| Return to Ask Energenie home page |
On most economizers, the mixed air temperature is set between 55 and 60 degrees Fahrenheit. More sophisticated control systems may vary the mixed air temperature according to the cooling requirements.
"The most common control strategy involves mixing the cooler outside air and return air to provide a cooler mixed air temperature," said Dennis Matteson, extension specialist in small business energy at Kansas State University. "This provides cooling without using conventional air conditioning equipment."
Determining proper damper operation requires an understanding of the particular system design.
Most automatic controls for outside air dampers are modulating or continually variable. They are designed to use outside air only when it is cooler than return air.
"When outside temperature is less than the return temperature, the damper positions will vary depending on the desired mixed air temperature," Matteson said.
"To determine if the dampers are operating correctly on a fixed mixed air temperature system, measure this air temperature when the outside air is about 10 degrees cooler than the return air temperature," Matteson said. "Then repeat this process when the temperature is 20 degrees less than the return air temperature."
If the mixed air temperature stays on the set point, the modulation controls are working.
On most systems, the outside air dampers should be at their minimum position when the outside air temperature is warmer than the return temperature.
"This can be verified when the outside air temperature is at least 10 degrees warmer than the return temperature," Matteson said.
To determine the amount of outside air entering a system, temperatures must be measured at three locations: the outside air (To), the return air (Tr), and the mixed air (Tm). The percentage of outside air is calculated as follows: Percent outside air = (Tr-Tm)/(Tr-To).
"It is more efficient to leave the thermostat in the automatic position," said Gene Meyer, extension mechanical engineer at Kansas State University.
The fan consumes only one-tenth the energy of the compressor, but when it runs continuously, the fan can cost up to $30 a month.
"This amount can be reduced by cycling the fan only when it's needed," Meyer said.
Additionally, the air conditioner will dehumidify the air when the compressor is running.
"If the fan remains on after the compressor cycles off, however, some moisture left on the cooling coil will evaporate back into the air stream," Meyer said. "This moisture must be removed during the next compressor cycle, which increases the energy consumption."
If air distribution is poor within the home or business and hot spots or very cold areas result, you can run the fan to even out the temperatures.
"The fan should be set to the auto position, however, when the building is unoccupied," Meyer said. "Even better, shut the air conditioner off or raise the thermostat setting when you leave the building."
Yes.
"Some of the larger electric utilities have at least two electric rates for residences," said Dennis Matteson, extension mechanical engineer at Kansas State University.
One of the rates is billed only by consumption, and the other rate is billed according to both consumption and demand.
Consumption is the amount of electricity used and is measured in kilowatt hours (kwh). A 1,000-watt load operating for one hour is one kwh.
"Demand, sometimes referred to as capacity, is the highest average energy usage for a specified time interval during the month," Matteson said.
The units for demand are kilowatts. This time interval is usually 15 or 30 minutes.
The purpose of the demand rate is to allow customers the opportunity to pay a lower electrical cost by minimizing their peak demand. The rate, which includes demand, has a lower consumption charge, but the customer also is charged for the demand during the month. For this reason, it is important to minimize the demand.
For example, consider a customer who consumes 1,000 kwh a month.
"Assume the rates available are either 7 cents per kwh for consumption or 5 cents per kilowatt-hour for consumption and $2 per kilowatt of demand," Matteson said.
By using the first rate, the cost would be 1,000 kwh times 7 cents per kilowatt-hour for a total cost of $70.
Using the second rate, the cost for consumption is 1,000 kwh times 5 cents per kilowatt-hour or $50. If the demand charge is less than $20, the second rate would be cheaper. In this example, the demand during the billing period needs to be less than 10 kilowatts.
"The demand rate gives customers a chance to save on their electric bills by offering a reduced rate for consumption," Matteson said. "But customers must be alert to their peak energy usage to save on the demand rate."
For residences, electrical demand peaks in the summer between 4 and 8 p.m., which is the warmest part of the day.
Customers can reduce the rate of energy use by avoiding operating large electric-consuming equipment simultaneously—such as the clothes dryer and air conditioner.
"Most window manufacturers are using low-emissivity coatings or films to conserve energy both in the winter and summer," said Bruce Snead, extension specialist in residential energy at Kansas State University.
Emissivity refers to the ability of a surface to emit—or radiate—heat. The "perfect radiator" has an emissivity of 1. Ordinary window glass has an emissivity of .84.
"This means a large amount of heat is transferred due to
long-wave emission," Snead said.
With low-emissivity coating on the glass, however, the emissivity drops to a level between .05 and .40.
"This allows the glass to reflect between 60 and 95 percent
of the radiant heat," Snead said.
In the summer, these coated windows reduce the cooling load by reflecting heat radiated from the surrounding buildings, sidewalks, and driveways.
"In the winter, the heat radiated from interior sources is reflected back in to the room by these windows," Snead said. "Thus, your heating load is reduced."
Although these windows reflect a high percentage of long-wave infrared radiation, they allow most of the visible light in. So, the glass will maintain natural light and views in the room.
"They also reflect most of the sun's ultraviolet rays—helping to reduce the fading of furniture and fabrics," Snead said.
Some manufacturers have introduced low-emissivity units with bronze-tinted glass to further reduce solar heat gain. This new coating is most effective on west-facing windows with full exposure to the afternoon sun in the summer.
No.
"Cycling the compressor and running the fan continuously does not increase efficiency," said Dennis Matteson, extension specialist in small business energy at Kansas State University. "In some instances, the efficiency may be decreased."
There are two components to cooling. The first is cooling the air temperature or "sensible cooling." The other component is removing the moisture or "latent cooling."
"When the compressor has cycled off and the fan continues to operate, some of the moisture that was condensed from the air is blown back into the room," Matteson said.
Energy was used to condense this moisture from the air.
When it is blown back into the room, the moisture has to be removed during the next compressor cycle.
"There can be reasons for cycling the compressor, however, other than increasing the efficiency," Matteson said. "Both customers and the electric utility can benefit from cycling the compressor."
Some utilities have programs that give a credit for allowing installation of a special device.
This mechanism cycles the compressor when the outside temperature is above a predetermined temperature," Matteson said. "By cycling the compressor off for short periods, they are reducing the total peak electrical load."
This can result in reduced cost to all customers on the utility.
"If your air conditioner runs continuously during the hottest days, adding the cycling device might cause an increase in room temperature during the warmest weather," Matteson said.
If your air conditioner cycles off for 15 minutes each hour during the hottest months, however, the difference of installing the device will not be noticed.