There’s a kind of air conditioning that’s remarkably uncomfortable. The air feels heavy and cold, but it’s also clammy. The thermometer is at a temperature that should be comfortable, but you want to turn it down lower. What’s going on with the system?

Let’s take a look at what’s going on in an HVAC system when this happens and what we can do to improve comfort and reduce energy costs for our customers.

Humidity Issues
The problem is humidity. When a house is cold and clammy, there’s too much moisture in the air. At 74F, it doesn’t feel as cool at 65% relative humidity as it does at 45% relative humidity, so the occupants keep lowering the thermostat setting in search of comfort.

Unfortunately, the lower the temperature is in the house, the less ability the coil has to cool and dehumidify the air.

For example, if we look at the “normal” return air temperatures shown on the typical cooling capacity tables of a 67F wet bulb, we find that a cooling system will operate at full capacity and have roughly a 70% sensible and 30% latent cooling capacity.

If your customer has elevated humidity in the home, they may keep turning down the thermostat trying to get comfortable.

When this happens, the air gets colder and colder. When the entering wet bulb drops to 57F, the system performance deteriorates in two ways. First the system total cooling capacity drops about 10% and second, the equipment’s ability to dehumidify decreases to almost zero.

When the return air gets very cold, it gets dangerously close to the temperature of the coil. Normally dehumidification happens best when hot air hits a cold coil. Before the air can cool, it has to shed much of its water vapor. So water condenses on the coil and flows out of the building through the condensate drain. But when the air is close to the coil temperature, very little water vapor is removed by the coil so the humidity passes right through the coil and back into the building. We’re left with cold humid air that is sticky and uncomfortable.

Return Duct Leaks
Most of us think the humidity comes from a shower or dishwasher. The primary cause of high humidity is most likely from outside the conditioned space…usually from a hot attic. Even if you think your duct system is inside the envelope, think again.

It’s not uncommon for attic air temperatures to be near 130F in the summer throughout most of the U.S. In the Southern U.S., 150F isn’t an uncommon temperature in an attic on a hot afternoon.

Return duct leakage is the culprit. Relative humidity is the percent of moisture that air can hold at a certain temperature. The warmer the air, the more humidity it can hold. At 130F plus degrees, imagine the amount of humidity contained in a return duct leak from an attic!

Under these conditions, the coil doesn’t have the ability to remove the adequate moisture from the air as it passes through it. So the moisture is carried into the building. Every minute the system runs, additional moisture is being poured into the building.

What’s the fix? Find and eliminate the return duct leakage. This can be done through measurement using an air-balancing hood or by simply crawling the duct system and repairing as you go.

Remember the first rule of duct sealing: NEVER seal ducts without first verifying duct system capacity. The net effect will be decreased system airflow and substantially decrease system cooling capacity.

Negative Building Pressure
The other source that draws moisture into a building is negative pressure in the building with reference to outdoors. This typically comes from supply duct leaks outside the building envelope. In other words, the supply ducts may be delivering 1500 CFM into a building while the return duct system may be pulling 2000 CF out of the building. The net effect is 500 CFM more return air being pulled out than the supply is putting in. Sooner or later the 500 CFM difference will be pulled from outdoors. On an unusually humid day, that equals 2250lbs. of humid outdoor air being pulled into the building per hour. It’s doubtful that the designer considered that additional load when he or she selected the equipment.

This problem is remedied by repairing supply duct leakage. Remember, there’s no assurance the repair is successful until the results have been measured and verified.

Rob Falke serves the industry as president of National Comfort Institute a training company with technical and business level membership organizations. If you're an HVAC contractor or technician interested in a procedure describing how and why to measure cooling system wet bulb temperatures, contact Rob at robf@ncihvac.com or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.