Thermal Distribution Efficiency - Legacy System

As documented in a companion report (Warner 2005), the Home Energy Saver uses the hourly DOE-2 thermal simulation model to estimate heating and cooling consumption. The treatment of air distribution duct losses in DOE-2 is very simple, allowing only a single value of duct losses (expressed as a percent of air input to the ducts) that applies to every hour throughout the year. Although it would be desirable to model duct efficiency as varying throughout the year, as a function of the ducts' environmental conditions, this would require a significant effort in modifying DOE-2.

Instead, we use an annual-average method for estimating the effect of duct materials and the type of space in which the majority of their duct system is located, since duct losses differ significantly depending on these factors. We used the ASHRAE 152P duct model to estimate duct losses for use as an input to DOE-2 (ASHRAE 1997a). Although this model is intended to calculate seasonal duct efficiencies based on detailed diagnostic testing, we assumed typical values for most of the inputs (such as duct surface area and number of return ducts) so that the number of inputs required of the user is more reasonable.

Users are able to specify whether or not the ducts are insulated and/or sealed, and the duct location. Insulated ducts are assumed to have R-5 insulation, while uninsulated ducts are assigned an insulation value of R-1 (to account for the thermal resistance of the external air film on the ducts). Unsealed ducts are assumed to have a leakage of 30% of the total air handler flow, based on field testing in existing California homes (Jump et al. 1996). Because concerted duct sealing efforts can typically reduce leakage by one-half, we assume that sealed ducts have a leakage rate of 15%. If users choose not to specify their duct location, we infer the location based on the type of foundation and typical building practices. Table 8 shows the default duct location that corresponds to each of the foundation types available in HES.

Table 8. Default Duct Location

To account for the effect of local climate on ducts located in unconditioned spaces, the ASHRAE 152P model uses design-day weather data from the ASHRAE Handbook of Fundamentals (ASHRAE 1997b). The model inputs are the winter 97.5% design dry-bulb and the summer 2.5% design dry-bulb, shown in Appendix C.

The ASHRAE 152P model generates seasonal duct efficiencies for both the heating and cooling seasons, which are then averaged together using weights corresponding to the HDD and CDD in that location, normalized to the national average degree-days (using TMY2 data). These weighting factors are shown in Appendix C, in the “duct factor” columns. A single annual average duct efficiency is passed to the DOE-2 model as an input to the hourly thermal simulation. This annual duct efficiency is determined based on the type of heating and cooling equipment in the house. The logic for determining the annual duct efficiency is captured in Table 9 by the intersection of heating columns and cooling rows based on the presence or absence of ducts for each type of equipment.

Table 9. Annual Duct Efficiency Based on HVAC equipment

Cooling Equipment

Doesn’t Have Ducts

Doesn’t Have Ducts

Has Ducts

Has Ducts

Heating Equipment

Doesn’t Have Ducts

Has Ducts

Doesn’t Have Ducts

Has Ducts

Duct Efficiency

100% (no duct losses)



Notes to Table 9:

DF = Weight factor based on relative proportions of heating- and cooling-degree days for location

HSE = Weighted heating seasonal duct efficiency from ASHRAE 152P model

CSE = Weighted cooling season duct efficiency from ASHRAE 152P model

Efficiency of Boiler Pipes

Boiler pipes are assumed to have a baseline efficiency of 90% (Wenzel 1997). Users are able to indicate whether their pipes are insulated. For insulated pipes we stipulate a 5% increase in efficiency.