Demystifying Dehumidification Loads

Does it seem like your house is more humid than you remember it (or other houses) being in the past?  If so, you may be right.  It turns out that a lot of factors contribute to how much dehumidification you need and if your air conditioner^† will be able to keep the air dry as well as cool.  It depends on climate, of course, but equipment type, equipment size, efficiency of the house, and more also come into play.  And those factors generally make dehumidification more difficult.  So let’s get started with a first look at demystifying dehumidification.

This article became a bit more technical than I thought it would, so if you want to jump right to the summary, see the second-to-last section.  It’s titled, Your air conditioner (probably) can’t do the whole job.

Sensible heat ratio

For cooling homes, we have two kinds of heat to deal with:  sensible and latent.  I mentioned this in last week’s article but let’s take it a step further now.  Sensible heat is what you need to remove to lower the temperature of the air.  Latent heat is the energy hidden in all those water vapor molecules that make up this thing we call humidity.

You often see cooling expressed as a ratio of the sensible cooling to the total cooling.  (That could be for either the load or the capacity.  See the next section.)  That is, you divide the sensible cooling number by the total cooling number.  In the pie chart above, the sensible heat is 12,000 BTU per hour.  The total would be sensible plus latent, or 12,000 + 3,000 BTU/hr = 15,000 BTU/hr for that particular example.  And that makes the sensible heat ratio (SHR) 12,000 ÷ 15,000 = 0.8.

Understanding this is the first step to demystifying dehumidification.  But what does that tell us?

Cooling load and cooling capacity

You can find out how much sensible and latent heat you need to remove—the cooling load—from the house through a cooling load calculation.  The amount of sensible and latent heat an air conditioner can remove is the cooling capacity of the equipment.  Got that?  Load tells you what the house needs.  Capacity tells you what the equipment can supply.  (More here.)

The sensible heat ratio (SHR) applies to both cooling loads and cooling capacities.  The cooling load SHR tells you how much of your load is related to changing the temperature (sensible heat).  If the pie chart above is for cooling load, the 0.8 SHR tells you that 80 percent of the total cooling load is sensible.  That means 20 percent is latent, or the part related to removing humidity.

We can break down the air conditioner capacity the same way.  Let’s say we’re looking to install an AC that has a capacity SHR of 0.9.  If the total capacity is exactly the same as the total load, then we won’t get enough dehumidification out of it.  Here are the numbers using the pie chart example above:

• 15,000 BTU/hr total capacity
• 0.90 SHR
• ==>  15,000 x 0.90 = 13,500 BTU/hr sensible capacity
• ==>  15,000 – 13,500 = 1,500 BTU/hr latent capacity

So the AC can do 1,500 BTU/hr of dehumidification (latent cooling), but the house needs 3,000 BTU/hr.  Ain’t gonna work!

Modern cooling equipment

As air conditioners have gotten more efficient, they’ve also gotten worse at removing moisture from the air.  Back in the old days when air conditioner efficiency was 10 SEER or lower, they came with sensible heat ratios of 0.7 or 0.75.  Or, in the context of dehumidification, 25 to 30 percent of its capacity was for latent cooling.  That’s a lot of dehumidification capacity!

These days, higher-end equipment often has latent capacities less than 10 percent of the total cooling capacity.  That makes it really hard to keep indoor humidity under control, even at the design conditions.

Timing of sensible and latent loads

I said at the start that this is about demystifying dehumidification.  So here’s the thing that doesn’t get talked about nearly enough.  A load calculation tells you the sensible and latent components of the total cooling load.  And that cooling load happens at the design conditions.  But guess what?  The actual design conditions occur only a few hours a year.

Even worse, the design conditions are about keeping the house cool when it’s about as hot outdoors as it ever gets in your location.  But every day during the cooling season, the temperature starts off low in the morning, climbs in the afternoon, and then drops again overnight.  So most of the time, the house is at part-load conditions.

Now, here’s that part that doesn’t get talked about:  The peak sensible load happens in the evening.  The peak latent load occurs at about 5 am.  (The graph above is from a 2016 report by the National Renewable Energy Lab (pdf) (NREL).)

