Ask Energenie

August 1998

Table of Contents

1. What can I do to cool my attic in the summer?
2. Should I let my furnace fan run continuously when I'm operating my central air conditioner or is it better to let it cycle on and off with the air conditioner?
3. What effect will an extension cord have on the operation of a window air conditioning unit?
4. What are the differences between screening (or short-term) tests and long-term radon tests?
5. The room temperature in our home always seems a few degrees cooler than the thermostat setting. Do I need a new thermostat?
6. Are mercury yard lights efficient?
7. Where can I get radon test kits and where are they analyzed?
8. I would like to install a whole-house fan in my house so I can cool my house with outside air on cool nights. How large a fan should I purchase? What else should I consider in selecting a fan?

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1. What can I do to cool my attic in the summer?

The heat gained through the ceiling from the attic can contribute significantly to the cooling load in a home. Proper levels of attic insulation are important in both winter and summer, with proper ventilation of attic space critical as well.

"Most attics can be ventilated without use of electric ventilators," said Richard B. Hayter, director of Kansas Energy Extension Service at Kansas State University.

"However, occasionally the geometry of the attic or construction of the roof prevents adequate natural ventilation, making it necessary to consider a power vent," Hayter said.

For proper natural ventilation, it is important that ventilators be placed along the low side of the attic for air to enter the space and along the high side for it to exit.

A number of ventilating products is available for both applications.

"Eaves or soffit vents can run continuously beneath the eaves or be a series of openings along the eaves," Hayter said.

"Prefabricated grills or screens are available for covering the eaves vents," he said.

Ventilators on the high side or along the ridge of the roof can include continuous ridge vent that is an opening the length of the ridge covered by a prefabricated rain cap. These ventilators also may include a series of openings cut along the ridge covered by rectangular or round caps, or turbine vents that rely on external breezes to enhance the ventilation from the attic.

"Vents in the gables of a roof may also be satisfactory," Hayter said.

Regardless of the type of ventilation system used, it is critical that there is sufficient, unobstructed ventilation area for proper attic cooling.

"The louvers or mesh that cover the ventilation opening can significantly restrict air flow through the opening," Hayter said.

When purchasing this equipment, notice the "free area" listed for the particular grill or louver.

"Ideal attic ventilation will be a minimum of one square foot of free area for every 300 square feet of attic area," Hayter said. "One-half of the opening along the low side is for air inlet and the other half is uniformly distributed along the high side for exhaust.

"For example, if your attic equaled 1,200 square feet, you would need two square feet of free area along the ridge or high side of the roof," he said.

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2. Should I let my furnace fan run continuously when I'm operating my central air conditioner or is it better to let it cycle on and off with the air conditioner?

"Unless you have a serious problem with air distribution and balancing in your home, you are better off letting the furnace fan cycle with the air conditioner, " said Doug Walter, president of Kansas Building Science Institute.

Continuous fan operation is expensive. A typical .five-horsepower furnace motor would consume 22 kilowatt-hours (kwh) per day, costing $1.66 at 7.5 cents per kwh. That's $50 per month.

"Continuous fan operation also adds heat and moisture to the air in your home," Walter said.

A .five-horse-power fan motor adds more than 3,000 Btu per hour to your home's cooling load.

"The moisture comes from condensate remaining on the cooling coil after the compressor shuts off," he said.

This moisture evaporates into the air stream as the coil temperature increases, raising your home's humidity.

"Air circulation is better provided by a portable fan or ceiling fan," Walter said. "These types of fans are more effective at creating air movement within a room and cost about a penny per hour to operate."

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3. What effect will an extension cord have on the operation of a window air conditioning unit?

"Because some window air conditioners have large power requirements, using an extension cord that is too small could cause the cord to overheat, or cause problems with the air conditioner," said Gene Meyer, extension mechanical engineer at Kansas State University.

"These problems could include failure of the compressor motor," Meyer said.

If the air conditioner is new, the instructions should give recommendations on the proper wire gage for an extension cord.

"If the unit is older, you should check the Full Load Amps (FLA) rating on the unit and use an extension cord with the required capacity," he said.

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4. What are the differences between screening (or short-term) tests and long-term radon tests?

"In common practice, a short-term radon test is usually from two to seven days," said Bruce Snead, extension specialist in residential energy at Kansas State University. "A long-term test is usually from three months to one year. Both are accurate and reliable if you follow instructions. "

A long-term test is a better indicator of the average radon levels in a home and more indicative of the personal exposure in that space. Short term testing serves the needs of real estate testing and is a good predictor of typical radon levels.

