Energy Conservatory Articles
Measured Efficiency of Forced Air Duct Systems in the Pacific Northwest.
(from TEC Update - Fall 95)
A 1994 study by Larry Palmiter and Paul Francisco of Ecotope, Inc. provides important new details on the efficiencies of residential forced air duct systems.
The study measured the efficiency of forced air duct systems in 24 electrically heated houses located in the Pacific Northwest, including both heat pump and forced air heating systems. The houses were closely monitored in order to calculate two separate measures of duct system efficiency. Aggressive duct sealing retrofits were performed on 6 of the houses and the efficiencies were remeasured.
The majority of the study houses (22) were chosen because they had over 50 percent of the their duct systems located outside of the conditioned space. Locating duct systems in unconditioned spaces is a common practice in large portions of the United States and has been shown in numerous studies to contribute to significant energy losses. Two additional houses were studied which had all of the ductwork located within the conditioned space in order to provide a reference to compare against the larger sample.
For each duct system, a Heat Delivery Efficiency was calculated by measuring the air flow and delivery temperatures at each supply register, and monitoring the energy input to the electric heating system. The Heat Delivery Efficiency is the percentage of energy input to the heating system which is actually delivered to the room via the supply duct system. Heat not delivered through the registers can be attributed to either duct air leakage losses, or duct conduction losses.
Additionally, a System Efficiency was calculated by comparing actual heating system energy use with a coheating system designed to duplicate the interior temperatures under normal operating conditions. The System Efficiency measurement accounts for any recapture of duct losses to the house, and therefore is a better measure of the overall operating efficiency of the duct system and its impact on energy use.
Note: The System Efficiency measurement does not include the effects of increased infiltration during fan-off times and differential pressurization due to door closure, therefore it can be assumed to be an upper bound on the actual duct system efficiency.
Exterior Duct Systems
For the base sample of 22 houses with exterior duct systems, the average Heat Delivery Efficiency was measured at 56.2%. This means that nearly 44 percent of the energy input to the heating system was lost due to air leakage and conduction before being delivered through the supply ductwork. The study found that on average, air leakage and conduction contributed equally to the efficiency losses. It should be pointed out that the duct systems in the base sample were quite well insulated, with a median duct insulation R-value of 7.0. Average duct leakage to the outside for these houses was measured at 436 CFM50 using a combined Duct Blaster and Blower Door test procedure.
The average System Efficiency of the 22 exterior duct houses was measured at 71.0%, indicating that the recapture of duct losses to the house increased the overall efficiency of the duct system by nearly 15 percentage points from 56.2%. Despite the recapture of duct losses, these 22 houses still used approximately 40 percent more heating energy than would identical houses with electric resistance heating (i.e. no duct system).
Interior Duct Systems
Equally intriguing were the findings on the sample of 2 houses with interior duct systems. The average Heat Delivery Efficiency for these 2 houses was measured at 66.8%. While this value is significantly higher than the Heat Delivery Efficiency for exterior duct systems, it still points to large duct losses for interior duct systems. However, the average System Efficiency for these two houses was measured at 97.9%, indicating that almost all of the duct losses for the interior duct systems was recaptured as useful heat to the house. This finding is particularly important because it suggests that efforts to seal duct leaks in interior duct houses may not be warranted from an energy efficiency perspective. Because of the very limited sample size however, this conclusion needs to be examined more closely through further research.
Duct System Retrofits
Extensive duct system repairs were performed on 6 of the exterior duct houses and the impact of the repairs on duct system efficiency was measured. The primary focus of the duct repairs was to reduce air leakage in the duct system. For the 6 houses, duct leakage to the outside was reduced by an average of 70%, from a pre-repair leakage rate of 541 CFM50 to 161 CFM50.
The actual reduction in space heating energy use under the test conditions was 16.4% for the sample. This level of energy savings is consistent with previous duct retrofit studies performed on exterior duct houses. Cost information for the retrofit work was not provided in the paper we reviewed.
The average measured System Efficiency for the 6 houses increased from 69.2% to 82.8% following the repairs. Despite extensive repairs to the duct system and fairly high levels of duct insulation, these 6 houses still use 20% more energy than would identical non-ducted houses.
Ecotope is a research and consulting firm located in Seattle, Washington.
