Assessment of Temperature-driven Pressure Differences with Regard to Radon Entry and Indoor Radon Concentration

A heavily instrumented and unoccupied research house is used to continuously monitor the temperature in each room, the supply and return ducts, HVAC thermostat location, attic and outdoors. Simultaneous measurements of the differential pressures have been performed across the house walls, indoorfattic zones, the house slab and the indoor zones. Indoor radon concentrations and meteorological conditions have also been monitored. Temperature measurements were conducted using thermocouple wires. All data were collected by computer-controlled data acquisition input boards. An analytical model was developed based on a linear approximation to the weakly exponentially dependent pressures between two temperature zones under hydrostatic equilibrium. The model is utilized to predict the temperature-induced pressure difference between two separated temperature zones. This model is then coupled with a semi-empirical formulation that utilizes pressure differences to predict radon entry into the structure and the air exchange rate across the house shell. The partial contributions of various temperatureinduced pressure differences are assessed on the model driving parameters. The contribution to the indoor radon concentrations are then obtained by incorporating the predicted changes in pressure differentials into the mass balance equation governing the time rate changes of the radon concentrations. It has been demonstrated that temperature differences associated with extreme weather conditions can generate pressure differences of several pascals that profoundly contribute to radon driving forces and indoor radon concentrations. The partial contributions of temperaturedriven radon entry rates and ventilation rates for a range of HVAC operation were also estimated. INTRODUCTION The transport of radioactive radon gas from the sub-slab area into structures such as residential housing can be described by convective and molecular diffusion processes. The soil gas has emerged as the main source of indoor radon (Nero and Nazaroff 1984, Nazaroff et. al 1988a. Crameri et. al 1989). Radon YRn) entry into residential structures occurs principally through pressure driven airflow processes. These processes are induced by pressure differential generating mechanisms that depend on interactions among environmental and indoor operational factors. Typical environmental interactions result in sources of house depressurization, such as temperature, wind and stack effects. House depressurization contributes to an increase in indoor radon concentrations as "'Rn-rich soil gas entry rates are increased compared to die increase in the air infiltration rates (Nazaroff et. a1 1987, Nero 1988). In many cases the concentrations of indoor radon exhibit a diurnal cycle driven by temperature dependent pressure differences (Gesell 1983, Kunz 1987). Furrer et al. (1991) studied the dynamics of Rn transport from the cellar to the living area in an unheated house and found that strong. long-term correlations between temperature differences and pressure differences exist between the indoors and outdoors Experimental verification, however, for this diurnal dependence is in most cases profoundly affected by uncontrolled pressure changes across the structure shell due to natural and mechanical ventilation caused by occupants' activities andfor the heating, ventilating and air conditioning (HVAC) system operation. Other researchers have given an increased importance to the transient effects of barometric pressure changes associated with changing meteorological conditions as contributors to radon driving forces (Owczarski et. 1994 International Radon Synlposium 111 6.1 a1 1990, Al-Ahniady 1992, Hintenlang and Al-Ahmady 1992. Tsang and Narsimhan 1992). Al-Ahmady and Hintenlang (1993) had developed a mechanistic model to characterize the temperaturedriven pressure differentials. The model is based on a linear approximation to the weakly exponentially dependent pressures between two temperature zones under hydrostatic equilibrium. In (his work, the previous model is utilized 10 assess temperature induced pressure differences contribution into the radon driving forces including radon entry and removal. A mathematical framework is developed to integrate the temperature-induced pressure differences model (AI-Ahmady and Hintenlang 1993) into a semi-empirical model for radon entry and indoor radon concentration. The latter model was developed by Hintenlang and Al-Ahmady (1994) to predict the radon entry and indoor radon concentration by the providing of pressure differences across (lie house shell and the slab. Parametric analysis is provided to estimate the temperature effects on the radon entry driving forces under unoccupied, unheated conditions and under heated conditions as provided by (lie HVAC systeni. The data presented were collected on the University of Florida Radon Research House (UFRRH) as part of the Florida Radon Research Program (FRRP). The research house was carefully chosen and heavily instrumented by a variety of devices. Thus, a number of key parameters could be measured or monitored simultaneously. The research effort by the University of Florida is dedicated to the development of building codes governing radon resistance for Florida houses and is designed to provide detailed characterization of the effects of HVAC on radon entry, as well as modeling of (lie radon entry and transport.