Airflow and Ventilation

Building Air Change Rates

The determination of building air change rates requires knowledge of airflow into a building through the building envelope whether intentional or unintentional. Unintentional airflow into the building is referred to as infiltration. The determination of air infiltration rates can be useful for a variety of purposes including the estimation of energy impacts due to building envelope leakage, the effects of weather, and the effectiveness of natural ventilation systems. Knowing infiltration rates, outdoor weather conditions, and desired indoor conditions, one can then estimate the contribution of infiltration to heating and cooling loads.

Infiltration analysis with CONTAM requires the definition of leakage paths in the building envelope that are distributed vertically over each facade along with the definition of wind conditions at each facade. CONTAM provides the ability to define leakage paths using several different models that relate pressure difference to airflow. These models provide the ability to define leakage paths using various physical descriptions such as cracks, orifices, and effective leakage areas. Wind pressure can be define using either surface-average methods or detailed methods with spatially varying wind pressures that are either constant or variable in time for each envelope penetration.

CONTAM also enables the analysis of mechanically-induced airflows via mechanical ventilation systems. These systems affect building air change rates directly and indirectly. Direct effects include those due to the intake of outdoor air by air handling systems. Indirect affects are less obvious and can include duct leakage and relative zone pressurization/depressurization. CONTAM provides three types of mechanical ventilation systems: fixed flow airflow paths, simple air handling systems, and detailed duct models. These systems are described below in more detail.

Ventilation Strategies

CONTAM provides a rich set of tools to analyze different ventilation strategies. Applications in this area include the ability to compare the use of different ventilation strategies within a given building, size air handling systems, analyze natural ventilation systems, and simulate demand-controlled ventilation (DCV) systems.

Comparison of Ventilation Strategies - CONTAM enables the analysis of ventilation systems to determine the impacts on building airflows, pressure differences, and contaminant concentrations. A given building model can be easily reconfigured to compare different ventilation strategies within a given building geometry. CONTAM provides several methods of simulating mechanical ventilation systems including: forced flow paths, a simple air handling system, and a duct system. Simple forced airflow paths can be defined to move air between adjacent zones at a fixed rate. For example, a bathroom exhaust fan or attic ventilation fan can be implemented to move air directly across the building envelope. The simple air handling system is provided to simplify the amount of data input relative to defining an entire duct system. This system enables the simulation of an air handler that can bring in varying amounts of outdoor air and re-circulate and exhaust return air. It provides the ability to distribute the supply air to and return air from any building zone. The duct system provides the most detailed method of modeling a mechanical ventilation system. An entire duct system can be defined including duct segments, junctions, transitions, terminals, dampers and fan performance curves. Each element of the duct system provides for the definition of friction losses, duct leakage, and dynamic losses.

All of these ventilation systems can be configured to move air at varying rates according to user-defined schedules. For example, the fraction of outdoor air intake of simple air handling systems, fan on/off cycles, and airflow rates through forced flow elements can be scheduled.

Sizing Air Handling Systems - CONTAM can be used to size ventilation systems including the determination of airflow requirements to obtain relative pressurization of building zones or determining ventilation flow rates for whole house ventilation systems.

While CONTAM is not a formal fan-sizing tool, it does enable an iterative approach of investigating multiple scenarios of building configuration and fan flow rates. It is fairly straightforward to establish building geometry and leakage characteristics for a given building within ContamW. Given required design conditions, e.g. minimum/maximum pressure differences between zones, one could use the ventilation system modeling capabilities to provide different airflow rates until these design conditions are met. Not only can the ventilation rate and delivery points be varied, but different envelope leakage characteristics can be investigated as well. Different leakage effects could include the opening and closing of doors and windows, construction tightness, building components with different leakage characteristics, or the use and placement of passive venting elements such as door transoms.

Designing Natural Ventilation Systems - Some of the fundamental aspects of designing a natural ventilation system are the geometry and orientation of a building and the sizing and placement of airflow paths to take advantage of prevailing weather conditions. CONTAM provides the ability to model a building to account for these features [Axley 1999].

CONTAM provides results in the form of airflow rates through each airflow (leakage) path that the user defines for a building. Using this feature one can perform simulations to investigate the differences in airflow rates obtained by varying different building features and weather conditions including the size and placement of ventilation openings in the building envelope, the orientation of the building to prevailing wind, the outdoor temperature difference and the size and location of ventilation stacks.

Demand Controlled Ventilation - Demand controlled ventilation (DCV) involves the control of ventilation rates based on the monitoring of occupancy levels. As an alternative to monitoring occupancy levels directly, carbon dioxide (CO₂) can be monitored since occupants are the dominant indoor source of CO₂. CONTAM can be helpful in addressing issues related to the design and operation of DCV systems [Musser 2000]. Given information on sources that might be found within a building, analysis can be performed to determine what contaminant levels might result based upon ventilation rates provided by a DCV system. Using CONTAM, one could perform such an analysis while accounting for the complex airflows associated with a multizone building. This analysis could take on various forms. In its simplest form, one could estimate minimum ventilation rates by performing steady state analyses for a range of source strengths that are likely to be encountered and determine the ventilation rates required to achieve desired contaminant levels. While this simplified approach tends to ignore the time-dependant behavior of the DCV system and contaminant sources, it can provide a reasonable estimate of a minimum ventilation rate required to control the contaminant in question. Transient simulations could also be performed to better account for varying airflows, adsorption/desorption of contaminants and time varying contaminant concentrations.

Building Flush out - Another concern with ventilation systems that might minimize or shut off ventilation during unoccupied hours is the build up of non-occupant related contaminants during this so-called night set back period. CONTAM can be useful in analyzing the levels of contaminant buildup that would occur under such situations and in determining the amount of ventilation required to purge, or flush out, the contaminants prior to occupancy [Musser 2000].