Energy Use - Knowledge is Power - 11/15/2004 - House Energy Efficiency

January 2001
Revised April 2004

Energy Use: Knowledge is Power

Consumer energy use has been a prominent issue in the public discourse over the last year, as the price of heating and cooling our homes and driving our cars has spiraled rapidly upward. Escalating utility prices are affecting many parts of the country and the price forecast for the future is uncertain at best. According to the Energy Information Administration (EIA), if the natural gas prices projected for 2001 are applied to households according to 1997 usage patterns, annual expenditures this year will be 35 percent higher. Electricity prices will also be volatile in the coming months and years, as shown by California's deregulation process.

This environment has quickly heightened public interest in energy efficiency in America's homes. The current demand for solar panels in California is unprecedented, production builders are marketing energy efficient models, and industry experts are making the morning talk and news show circuit to discuss home energy issues.

But how is home energy use evaluated and measured? This information is critical to builders, manufacturers, homeowners, government agencies, and many other groups. After all, how can a builder guarantee his buyers a maximum heating and cooling bill without baseline measurements? How can a utility program certify that one home is energy efficient and another is not without some basis for comparison?

Measuring Home Energy Use

Home energy use can be measured and characterized in several ways. Utilities use meters to track electricity and natural gas usage in homes. Building scientists may utilize electronic sensors and meters to assess the energy performance of specific components like an air conditioner, hot water heater, or exhaust fan. Builders, too, have tools at their disposal to measure and characterize the way the homes they build use energy.

The tools used to evaluate energy performance depend on the objective. Common reasons for measuring or characterizing energy use in homes include:

• Identifying, diagnosing, and correcting building performance issues;
• Prioritizing energy improvements; and
• Determining a home's compliance with an energy efficiency program.

Also, a builder might simply want to assess the performance of his homes relative to other homes in the area. Existing databases available from the local utility or national databases may be helpful in carrying out this comparison. Both the Department of Energy (www.energy.gov) and EPA's Energy Star Program (www.energystar.gov) have tools on their sites that allow a comparison of energy costs to local averages. These tools require only a basic description of a home's building components to provide information.

More sophisticated performance measurements require specialized testing equipment and carry increased costs, but are far more accurate and informative than these general comparisons. One of the most common field tests conducted to assess energy performance is the blower door test. This test involves using a powerful fan (maximum flow = 6,400 ft³/minute) to pressurize or depressurize a house. While the house is subject to this elevated pressure, the airflow moving through the fan is measured to quantify how much the building envelope leaks at a certain pressure. This information tells a builder how "leaky" a building is under normal conditions.

Air that leaks into or out of a building is commonly called air infiltration. Air infiltration is a critical part of the energy performance of a house because it typically makes up one-fourth to one-third of the total heating load for a house. It's also an important element to measure because it is fairly easy and inexpensive to make a house more airtight while it's being built.

Blower door test results are expressed in terms of an "ACH" estimate—the air changes per hour (ACH) that would take place in a house under normal conditions. An ACH is the number of times each hour that an entirely new volume of fresh air makes its way into the house. If a house with an indoor volume of 16,000 ft³ (2,000 ft² floor area and 8 ft ceilings) experiences 8,000 ft³ of fresh air infiltration every hour, it has an air exchange rate 0.5 ACH; a rate of 0.25 ACH would mean that 4,000 ft³ of fresh air would enter the house each hour. ACH values for new homes may range from as low as 0.10 ACH up to 0.75 ACH.

Another building test which is similar to the blower door test is a duct blaster test. This diagnostic test also uses a fan to pressurize the ducts in a home to measure the leakage rate from the ductwork. Like air leakage through the building envelope, duct leakage can have a major impact on the performance of heating and cooling systems. Duct blaster results are often reported as CFM25, which is the duct leakage flow rate in cubic feet per minute when the ducts are pressurized with the duct blaster fan. Duct leakage results can also be expressed as a percentage of the area of the house (i.e., 250 CFM25 duct leakage / 2,000 ft² house = 12.5%). Some energy efficiency programs establish a level around five percent as a requirement.

Tests like the blower door and duct blaster tests can be performed by local utilities or energy audit companies. The cost for such tests starts around $100, although sometimes utilities may offer these tests for free. Certification for energy efficient home programs often requires blower door or duct leakage testing.

Another test that can be used to assess energy performance is an infrared scan. Thermal photography displays the temperature of surfaces on the building shell, which can be a very informative tool in evaluating how well a building is insulated and air sealed. This test is performed when the indoor to outdoor temperature difference is significant so that heat loss (i.e., cold surfaces) will be more pronounced. The color of a surface indicates its temperature, so cold spots are visually identified. Some builders use the results of infrared testing to identify and correct any deficiencies in the building shell as a home nears completion.

In addition to these one-time tests, energy monitoring uses sensors and meters over extended periods of time. So instead of a "snapshot" of energy conditions in a home, a more thorough record of how a house performs over time is collected. Energy monitoring can be as basic as sub-metering the electrical energy use of a light fixture or tracking the cumulative run-time of a furnace during the winter, all the way to recording dozens of temperatures, humidity levels, and power levels for years. The monitoring layout depends on the objective.

At the NAHB Research Center field projects often involve measuring the energy consumption of a whole house or particular systems, indoor and outdoor air temperature and humidity, HVAC system operating characteristics, and possibly hot water consumption or appliance use. Research of this type often involves a datalogger, which serves as the control center for all of the monitoring apparatus. The datalogger is programmed to read all of the sensors and meters, and process and store this information. Typical sensors and meters used in these projects include temperature and relative humidity sensors, watt-hour meters, current activated switches (which indicate run-time), and pyranometers (which measure solar radiation).

To learn more about any of these tests or participating in future research projects, contact Tom Kenney at tkenney@nahbrc.org.