible heat. Technically speaking, sensible heat refers to the amount of energy needed to increase or decrease the temperature of some substance, independent of phase changes (like a gas-to-liquid phase change).
That might sound complicated, so think of it like this: Sensible heat is reflected by the temperature that appears on your thermostat. When you see it's 75 degrees after running the AC, you're seeing the result of your air conditioner's removal of sensible heat.
Different factors affect the amount of sensible heat your air conditioner needs to remove from your home:
- Insulation volume and quality (R-value)
- Window glass types (low-e or standard), quantities, and sizes
- Number of stories in your home
- Orientation of your house relative to the sun
- Lighting and appliance specifications
- Roof overhang measurements
And so on. When you turn on the AC and it lowers the temperature you see on your thermostat, you've mitigated the effects of heat gain from these sources. Basically, your air conditioner has removed a certain amount of sensible heat.
And yet, you might still be uncomfortable. Why is that?
Latent heat matters, but most air conditioners don't remove enough of it.
Your air conditioner might not be removing enough latent heat from the air. This is a common problem that leads to serious discomfort in spring and summer. What gives?
Latent heat is the heat the results from an increase or decrease in the amount of moisture held by the air. Specifically, it's the amount of energy needed to cause a phase change (for our purposes, liquid-to-gas or gas-to-liquid) without changing the actual temperature of a substance.
Humidity itself isn't latent heat, but humidity contains latent heat. When your thermostat displays a temperature - 75 degrees, say - it's not showing you how much latent heat is contained inside the air. When it's really humid inside your home, you probably feel a lot of latent heat in addition to sensible heat. So it might feel like 80 degrees even though your thermostat reads 75. That's because the thermostat doesn't measure the amount of latent heat in the air. It's only monitoring sensible heat (temperature).
If you've ever complained about humidity, you were actually complaining about the latent heat inside the humidity. Now you know!
Your home's latent heat load depends on several factors:
- Air infiltration through gaps, cracks, and holes in the building envelope
- Household activities like cooking, bathing, cleaning, and exercising
- The number of people inside your home at a given time
When an air conditioner is properly sized for your home, it should remove a sufficient amount of latent heat to keep you comfortable. For example, maybe your air conditioner has a cooling load specification for 80% sensible and 20% latent heat. If that's in line with the actual sensible/latent heat ratio in your home, then you should be in good shape. When you set your thermostat to 75 degrees, it will actually feel like 75 degrees.
Unfortunately, most residential air conditioners aren't properly sized. They're way too big. And when your air conditioner is too big, it's not going to remove enough latent heat from the air.
The solution? Eliminate points of entry for latent heat and remove the rest
If you're already feeling the effects of high indoor humidity this spring, your air conditioner clearly isn't keeping you comfortable. Thankfully, there are still things you can do to make improve your situation (and get rid of that pesky latent heat).
For starters, consider air sealing your home. There are likely several gaps and holes between your living space and your crawl space. If you've got ductwork in your attic, you can bet there are gaps there as well. Humid air gets in through these holes and makes you uncomfortable, but sealing them stops the infiltration.
A whole-home assessment will reveal your biggest sources of air infiltration. With the results in hand, you can prioritize air sealing opportunities. After you've sealed things up, consider the following methods for removing humidity (in rough order of effectiveness and affordability):
- Switch to a combination thermostat and humidistat. Not only will your AC kick on when the temperature exceeds your preferred setting - it will turn on when the relative humidity (RH) inside your home exceeds a specific percentage. To stay comfortable during warmer, humid months, we recommend keeping your RH below 55%. Just be aware that the lower you set it, the more often your air conditioner will run. This setup works best when you have an air handler with variable airflow. The fan runs for longer periods of time and removes more moisture from the air than standard units.
- Install a whole-house dehumidifier. These devices connect to your existing ductwork and pull out humidity without lowering the temperature inside your home. After your home cools to the desired temperature, the dehumidifier takes care of any remaining humidity that the AC failed to remove. Most whole-house dehumidifiers also connect to the outdoors, ventilating your home with fresh air in addition to removing humidity.
- Upgrade your air conditioner to a two-stage or variable speed unit. If your heating and cooling system is already past its prime, consider upgrading to a two-stage or variable speed unit. Your old AC probably operated at one speed: full blast. Two-stage units run at a slower speed for most of the season, only ramping up when the load requirements are at their most extreme (peak summer, usually). Variable speed units change speeds across a wide spectrum depending on the load requirements at any given time. Both types of equipment offer longer AC run times but fewer cycles overall. As a result, they remove more humidity than standard, single-speed systems. They're more efficient, too.
It's not the heat that's uncomfortable…
…it's the humidity! You hear this all the time in the South.
In reality, it's not the humidity that's so oppressive. It's the latent heat inside the humidity that keeps you sweaty and uncomfortable from April to October. You could start saying, "That darn latent heat is getting on my nerves," but that's the sort of comment that might attract strange stares from passerby.
Better to blame the humidity like everyone else while secretly cursing the latent heat. Anyway…
Understanding the correct cooling loads for sensible and latent heat is a balancing act that requires a lot more than rules of thumb (the way most HVAC "pros" approach system sizing) and playing "set it and forget it" with your thermostat. In addition to properly sizing your HVAC system, it's about tightening up the home envelope and using equipment that mitigates issues with excess humidity (and the latent heat within).
Now that you understand how it all works - and why it matters - you're in a better position to take action. Who knows? This summer might be your most comfortable one yet.