West / Regional Reports

Predicting Air Conditioning Energy Bills Can Be Tricky

May 19, 2002
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LAS VEGAS, NV — A team of researchers recently analyzed the utility bills of Las Vegas homeowners living in new Building America houses to see if predicted energy savings were being achieved. The analysis turned up some surprises, and the lessons learned should help refine predictions for hot climate energy use.

The analysis was conducted by the Building Science Corp., one of four Building America teams working with production builders to improve the energy efficiency of new homes. The researchers examined the utility bills from hundreds of Building America houses built by Pulte Homes in Las Vegas.

Working with Pulte Homes, Joe Lstiburek, president of Building Science Corp., said his Building America team developed a package of building envelope and hvac improvements designed to lower energy costs significantly without increasing construction costs. These Energy Star homes differ from typical Las Vegas homes in several ways, including:

  • Instead of OSB sheathing, the houses have 1-in. rigid foam sheathing with taped joints.

  • Walls are framed with 2x6 studs 24 inches on-center, using advanced framing techniques (no jacks or cripples, and only a single top plate).

  • The building envelope improvements allowed the air conditioner to be downsized, saving $1,500 per home — more than enough to pay the $1,000 cost to move the ceiling insulation to the sloped roof.

    “Hvac designers are committed to the institutional over sizing of air conditioning equipment,” said Lstiburek. “The average system is sized at 150% to 200% of the requirement of Manual J. They oversize because they don’t know what they will get for a building envelope, and to compensate for duct leakage and inappropriate refrigerant charge.

    “If you size according to Manual J, there is already a fudge factor built in. But most designers then add another fudge factor.”

    Lstiburek said that while their calculations showed that the Building America houses in Las Vegas could have had hvac systems sized at about 60% of Manual J, his team decided to size the systems conservatively at 80% of Manual J.

    The Building America team had hoped to achieve a significant reduction in utility bills for air conditioning, compared to conventional Pulte-built homes. However, the end results did not necessarily show this.

    “Although we saved more than expected on the gas used for heating, we saved hardly any money on air conditioning,” said Lstiburek. “So, we asked ourselves, ‘What happened?’”

    TIGHTER DUCTS THAN EXPECTED

    A close examination of the utility bills showed that not only were Building America houses using more electricity than expected for air conditioning, but the control houses were using less than expected. Lstiburek had two reasons for this development:

    1. The ducts in the control houses were not as leaky as assumed, and

    2. The homeowners in the control houses were drawing their drapes to block out the sun more than the homeowners in Building America houses.

    “The standard houses performed much better than we thought,” he said. “Our predictions were way off for a bunch of reasons. Between the time we started and now, there has been a five-year interval, and duct installation practices have changed dramatically.

    “For installers working for production builders in Las Vegas, the typical duct installation is now coming in at 10% leakage. Some are even at 5% leakage. We had assumed the base-case houses would have 15% duct leakage.”

    In other words, the base-case houses were using the same hvac contractors as the Building America contractors, he said. This meant that ducts were being sealed on every job “because it dramatically reduces their callback.”

    DRAWING THE DRAPES

    The Building America team had also assumed that the inexpensive windows used in the control homes would have a large energy penalty compared to the spectrally selective low-e glass used in the Building America houses.

    “The huge reduction in energy that we expected from going to high-performance glass didn’t appear,” said Lstiburek. “People aren’t stupid; houses with lousy glass are hot and ugly, so they close the drapes. It turns out the lousy windows with closed drapes works was well as a good window with open drapes. Once we realized this was happening, we surveyed the base-case houses by driving around, and we noticed that they all had their drapes closed.”

    Lstiburek was quick to point out that one shouldn’t conclude that it’s not worth investing the money on low-e windows, “because a house with lousy windows needs a big air conditioner. You have to size the air conditioner for when the drapes are open.”

    TOO MUCH HOT VENTILATION

    During the short Las Vegas heating season, the Building America home performed well.

    “These homes had lower heating bills than we anticipated, because the unvented roofs performed much better than we anticipated,” said Lstiburek.

    Such was not the case during the cooling season.

    “The main reason for the higher air conditioning bills was that we were over ventilating the houses,” he explained. “We were ventilating the houses by providing outside air to the return side of the air handler. Since the air conditioners were correctly sized, they had a longer-than-usual duty cycle, which meant we were bringing in more outside air than typical”

    Lstiburek confessed that the Building America team now addresses the over ventilation issue by installing a motorized damper on the outside air duct.

    “We had always had a base control of at least 10 minutes of ventilation per hour, but we didn’t have a peaking control. Now the ventilation is controlled at 10 minutes per hour, period. This change alone may result in a 15% reduction on the air conditioning bill.”

    LESSON LEARNED

    The Las Vegas study reveals that there are many levels of complexity in predicting utility bills in hot climates.

    “It has been a struggle to predict the data for air conditioning,” admitted Lstiburek. “The technology for cold-climate construction is not a big deal, and it works. But we have a lot to learn in hot climates.

    “Data is not readily available. We are learning important lessons. For instance, in hot climates, don’t expect much savings in utility bills from low-e glazing, because people use drapes.”

    For more information, go to www.buildingamerica.net.

    Publication date: 05/20/2002

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