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Let's do the Sensible Heat Shuffle (1997)
An article by David Debien
This is not a new dance, but something that everybody does every day when you drive. When you first get in your car, it is very hot. You turn on the A/C to maximum until the heat is reduced. As you slow the airflow down, you change the sensible heat ratio from near 90% dry heat removal to 60% dry heat removal. As your auto continues to cool an even lower speed becomes too cold, you raise the coil temperature by changing the setting on the adjustment from colder to warmer. All you have done is shuffle the sensible heat ratio to improve your comfort. Why don't we do this in our homes? The same needs exist.
In recent years, most manufacturers have designed variable speed units that can be installed to address this problem. What about existing systems that are not variable speed? Central City has attacked this design problem and came up with three solutions. Although these do not violate the laws of Physics, they are revolutionary in the sense that no one in this area has applied these fundamentals. Here they are!
- Install a humidistat to measure the humidity in the conditioned space. When the humidity is high, the inside fan will slow to increase temperature drop across the cooling coil. When the humidity low, the indoor fan will run at a higher speed and a lower temperature drop across the cooling coil.
- A similar result can be obtained by using a 2-stage thermostat on a single speed system. The indoor fan speed is controlled by the two stages of the thermostat. The fan starts on low and progresses to high based on the cooling needs of the home.
- The third method involves a two-speed control with a single speed thermostat and single speed compressor. The indoor fan starts on low and after a set time, it ramps to high speed. The fan stays in high speed until the thermostat is satisfied. On the next cycle the fan again starts in low speed. This allows modification on an existing system by adding only a relay and a timer.
This shuffle changes the sensible heat ratio according to the needs of the home. The system must be adequately sized to be able to satisfy the needs of the space at both speeds. This system uses a single speed compressor and a matched coil. The efficiency will be below the SEER rating at low speed and at the SEER rating on high speed. The comfort will be maximized and allow the system to be operated at a higher thermostat setting thus improving comfort. The total electric bill will be lowest when the customer is comfortable, both heat and humidity, at the highest thermostat set point. This is the reason for the Sensible Heat Shuffle. It is especially effective when the humidity levels in the house are high. This occurs mostly in the evening or at times when the heat load is low. It also occurs in homes that are built to tight building standards with state of the art vent skinning, radiant barrier, and ridge and soffit venting. As buildings are built tighter, humidity levels will rise. Thus, the need for even more Sensible Heat Shuffling.
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