As your cooling system kicks on, you’ll soon experience a type of air conditioning that is remarkably uncomfortable. The air feels heavy and cold, but it’s also clammy. The thermostat says it should be comfortable, but you aren’t. What is that feeling, and what’s really going on with the system?
Let’s look at what causes this condition and how some simple repairs with a little instruction to your customers can help them avoid that Cold and Clammy Feeling.
What’s the Cause?
Excessive humidity is the culprit and your customer’s response to it is to turn their thermostat down lower. Unfortunately, this may cause colder and clammier air in the house and a sizable increase in utility costs, resulting in a permanently grumpy customer.
The Problem is Humidity
When the cold and “clammies” hit, there is too much moisture in the air.
Air at 74 degrees, doesn’t feel as cool at 65% relative humidity (RH) as it does at 45% RH. Occupants often lower the thermostat setting in search of comfort, without knowing what’s going on. But by lowering the temperature in the house, they decrease the system’s ability to cool and dehumidify the air.
For example, if you look at the typical cooling capacity tables, you’ll see at the “normal” return air temperature of 67 degrees wet bulb, a cooling system will operate at full capacity and have roughly a 70% sensible and 30% latent cooling capacity. This means 70% of a system’s capacity goes toward lowering air temperature while the other 30% is dedicated to humidity removal.
By turning down the thermostat, the air gets colder and colder. As coil entering wet bulb temperatures drop to 57 degrees, system performance deteriorates in two ways. First, the total cooling capacity drops about 10%. Second, the equipment’s ability to dehumidify decreases to almost zero.
In other words, when return air gets colder, it becomes closer to the coil temperature and the ability to dehumidify the air decreases significantly. Under these conditions, which are completely off a cooling performance chart, the supply air leaves the coil at 90%+ RH.
Normally, dehumidification happens best when hot air hits a cold coil. Before the air can cool, it must shed much of its water vapor. So, water condenses on the coil and flows out of the building through the condensate drain.
When air is close to the coil temperature; very little water vapor is removed. This leaves your customer with cold humid air that is sticky and uncomfortable.
Return Duct Leaks
Most of us think the excess humidity comes from a shower or dishwasher. Sorry, but under this condition, probably not. The primary cause is most likely hot attic air being pulled in through return duct leaks from outside the conditioned space. This invisible source of humidity is difficult to identify and correct.
It’s common for attic air temperatures to be near 130 degrees in the summer throughout most of the U.S. In the south, 140 degrees is a common in an attic on a hot afternoon.
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 130 plus degrees, imagine the amount of humidity contained in a return duct leak from an attic!
Under these conditions, the coil cannot remove adequate moisture from air passing through it. So, the moisture is carried into the building. Every minute the system runs, additional moisture is 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 repair as you go.
Remember the first rule of duct sealing is to NEVER seal ducts without first verifying adequate duct system capacity. The net effect will be decreased system airflow and substantially decreased system cooling capacity.
Negative Building Pressure
The other source that draws moisture into a building is negative pressure. This typically comes from supply duct leaks outside the conditioned space. In other words, the supply ducts may be delivering 1500 CFM into a building while the return ducts may be pulling 2000 CFM out.
The net effect is 500 more CFM of return air being pulled out than the supply is delivering into the building. Sooner or later this difference will be pulled in through windows, an attic hatch, or any opening to the outdoors.
On an unusually humid day, that equals 2250 pounds of humid outdoor air being pulled into the building per hour. It’s doubtful system designers considered this additional load when they selected the HVAC equipment.
This problem is remedied by repairing supply duct leakage. Remember, there is no assurance any repair has been successful until the results have been measured and verified.
Rob “Doc” Falke serves the industry as president of National Comfort Institute, Inc., an HVAC-based training company and membership organization. If you're an HVAC contractor or technician interested in a free Humidity Measurement Procedure, contact Doc at [email protected]or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles, and downloads.