Reference Publication: Parker, D., Fairey, P., McCluney, R., Gueymard, C., Stedman, T., McIlvaine, J., "Rebuilding For Efficiency: Improving the Energy Use of Reconstructed Residences in South Florida", Prepared for U.S. Department of Energy, Florida Energy Office, and Florida Power & Light Company, FSEC-CR-562-92, December 1992.
Disclaimer: The views and opinions expressed in this article are solely those of the authors and are not intended to represent the views and opinions of the Florida Solar Energy Center.
Improving the Energy Use of Reconstructed
Residences in South Florida
Analysis of Energy Losses of Thermal Distribution System
of Impacts of Duct Leakage and Heat Transfer for use with DOE 2.1D
Prior to the analysis, we used a detailed simulation of a residential prototype building to determine the impact of duct leakage and duct heat transfer on the overall system efficiency of residential heating and cooling systems in Florida. This is important since DOE 2. 1D cannot account either for duct air leakage or heat transfer to the duct system. Since ducts are most often located in the hot attic environment in Florida, these questions are very pertinent to any analysis of potential savings associated with the HVAC distribution system. In turn, the efficiencies of the heating and cooling system used in the analysis have systematic effects on the savings impacts of most building shell and HVAC measures.
The building used has a 1,500 square foot rectangular floor plan as described in the report. The finite-element model features a detailed description of the attic and duct system which is described in two recent papers.’ 2 The simulation, FSEC 3.0, runs on an hourly time step.
We analyzed a number of cases on both the peak heating and cooling days on the Orlando TMY tape (January 25th and August 1st). We also completed annual runs of the same building with the various configurations. The cases of greatest interest are as follows:
Annual results from the simulations are attached. They show that the cooling equipment efficiency (in DOE 2. JD, Cool-EIR and Heat-EIR) should be de-rated by the following numbers to achieve an accurate reflection of the influence of the distribution system:
to DOE 2/1D HVAC Efficiencies to
Reflect Thermal Distribution System Interactions
|2. R Leak||
|3. No DHX||
|4. R Leak/No DHX||
indicate that duct leakage and heat transfer degrades cooling system efficiency
by about 17% and heating system efficiency by about 14%. Duct heat transfer
is responsible for a 6% reduction to cooling efficiency and an 7% penalty
to heating efficiency. A 70% sealing of duct leakage results in slightly
better improvements than does eliminated heat transfer. Duct leakage degrades
heating efficiency by 8% and cooling efficiency by 12%. Note that the combined
effects are not strictly additive. Although not analyzed, the similarity
of the heating and cooling modifiers in the table lead us to believe that
the results are climatically robust for Florida.
Since case 3 is generally unrealistic (without an infinite thickness of duct insulation while preserving leakage), cases 1, 2 and 4 generally describe the factors needed for the DOE 2. JD analysis. The base case would be case 1, duct leakage repair would be case 2 and an interior mounted duct system would be case 4. This means that the base case analysis would assume a resistance heating system efficiency of 85.7% and a cooling system SEER of 8.27 with a nominal SEER 10 unit. Our only qualification to these results are for systems that strongly interact with duct heat transfer. Although not included, our analysis indicates that the use of an attic radiant barrier or reflective roof serves to greatly reduce the magnitude of heat transfer during summer conditions to the duct system (compare the RBS and RBS/NoLk cases). We therefore recommend that EIR multipliers of 0.98 for cooling be used when a radiant barrier or reflective roof is in place and the duct system has been sealed, but is still present in the attic.
FSEC ‘ s studies are not the only works indicating the magnitude of losses associated with thermal distribution systems. Other recent investigations by Lawrence Berkeley Laboratory and Brookhaven National Laboratory has found similar (or worse) effects of duct systems on efficiency through the use of another very detailed simulation of the duct distribution system.
Field measurements also show that the magnitude of energy losses from thermal distribution systems are quite large. A field study of duct repair to 12 homes and 66 apartments in climatically similar Los Angeles showed a measured reduction in air conditioning energy of 30.1 % . A study of 15-randomly selected homes in the Pacific Gas and Electric Service territory showed an 18% savings in measured space cooling energy consumption after duct sealing.6 Perhaps most interesting of all, a utility field test of interior located duct systems in four apartments versus their previous existing external duct systems showed a 40% reduction in space heating energy.
The implications of such results from many diverse agencies around the country shows that the utilized levels of efficiency improvement from duct system sealing and elimination of are veiy conservative. The recommended values would indicate a 21% reduction in cooling energy use (system efficiency is reduced by 17%) from an internal well-sealed duct system. Duct sealing alone would reduce cooling energy use by 14%. These adjustments were consistently used in the DOE 2 analysis of efficiency options associated with the thermal distribution system for this report.