Ask Energenie

May 1999

Table of Contents

1. What maintenance does our central air conditioning unit need prior to the cooling season?
2. What is the heat index?
3. Should I operate my furnace fan continuously when I’m air conditioning, or put it in the auto position?
4. I have had a number of energy improvements installed in my house, but the dollar savings do not seem very significant. What is the problem?
5. Why is there a concern about shading east and west windows during summer, but not south windows?
6. What cooling season maintenance do you recommend for the roof-mounted heating and cooling unit of our business?
7. When it gets really hot, my neighbor turns the sprinkler on his air conditioning unit. He claims this improves its performance. Is he right?
8. How can I shade a sunspace or solar greenhouse to prevent overheating in the summer?

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1. What maintenance does our central air conditioning unit need prior to the cooling season?

"Maintenance of both the exterior and interior portions of your central air conditioning system is important," said Gene Meyer, extension mechanical engineer at Kansas State University. "Checking its operation prior to the cooling season can prevent system failures and also improve your unit's efficiency.

Meyer offered the following tips for exterior maintenance:

• Shut off the electrical power to the unit.
• Check the manufacturer’s recommendations for owner maintenance.
• Check the condensing unit for accumulation of grass clippings and tree "fuzz" or debris.
• Clean the condensing unit as needed, taking care not to damage the fins.
• Check for damaged or bent fins on the coil. Some units have guards over the fins, but lawn mowers, hail and children can damage unprotected fins. Use a coil comb, available at hardware stores, to straighten bent fins.

For interior maintenance, he suggested these tips:

• Oil the blower and motor of the interior unit (blower section).
• If belt driven, check belts for tightness, and replace if worn excessively.
• Check the filter and replace if necessary.
• Check the evaporator section condensate drain. If water is running out of unlikely locations (such as out of the furnace instead of the drain) during cooling, the drain may be plugged and you may have to clean or replace it. Due to the cramped location of some units, this step may require a service technician.

"You can follow these maintenance tips on your own. However, if your system does not cool as effectively as in previous years, call a qualified service technician," Meyer said. "Technicians can detect leaks in the refrigeration system, and many service contractors charge less for preseason checks."

"Servicing your unit between heating and cooling seasons will assure proper operation when the cooling season arrives again," Meyer said.

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2. What is the heat index?

"In many ways the heat index is much like the wind chill index," said Richard B. Hayter, director of Engineering Extension at Kansas State University.

"Both attempt to form a single index by combining multiple factors that affect the thermal sensation experienced by someone in a particular environment or set of conditions," Hayter said.

A number of indices have been developed to show the potential level of heat stress using factors such as dry bulb temperature (air temperature), air movement, humidity levels and exposure to direct sunlight.

"The index, which seems most popular in media weather reports, is the heat stress index, more commonly referred to at the heat index," Hayter said. "This index combines air temperature and dew point into a single unit called the apparent temperature."

Developed by R.G. Steadman of Colorado State University and first published in 1979, the index measures what hot weather "feels like" to the average person for various temperatures and relative humidities.

For example, at an air temperature of 90 degrees Fahrenheit at 60 percent relative humidity, it feels like 100 degrees. The comparison is based on the thermal sensation you would experience at a constant dew point of approximately 55 degrees.

In addition to the thermal sensation, heat stress categories have been developed, based on apparent temperatures.

"The lowest would be a category IV, which includes apparent temperatures from 80 degrees to 90 degrees," Hayter said. "This means fatigue is possible with prolonged exposure or physical activity."

The most severe category is category I, which is an apparent temperature of 130 degrees or higher.

"In these conditions, heat or sunstroke is likely with continued exposure," he said.

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3. Should I operate my furnace fan continuously when I’m air conditioning, or put it in the auto position?

"You should leave the fan selector switch on auto," advised Gene Meyer, extension mechanical engineer at Kansas State University.

In the auto position, the blower fan starts running whenever there is a call for cooling. Thus the blower fan operates on less energy.

Having the fan on auto also helps control moisture in your home.

"The reason you have better moisture control is that the evaporator coils are still wet with condensate after the air conditioner has cycled off," Meyer said. "So, if the fan runs during this time, moisture from the coils re-evaporates, increasing the amount of moisture in the air."

If your air distribution system does not provide even temperature control in all the occupied spaces of your home, it is best to leave the blower fan running. The blower fan will distribute cooled air throughout your home.

"Do not use the central fan to increase the air velocity in the room and improve the sensation of comfort," Meyer said.

If you want additional air velocity so you can maintain the thermostat at a higher temperature, Meyer suggested the use of a small box, oscillating or ceiling fan in the occupied area.

"The motor size on these fans is considerably smaller than the blower motor and provides more noticeable air movement than the air conditioner blower," Meyer said. "It also uses less energy."

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4. I have had a number of energy improvements installed in my house, but the dollar savings do not seem very significant. What is the problem?

The failure of energy improvements to yield the expected energy savings can cause problems for both owners and contractors.

"Sometimes this is the result of unrealistic expectations on the part of the homeowner, or exaggerated advertising on the part of business," said Bruce Snead, extension specialist in residential energy at Kansas State University.

"Numerous factors influence energy consumption," he said.

When you install several measures together, net savings is often less than if you had installed and evaluated them individually.

"Let's assume that you invest in extra wall insulation at the same time as you install a set-back thermostat," Snead said. "If the thermostat sets back temperatures at night or during unoccupied hours, your savings from the extra insulation will be less."

Another explanation for energy savings being less than what you expected is exaggerated advertising claims.

