Here's a new term home buyers should learn. It's called a HERS rating and I just spent the morning witnessing all the exams required to provide an East English Village home with one. HERS stands for Home Energy Rating System. A score of 100 means that the home's energy performance is equal to that of a home built to minimum energy code standards. A score of 85, which is required for any home meeting Energy-Star standards, means the home will perform 15% better. In other words, the lower the score, the better energy performance.
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| Minneapolis Blower Door Source |
The Blower Door Test
To perform this test, the home is de-pressurized by sealing all openings except one, typically the front door. A fan is placed within this opening. When the fan is turned on, it pulls air out of the house. The fan pressure is raised to 50 Pascals (Pa) and using the gauges attached to the fan, the amount of air being pulled into the house through cracks and openings is measured. The rate of leakage is measured as the number of air changes in an hour, or ACH. Current energy code requires all new homes to have a maximum of seven (7) ACH. For a typical home, like mine which was built in the 1960s, this number is typically around 14 ACH. Today's renovated home performs well with 8 ACH.
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| Duct Leakage Test Source |
Part 1 - Pressurized Duct Test
Next we move to duct performance. To perform this test, covers are placed over all supply registers sealing the entire duct system. The furnace cover is removed and a fan pushes air into the ductwork. Hoses attached to the fan and to nearest supply register measure pressure. The difference between the two pressures is converted to a leakage rate. The higher the pressure, the more accurate the measurement. Typically, the pressure must be raised to at least 25 Pa to get the most accurate measurement.
Today's test indicates a leakage rate of 2350 cfm, but at only at a pressure of 3 Pa. The fan is unable to raise the pressure above 3 Pa. In other words, the system is very leaky. 2350 cfm of air is being lost through leaks within the home's ductwork system. Unfortunately, these results are more the norm than atypical.
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| Infrared camera KT-384 made by Sonel Source |
Next, the ducts are pressurized at the same time the blower door fan pressurizes the house. This requires reversing the fan direction so that air is blown into the house. The pressure within both is equalized and a gauge at the blower door fan measures the difference between the two fan rates. The difference is converted to the rate of air leaking from ducts outside the thermal envelop. In other words, this test measures the rate of air being leaked from ducts within unheated spaces like attics and crawl spaces.
Today's test indicates 186 cfm of air is being leaked through ducts that are located out side the thermal envelop.
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| Thermogram of two buildings. Darker colors indicate cooler temperatures. Source |
The final test again pressurizes the supply duct system and pressurizes the entire house using the blower door fan. In this test, however, the difference in pressure between the blower door fan and the pressure taken at various supply registers throughout the house is compared. For a good duct system, the differences will never be more than 2 Pa.
Today's home performs well. The pressure difference at two 1st floor supply registers is measured at no more than 0.8 Pa.
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| Foam used to seal air infiltration points at the rim board |
During the performance of the blower door test, a thermal imaging infrared camera is used to locate areas of air leakage. Pointing the device at surfaces within the home, the camera measures surface temperature differences. On a cold winter day, where outside temperatures are hovering around 20 degrees and indoor temperatures are around 68 degrees, the camera reveals temperatures below 68 degrees in varying gradations of dark colors. The darker the color, the colder the surface. The colder the surface, the more likely the location is a source of cold air leakage.
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| Poorly sealed fireplace doors were a major source of air infiltration |
Conclusion
With the ultimate goal of improving home energy efficiency, improving homeowner comfort and reducing homeowner utility bills, more and more homes are going to be tested for air and duct leakage. Homeowners will expect architects to use tests as described above to push contractors to raise their own standards. And let's be clear, the savings that can be achieved by a homeowner are significant. And the same way buyers use MPG (miles per gallon) to gauge the fuel efficiency of cars, HERS ratings will begin to educate home buyers on home energy performance.
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