SERVING 
Here in the Atlanta area, summer humidity is just as brutal as the summer heat. Thank goodness for air conditioners, right? Assuming your AC is properly sized and in good working order, it should do a pretty good job of cooling your home and removing humidity.
And yet, we get lots of calls from homeowners wondering why their air conditioners don’t remove enough humidity. What gives?
There are four reasons your air conditioner will fail to sufficiently lower your home’s relative humidity, or RH:
- Setting: If you set the fan to “on,” you’ll probably have high RH most of the time.
- Sizing: Many air conditioners are simply too large for the homes where they were installed. It’s a common problem, and it leads to high summertime humidity.
- Preferences: You might prefer an RH level that’s lower than your air conditioner can deliver.
- Setup: If your air conditioner wasn’t configured to move the proper amount of air per ton, it might not do a very good job of removing humidity.
Let’s explore all four of these issues, paying particular attention to the fourth one. If your humidity problem relates to improper setup, a simple adjustment might be all that’s needed to upgrade your comfort.
Setting your air conditioner fan to “on” vs. “auto”
If you run your HVAC system’s fan 24/7, you’re bound to have high RH. Set the fan to “auto,” and you might just solve your humidity problem.
Here’s why running the fan causes humidity spikes. Every time your air conditioner turns on, it removes moisture from the air. That moisture condenses on the indoor coil, where it eventually flows into a drain line and out to the ground. In the meantime, the indoor coil is wet. When you continue operating the fan after the air conditioner stops running, you’re blowing air over that wet coil. The fan blows moisture back into your home, leading to an increase in RH.
Before you try anything else to solve your humidity problem, check your thermostat. Is the fan set to “on?” If it is, set it to “auto.” Now your fan will only run while the air conditioner is busy removing humidity.
Air conditioner sizing and home attributes
Before we discuss AC adjustments, let’s talk about HVAC sizing. One of the most common reasons an air conditioner can fail to remove sufficient humidity from your home is because it’s too big.
Depending on its size, an AC might not have to run very long to cool your home. However, it does have to run for a while if you want it to remove enough moisture to keep you comfortable. Oversized units are more than powerful enough to satisfy your home’s thermostat setting, but they cool the air very quickly.
In other words, they lower the temperature just fine but don’t solve your humidity problem.
To prevent oversizing, an HVAC contractor should perform a Manual J load calculation before installing anything. We’ve actually written an entire article series on load calculations, so we won’t go into too much detail here. The bottom line is that the various attributes of your home will determine which size of AC you need
If a pro doesn’t take the time to measure all of those things and crunch some numbers (most don’t, by the way), you’re likely to end up with a system that is too large for your home.
Contractors who don’t perform a load calculation are more likely to oversize than undersize. That way, they can at least say that the system they sold you cooled your home. It’s why HVAC oversizing is far more common than undersizing.
When your air conditioner is too big, there’s little you can do to make it remove more humidity. You’ll either need to get a new, properly sized system or install a whole-house dehumidifier.
Humidity and comfort preferences
Sometimes, high humidity is a problem even when your air conditioner is the right size for your home. This predicament is often a matter of personal preference.
Most people are comfortable when relative humidity (RH) indoors is 45 to 55%. All else being equal, a properly sized air conditioner should keep your RH within this range. Some will keep it even lower than that, depending on your thermostat setting.
However, some people simply can’t handle any RH over 50% and may insist on keeping humidity below that threshold all the time. When you need additional humidity removal beyond what your air conditioner can offer, a whole-house dehumidifier is usually the way to go. It kicks in when your AC shuts off to remove additional humidity.
Now about that air conditioner adjustment…
As promised, here’s an alternative method for removing humidity: have an HVAC professional tune your AC for optimal operation.
Your air conditioner should be moving a specific amount of air per ton. The proper amount, expressed as cubic feet per minute (CFM), depends on a lot of factors, including HVAC equipment size, ductwork size and design, the amount of interior space you’re cooling, and so on. If the system wasn’t set up properly, it might be moving a lot more air than necessary – even if it’s technically the right size for your home!
If the system is moving air at a rate of 450 CFM and it only needs to move air at a rate of 350-400 CFM, it’s probably meeting your thermostat setting quickly. Unfortunately, it’s not running long enough to remove a lot of moisture from the air. It’s kind of the same problem as oversizing, but it can happen when the system isn’t oversized!
The solution is to lower the fan speed so that the system moves the proper amount of air for an air conditioner operating in a hot, humid climate zone… like ours.
As a result, the air conditioner will run for longer cycles. Sure, it will take a little longer to satisfy your temperature setting, but you’ll be more comfortable because the longer AC cycles get more moisture out of the air.
Just to be clear, making this happen isn’t as simple as turning a dial from “high” to “low.”
When a technician lowers an AC’s fan speed, he or she also needs to check the static pressure and determine how much air the system is moving. CFM will be lower, but it still needs to be high enough to cool the home. The technician also needs to check the temperature drop across the indoor coil before and after adjusting. In addition, he or she should check the refrigerant pressures to ensure the system can cool properly at the lower fan speed.
The best part? You might not need a new air conditioner or a whole-house dehumidifier.
You can often solve your humidity problem by adjusting the AC to move the right amount of air. Needless to say, a simple AC tune-up will always be less expensive than a big equipment purchase!
This fix won’t work for everyone, and some people do need a new, properly sized air conditioner or a whole-house dehumidifier to feel comfortable. But it does work for a lot of air conditioners in a lot of homes around Atlanta.
People with this problem tend to be super grateful that we were able to fix it so easily! At PV Heating, Cooling & Plumbing, we will always analyze your AC’s operation before recommending a more costly solution.
