Framework for Coupling Room Air Models to Heat Balance Model Load and Energy Calculations (RP-1222)

Abstract

Most energy and load calculation procedures have assumed that room air is well mixed, which may lead to significant errors in sizing HVAC systems, estimating building energy use, and predicting thermal comfort for buildings with buoyancy-driven room airflow. This investigation has developed a framework and computer code for coupling detailed air models with building energy and load calculations as an extension to the ASHRAE Toolkit for Building Load Calculations. Two nodal models and a momentum-zonal model were selected for testing the coupling framework in a program for hourly load calculations of a single thermal zone. The heat balance model for load and energy calculations is reformulated to use zone air temperature as a variable defined separately for each surface. Air system flow rates are determined using air model predictions for temperature at the air system returns and a room air control location. The effect of air models on sensible load was found to be minor except when aggressive diurnal thermal mass strategies were involved. Nodal models appear practical to implement in load and energy programs and should improve results for air system flow rate and return air temperatures. Results show increases of about a factor of four in computing time for nodal models compared to the well-mixed model. Computing time is increased by two orders with the three-dimensional momentum-zonal model.