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. |
REBUILDING
FOR EFFICIENCY:
Improving the Energy Use of Reconstructed
Residences in South Florida
Appendix
B
Analysis of Energy Losses of Thermal Distribution System
Analysis
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.
Simulation Analysis
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:
Results
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:
Adjustments
to DOE 2/1D HVAC Efficiencies to
Reflect Thermal Distribution System Interactions
Case Description |
Heating EIR Multiplier |
Cooling EIR Multiplier |
|
||
1. Base | 0.857 |
0.827 |
2. R Leak | 0.932 |
0.945 |
3. No DHX | 0.923 |
0.880 |
4. R Leak/No DHX | 1.000 |
1.000 |
These numbers
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.
Justification
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.