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HVAC Humidity Control: 5 Reasons Your AC Is Ineffective

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Isn’t air conditioning supposed to remove humidity? So why is yours not doing the job?

If you’re sweating your way through another muggy August, read on to learn more about HVAC humidity control and why your AC might not be up to the task.

Then we’ll tell you 4 ways you can get an older unit to remove more moisture so your space doesn’t feel like a tropical jungle.

HVAC humidity control problem #1: Your AC unit is too large for the space

First of all, when we talk about AC size, it’s about cooling capacity, not the physical size of the unit. That’s not the only part that people misunderstand about AC size.

If you’re not an HVAC expert, it may seem counter-intuitive to say that your air conditioner is too big to do the job effectively. Wouldn’t a bigger system be able to provide better HVAC humidity control as well as cooling?

Unfortunately, the answer is no. The fact is, an oversized AC system does a poor job of controlling both temperature and humidity. That’s because the powerful compressor turns on an off frequently, so the system never runs long enough to remove humidity from the air.

Your AC size needs to be carefully calculated using a load calculation process. If your AC was chosen and installed by a builder rather than a qualified HVAC company, there’s a good chance that it was done incorrectly. That’s how many homes and businesses end up with oversized units that provide poor HVAC humidity control.

You may actually need to install a smaller capacity AC system to fix this problem.

Related article: New York HVAC Systems: 8 Reasons Bigger is Not Always Better

HVAC humidity control problem #2: Your AC is limited to a single speed

An air conditioner that’s only capable of running at one speed— basically, on or off—can have the same problems as one that’s too large when it comes to HVAC humidity control.

When the unit can only run at one speed, it turns on and runs full blast until the air reaches set temperature. Then it turns off until the temperature once again goes above the thermostat threshold. That often means the unit doesn’t run consistently enough to remove humidity.

That’s especially true at times when the temperature isn’t too high, but the air is very humid.

When it’s time for a new AC system, think about investing in a modulating HVAC system such as a VRF system. A VRF system can run longer at a lower capacity, so it does a much better job of HVAC humidity control (and providing more consistent temperatures). Another plus: you’ll notice lower energy bills as well.

Related article: 7 Reasons to Choose VRF Technology for Your New Air Conditioning

HVAC humidity control problem #3: Negative air pressure in your space

Negative air pressure is the result of a poorly designed ventilation system. In a nutshell, it means that you are venting too much air from the space.

That can happen, for example, in a restaurant with a powerful kitchen exhaust system.

When you have negative pressure, the air tries to balance itself by drawing in more outside air any way that it can. For example, whenever someone opens an outside door, a gust of wind seems to enter the room. Outside air is also drawn in through every little opening in the building.

So what happens when the relative humidity outside gets into the 70’s, 80’s and above?
You guessed it, all that muggy air is being drawn into the building. If the problem is bad enough, your AC may not be able to keep up, and HVAC humidity control is compromised.

Changes to your ventilation system design may be needed to correct this issue.

HVAC humidity control problem #4: You’re using the wrong thermostat setting

You may think you’re getting rid of more humidity by using your thermostat’s FAN ON setting. When you use this setting, the fan runs continuously even when the AC is not running.

The fan does move air even without the AC running, so you might feel like you’re getting some cooling benefit. But you’re making the humidity conditions worse. You can end up blowing the moisture your AC has removed back into the space before it has a chance to drain away.

HVAC humidity control problem #5: Your older unit simply can’t handle the load

As your air conditioner ages, parts wear and it may not run as efficiently as it used to. That tendency is increased when the system hasn’t been regularly maintained. In addition to inconsistent temperatures and inadequate cooling, you’ll start to notice poor HVAC humidity control.

At this point, you may be facing a repair or replace decision. Can some system tweaks be enough to restore adequate performance, or is it time to bite the bullet and invest in a new one? Here’s a resource that can help you decide: Repair or Replace? A guide to making an informed choice when your HVAC system is down.


Should you repair or replace that broken HVAC equipment? Download the complete guide.

If you’re not quite ready to replace your air conditioner yet, you can try these measures to boost your system’s ability to fight that indoor mugginess.

4 strategies for enhancing HVAC humidity control in an existing system

1. Clean the coils
This is the easiest thing to try first for getting better HVAC humidity control from your older system. Humid air outdoors is rough on your AC’s condenser coils, and they can get grimy in a hurry. That layer of dirt and debris hurts the unit’s ability to release heat. As a result, the AC does not operate as efficiently and may have more trouble removing humidity from the air inside.

2. Get a preventative maintenance plan
Cleaning only the coils may not be enough to restore performance and better HVAC humidity control if the rest of the system hasn’t been properly maintained.

When you invest in a preventative maintenance plan for your air conditioner, you get a regular inspection, tune-up and cleaning of system components. Doing so keeps it running as efficiently as possible to provide better HVAC humidity control.

Your tech will also be able to spot and fix impending problems before they worsen and cause the system to fail completely.

Related article: 8 Ways AC Preventative Maintenance Keeps the Repairman Away

3. Adjust the airflow
It’s sometimes possible for a qualified HVAC technician to adjust your system’s air flow in a way that makes the evaporator coil colder when humidity levels increase. That can result in more moisture removal and better HVAC humidity control.

This is possible only for certain types of AC systems, and it does carry some risk of ending up with a frozen evaporator coil. Talk to your AC professional to see if this might work for you.

4. Add a dehumidifier to your system
If all else fails, and you’re not ready for a complete new air conditioner, you can add a dehumidifier to your air conditioning system for better HVAC humidity control.

A dehumidifier removes excess moisture from the air before it travels through your ductwork and into your space. Ask a knowledgeable HVAC professional to recommend a dehumidifier that’s compatible with your air conditioning system.