[BLDG-SIM] specifying duct efficiency in model

[Ian Doebber]

I am wondering how current energy modelers incorporate duct efficiency in
homes.  Currently, I have specified all my ducts to be within my
unconditioned basement.  Using the defaults from the "Building America"
program in the ASHRAE standard 152 duct efficiency calculations, my
"Delivery Effectiveness" and "Distribution System Efficiency" results are

"Delivery Effectiveness"                                   "Distribution
System Efficiency"
Heating, design 0.76                                        Heating, design
0.72
Heating, seasonal 0.78                                     Heating, seasonal
0.74
Cooling, design 0.84                                        Cooling, design
0.79
Cooling, seasonal 0.87                                     Cooling, seasonal
0.83

My first question would be whether I should use the "Delivery Effectiveness"
or "Distribution System Efficiency" to specify the efficiency of my thermal
distribution system.  My second question would be should I just use the
heating and cooling average of the design or seasonal efficiency or
something else entirely.

My final question pertains to: once I choose my duct efficiency, how to
include it in my model.  Should I assign some percentage of the loss to duct
air leakage and thereby model some of my air supply going directly to the
basement.  Then account for the rest of the loss, conduction loss, by having
the air temperature leaving the registers lower/larger from the air
temperature leaving my system heating/cooling coils respectively.

Thank you
Ian Doebber
Mechanical Engineering M.S.
Virginia Tech


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