Saturday, May 15, 2010

Dry Out!

Its spring, the weather is warming up, flowers are blooming, and our thoughts turn to dehumidification. In humid climates like the southeast, removing humidity from the air in warm weather is just is just as important to comfort as reducing its temperature. Being warm blooded, our body normally produces more heat than it needs and then regulates our temperature using different cooling mechanisms. The primary cooling mechanism is evaporation of perspiration from our skin. Dry air makes us feel cooler because it accelerates the evaporation from our skin. Humid air makes us feel warmer because the evaporation process is slowed down. Dehumidification can be the difference between being comfortable and being uncomfortable at 78°F. Dehumidification can save energy by reducing the amount of sensible cooling required for comfort. Many people with oversized air conditioning systems essentially over cool their house to be comfortable because their systems do not run long enough in mild weather to reduce humidity, so they do not feel comfortable until they reach temperatures of 70°F in their house. It takes several minutes for most air conditioning coils to get cold enough to sweat. An over sized system will often satisfy the thermostat and shut off shortly after the coil reaches dew pont. A properly sized air conditioning system will run longer, allowing longer operation with an evaporator operating below dew point and removing more water from the air. Systems with ECM blowers and thermidistat controllers have a special dehumidification mode that reduces system airflow for dehumidification. This increases the latent system capacity and decreases its sensible capacity. Two stage cooling systems can help by allowing longer system operation at moderate loads. All of these are a big improvement over the typical oversized single capacity system with a PSC blower. However, an air conditioner is still not a dehumidifier.

If you are really serious about dehumidification you need a dehumidifier. In a nutshell, a dehumidifier is an air conditioner with a single blower that moves air first over the evaporator, and then over the condenser. The air first passes over the evaporator where it is cooled to dew point to remove water, and then the same air passes over the condenser where it is reheated to a temperature slightly above its original temperature. If you have a basement in the southeast you NEED a dehumidifier. My basement stays below 80°F all the time without air conditioning, but it feels warm without my dehumidifier operating. In the past most dehumidifiers have been small console types that are noisy, inconvenient, and typically undersized. Several companies now offer whole house dehumidifiers that can be integrated into a complete comfort system for your house. They have enough capacity for a house, are far quieter, and do not require you to empty a bucket twice a day. Therma-Stor has a good short animation on why basements have high a relative humidity and how a dehumidifier addresses this problem. You will find other articles on their website which help address specific dehumidifier applications. They also have a psychrometric chart in a round format that looks like a ductulator that are great for teaching psychrometrics. To read more about the effect of humidity on comfort, check out Unit 61 Fundamentals of Psychrometrics in Fundamentals of HVAC/R. To download an interactive pschrometric chart for free, go to HandsDown Software.

1 comment:

  1. Some great points here Carter! The RSES Journal also recently ran a three-part series on the Psychrometrics that offered some great info on how humidity affects indoor comfort. Thanks for sharing!