"Short term tests are much more commonly done by homeowners and testing service providers because of the desire to find out about current radon levels in the house," Snead said.

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5. The room temperature in our home always seems a few degrees cooler than the thermostat setting. Do I need a new thermostat?

"It is possible that the thermostat just needs leveling," replied Gene Meyer, extension mechanical engineer at Kansas State University.

"Many thermostats use a mercury bulb type contact," he said. "If the thermostat is not level, room temperature will be maintained above or below the selected temperature."

Releveling a thermostat is easy and inexpensive.

Remove the cover and look for leveling marks on the thermostat. One of the attachment screws should be in a vertical slot. Loosening this screw allows you to raise or lower that end of the thermostat. Once you have straightened the thermostat, use a small level to check for proper level. Retighten the screw and replace the cover.

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6. Are mercury yard lights efficient?

"Mercury vapor lamps are more efficient than incandescent lamps, but if you want to substantially improve the efficiency of outdoor lighting, use high pressure sodium lamps," said Gene Meyer, extension mechanical engineer at Kansas State University.

Lighting efficiency is a measure of the amount of light from a lamp, in lumens, divided by the power to the lamp, in watts. A 100 watt mercury lamp has an efficiency of 38 lumens per watt. The efficiency of an incandescent lamp is about 16 lumens per watt.

"A Sodium lamps producing about the same light as a 100 watt mercury vapor has an efficiency of 70 lumens per watt, over four times more efficient than incandescent lamps, and twice as efficient as mercury vapor," Meyer said.

The smallest sodium lamp is a 35-watt lamp. It will produce more light than a 100-watt incandescent.

"It takes about five minutes for a sodium lamp to brighten," Meyer said, "so they shouldn't be used where they will be turned on and off frequently."

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7. Where can I get radon test kits and where are they analyzed?

"You can get test kits in large discount stores, hardware stores or home supply stores," said Bruce Snead, extension specialist in residential energy at Kansas State University. "Other sources to check are local health departments and your county extension office. When you purchase kits from the extension service, the results will add to the state database by zip code and help us understand the distribution of radon levels."

Test kits should have "EPA Listed" on the packaging, indicating that the laboratory is a participant in the federal government's voluntary quality assurance program.

"The laboratory that produced or handled the initial preparation and analysis of the kit must do the final analysis," Snead said. "This is essential for consistency and accuracy of the test."

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8. I would like to install a whole-house fan in my house so I can cool my house with outside air on cool nights. How large a fan should I purchase? What else should I consider in selecting a fan?

"The size of whole-house fan you need depends on the volume of air in your house," said Doug Walter, president of Kansas Building Science Institute in Manhattan. "Ideally, a whole-house fan should change the air in your house 40 times in an hour."

You can determine the fan size you need by multiplying the volume of the conditioned area of your house by 2/3.

"This will give you the size of whole-house fan you need in cubic feet per minute (cfm), which is how they are rated," Walter said.

For example, if your house has 1,500 square feet of living space, the volume is 1,500 X 8, or 12,000 cubic feet. Two-thirds of 12,000 is 8,000 cfm. You would select a fan rated at 8,000 cfm.

"If your whole-house fan discharges into the attic, make sure you have the attic vent area to handle discharge air," he said.

You will need one square foot of unobstructed vent area per 750 cfm of air flow. For a fan rated at 8,000 cfm, you would need 10.7 square feet of net vent area.

"The fan should be located centrally in your house, but not directly outside a bedroom door if mechanical noise bothers you at night," Walter said. "The noise can be reduced by building an air-tight frame that supports the fan a foot or so above the ceiling louvers."

Mounting the fan on foam rubber weatherstripping helps to reduce vibration noise.

"Consider installing a 12-hour timer switch to control the fan so you can have it shut off automatically during the night," Walter said.

"This is especially useful during the fall and spring months when night-time temperatures can get quite low," he said.

You also should install a disconnect switch in the attic near the fan. If you need to work on or near the fan, the attic disconnect assures that no one will inadvertently turn the fan on while you are near it. Last, provide a means to seal the fan in the winter.

"This could be an air-tight, insulated cover that is installed in the attic, or an air-tight panel that installs over louvers from the room side," Walter said. "If you install a cover in the attic, use the disconnect switch to shut off power to the fan until you remove the cover in the spring."

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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.