"For example, energy savings from replacement windows are highly touted," Snead said."If you replace a single-glazed pane, including storm window, with a sealed, double-glazed unit, you may reduce air leakage. However, even with proper installation, there will be little effect on conducted or radiant heat loss."

Weather fluctuations and increases in utility rates can negate energy savings. Also, changes in family size may alter comfort demands in the home, leading to different thermostat settings or energy use patterns.

"You must deal with such variables as they come since you have no control over them," Snead said.

"Having a reasonable expectation of energy savings and knowing how to take full advantage of improvements is the best way for both homeowners and contractors to gain from energy improvements," Snead added.

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5. Why is there a concern about shading east and west windows during summer, but not south windows?

"During summer, the sun is high in the sky," said Gene Meyer, extension specialist in small business energy at Kansas State University. "At its maximum, the sun is approximately 70 degrees above the horizon, compared to 30 degrees in winter. As a result, very little sunlight enters a south window, especially if there is an overhang."

In the morning and late afternoon, the sun is lower in the sky, but to the east or west. Again, little sun hits south windows, but a considerable amount enters east or west windows. Reducing west exposure is most critical.

"The amount of solar energy entering your home is no greater in late afternoon than in the morning, but temperatures are higher," he said. "So, an unshaded west window creates the heaviest air conditioning load."

Overhangs do not effectively shade east and west exposures.

"Exterior shades such as awnings or trees are most effective," Meyer said. "Light-colored, reflective interior treatments also help."

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6. What cooling season maintenance do you recommend for the roof-mounted heating and cooling unit of our business?

"As with any heating or cooling system, you should regularly inspect your roof-mounted unit so it gives you optimum performance," said Gene Meyer, extension mechanical engineer at Kansas State University. "The following tips will help you establish your own inspection routine," he said.

1. Before servicing, turn off the power to the unit. Check unit's exterior casing for holes or rusted out areas. If they exist, they can leak conditioned air to the outside.

2. Clean outside air dampers, economizer and condenser coil, removing all leaves, debris and airborne material such as cottonwood "fuzz." Adjust dampers for proper operation, and oil them.

3. Clean the condenser coil by brushing downward and then blowing out with compressed air.

Meyer suggested using a coil cleaning detergent if the coil is extremely dirty. After washing it, flush thoroughly with water. Do not use wire brushes or power washers because they can damage the coil.

"Also at this time, you should straighten bent coil fins," Meyer said. This is done with a coil comb. It assures full airflow.

"Now that you have checked the exterior parts, you can open the unit and inspect the filter, belts and evaporator coil," he said. "But before removing unit panels, make sure the disconnect switch and breakers are off."

4. Clean permanent filters and replace disposable ones regularly, based on installation requirements. Meyer said it is a good idea to check them monthly.

5. Oil fan and motor according to the manufacturer's requirements. Check condition, alignment and tightness of belts. The belt should give about an inch when you depress it midway, using moderate pressure. Check blower fins for accumulated dirt, which will decrease unit performance.

6. While the unit is open, check the evaporator coil. Unless you have neglected the filters, the coil may need only a light brushing. If it needs additional cleaning, spray it with coil cleaning solution approved for interior use, and then wash with water. Units have a drain pan to catch condensation, so the water will flow out.

7. Clean dirt and scale that have accumulated in the drain pan, and check the drain line to make sure it is open.

"Rooftop units are found in many installations, including commercial, industrial and institutional," Meyer said. "They provide reliable service when maintained, but due to their remote location, they may not be noticed until they fail to operate."

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7. When it gets really hot, my neighbor turns the sprinkler on his air conditioning unit. He claims this improves its performance. Is he right?

"Your neighbor is cooling his condenser and is improving his air conditioner performance," said Gene Meyer, extension mechanical engineer at Kansas State University.

The temperature of the condensing unit has a strong effect on the equipment's performance and capacity.

"When it is extremely hot outside, efficiency and capacity go down," Meyer said. "By spraying water on the condensing unit, your neighbor effectively lowers the unit's temperature — thereby improving efficiency and increasing capacity."

However, a problem may develop with your neighbor's system since city water contains many minerals and solids.

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8. How can I shade a sunspace or solar greenhouse to prevent overheating in the summer?

"Ideally, whatever measures you take to control sunlight should be adjustable," said Bruce Snead, extension specialist in residential energy at Kansas State University.

This admits sunlight during cold spring periods but shuts it out during a warm fall season.

"Natural shading by trees and vines works best if the plants are tall enough and placed correctly," Snead said.

Solid roofs and overhangs are the best permanent protection against overheating. Overhangs can be designed for the desired shading.

Exterior shades create flexibility, but are hard to mount and vulnerable to weather," Snead said.

Bamboo screens, wood or aluminum slats and woven shade cloth are common materials, usually operated in roll-down systems. While shade paint can be used on glass systems, it is advisable to check with the manufacturer of plastic glazing systems for compatibility with shade paint.

"Interior shades are less effective than exterior shades in blocking solar gain," Snead said.

Cutting nighttime heat loss is possible if there are tight edge seals on insulating materials.

"You can also help cool the space by using natural or mechanical ventilation with your shading system," Snead said.

"Another factor is careful placement of low inlets and high outlets coupled with good prevailing winds," he said. Installation of a fan can also force ventilation to adapt to varying conditions.

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Ask Energenie is produced by the Kansas Energy Extension Service through Kansas State University. This material was prepared with the support of the U.S. Department of Energy (DOE) Grant No. DE-FG47-92-CE60210. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the author(s) and do not necessarily reflect the view of DOE.