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[FIGURE 1 OMITTED]Simulation Strategy & Boundary ConditionsPerformance is assessed primarily through numerical modeling, both using VENT-II and Computational Fluid Dynamics (CFD). VENT-II software was developed for the Gas Research Institute (GRI) during the original development of the venting tables (DeWerth, 1983). FLUENT version 6.3 is used for CFD, modeling fluid flow, heat transfer, and phase change. As VENT-II is a simpler 1-D modeling tool with a Graphical User Interface (GUI), it has been a useful tool in venting research. With its finer resolution and more sophisticated algorithms, CFD is used to assess the relative veracity of VENT-II in modeling venting systems for varied boiler efficiency, firing rate, ambient condition, number of exposed chimney walls, and presence of chimney liner. Simulation results are compared with experimental data from a full-scale test masonry chimney, using the metric Boolean continuously "wet" or "dry" conditions as used in the NFGC.In the baselining of this condensation modeling methodology, a non-compliant common venting of a fan-assisted furnace and natural-draft gas-fired water heater into both interior and exterior masonry chimneys is simulated with VENT-II and CFD. Comparing these simulation results, the interior and exterior masonry chimney simulations result in net condensation gains within the chimney segment above the roofline within 43% and 22% of each other (with CFD showing consistent overestimation), showing order of magnitude agreement. As the primary criteria of simulation-based venting table development was the Boolean "wet" or "not wet", this agreement is sufficient, and this modeling method was adopted.* The cycle rate at 50-percent load is 2 cycles per hour for a boiler (this directly affects furnace on-times as well), and* A boiler is typically equipped with a draft hood and a stack damper.2. Reviewing similarly employed condensation models in the literature, they fit into one of three categories in increasing complexity: simple mass flux, mass diffusion via an empirical diffusivity, and heat diffusion based upon film or drop-wise condensation on various geometries. After performing extensive modeling of flue gas condensation, Perujo et al. (2004) found that for a sufficiently fine mesh, the mass flux and mass diffusion models yield similar results within 10% of one another, which due to its simplicity was selected (Perujo, 2004). This model states conservatively that water vapor in excess of the saturated cell condition will condense. This is determined after calculating the water vapor saturation pressure at each cell (ASHRAE 2009), the actual and saturated water vapor concentration is determined.

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