What!?  You mean we’re selecting air conditioners that by definition do NOT have enough dehumidification capacity when the dehumidification loads are at their peak?

Yep!  This is a game that’s rigged against the air conditioner.  Now our journey of demystifying dehumidification is a lot more complete.

Energy efficiency of the house

Another change that affects indoor humidity is the energy efficiency of the house.  A large number of the homes built in the past 20 years are significantly more efficient than those built in the past.  So how do you think that has affected dehumidification?  Are air conditioners better at dehumidifying efficient homes?  Or worse?

That NREL report mentioned above investigated this, too.  The graph below shows how energy efficiency affects the latent loads by city and by efficiency level of the house.

The Code Home (built to the 2009 International Energy Conservation Code) is the least efficient and has the highest latent loads.  The Prototype Home is the most efficient and has the lowest latent loads.

Infiltration and return duct leakage are the primary factors that change the latent loads as the efficiency level changes.  Both of those things pull humid outdoor air into the home, so there’s not surprise that reducing or eliminating them reduces the latent load.  The internal gains (from breathing, cooking, showering, etc.) and mechanical ventilation loads are pretty much the same across all three efficiency levels.

How much dehumidification can the AC handle?

And how does this relate to whether you need supplemental dehumidification or not?  Well, the graph below shows what percentage of the dehumidification—at the 5 am peak latent load—the air conditioner is capable of handling.

As you can see, only the houses in the dry climates are hitting the 100 percent line.  In the humid cities, the code-built home comes in between 50 and 60 percent.  The air conditioner in the ENERGY STAR home is able to do a bit more than 60 percent.  And the Prototype Home is a bit higher and is close to or above 70 percent in some humid cities.

So when you’re sweating in bed in the morning and throwing the sheets off of you, this is the reason.

Your air conditioner (probably) can’t do the whole job

This has become a more technical article than I thought it would be, so let me summarize what this all about in a way that anyone can understand it.

• An air conditioner does two things:  lowering the temperature and removing water vapor.
• A cooling load calculation tells you how much heat needs to be removed to lower the temperature (sensible heat)
• It also tells you how much heat needs to be removed to remove water vapor (latent heat).
• Modern air conditioning equipment isn’t as good at dehumidifying as older equipment was.
• The cooling load calculation is valid for conditions that occur only a few hours a year, the 1 percent design condition.
• For 99 percent of the time, the cooling load is lower than the design load.
• Worse, the peak latent load happens when the sensible load is low (about 5 am in the NREL study).
• In humid climates, the air conditioner can handle only 50 to 70 percent of the dehumidification needed.
• Energy efficient houses are better at controlling humidity than code-built homes.

And there you have it.  If you’re in a humid climate, you probably need a dehumidifier in addition to your air conditioner to control humidity.  And controlling moisture is one of the 7 steps to good indoor air quality.  Oh, and it also helps with comfort.

About that NREL paper

The NREL paper contains a lot of good information that helps with demystifying dehumidification.  Of course, they had to do that part because the main objective of the paper was to come up with a method to size dehumidifiers.  The reason they looked at the 1%, 2%, and 5% dew point (DP) conditions was to decide which would lead to better sizing of dehumidifiers.

The answer was 2 percent.  In other words, they used the dew point temperature that was exceeded only 2 percent of the hours in a year to do their dehumidification load calculation.  And they did the calculation two different ways.  So using what they called Method 1 with the 2 percent dew point condition led to the best results for sizing dehumidifiers.

As you can see, the parts of their paper that help with demystifying dehumidification dominate this article.  I haven’t gone into the details of their calculations here, but you can download the paper (pdf) and read about them yourself if you want to understand that part.  Or maybe I’ll cover that in a later article.

 

^† When I use the term “air conditioner” here, I’m talking about heat pumps in cooling mode, too.

 

Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia.  He has a doctorate in physics and is the author of a bestselling book on building science.  He also writes the Energy Vanguard Blog.  For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.

 

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