DUCT LEAKAGE TESTING
Similar to the envelope air leakage problems, leaky mechanical air ducts were found to be huge source of energy loss. Unless installed and tested by a certified Home Performance Contractor and then verified by a HERS rater, residential duct system leak at a rate between 30% to 50%, and this is true for new and existing homes. Plumbers have no tolerance, their systems leak 0%. Accepting leaky mechanical ducts, at these high percentages, is equivalent to accepting a leaky gas tank on your car. Energy Efficient Design requires duct leakage to be 0% - 6%.
Understanding this huge energy loss when delivering heated/cooled air from the furnace/air handler to the rooms of the house is referred to as distribution losses. Far too often, home owners upgrade the efficiency of their furnace from an old 78% efficient furnace to a new 92% efficient one without improving the leakage rate of the mechanical ducts. When this is done, the real upgrade in furnace efficiency is 14% (92%-78% Furnace) x 30% (Duct loss) = 4.2% increase in efficiency. It is much less expensive to fix the 30% leaky ducts and keep the old 78% efficient furnace, which results in an overall improvement of 16% - much better than 4.2%.
Testing the leakage rate of mechanical ducts is done with an applied building science instrument called a Duct Blaster. This tool is similar to the Blower Door used in envelope leakage testing, but done at a much lower testing pressure of 25 pascals. With the mechanical registers sealed and the Duct Blaster connected to the return air register, a measurement is made of the CFM leaking from the sealed system. This value is compared to the required CFM of the mechanical system as prescribed by the manufacturer, resulting in the percent leakage of the duct system.