Field study measurements evaluating radon concentrations under different ventilation scenarios

Increasing indoor ventilation has the potential to dilute indoor radon and may be an appropriate first step when measured indoor radon concentrations are close to the mitigation threshold for an existing low-rise house that lacks balanced mechanical ventilation. A ventilation system that includes a heat exchange core is recommended in cold climates to reduce the energy loss associated with replacing stale indoor air with outdoor air that must be either cooled or heated to maintain thermal comfort. A field study of the effectiveness of heat recovery ventilation systems (HRVs) at reducing moderate indoor radon concentration was conducted in 16 houses between 2020 and 2023 in the National Capital Region in Canada.   
The average indoor radon concentration for different HRV settings within a season were compared to estimate the effectiveness of radon reduction.  Hourly measurements on lower and upper floors were made using continuous radon monitors and averaged over a one-month period for each HRV setting evaluated:  off, running continuously or periodically (typically 20 minutes per hour), and with a fan speed of high or low when running. The effect of the HRV system on indoor radon concentrations was evaluated for sixteen houses, while a comparison of the effect of operating both the passive depressurization system (PSD) for radon mitigation and the HRV was also evaluated for four of the study houses.   
Results and Conclusions: The initial radon concentrations measured on the lower floor in the 16 study houses with the HRV off were moderate, ranging from 91 to 312 Bq/m³, and were roughly lognormally distributed with a geometric mean of 166 Bq/m³. The arithmetic mean indoor radon was reduced by 40% for continuous HRV operation in the 13 houses with forced air furnace heating systems, with the reduction effectiveness characterized by a normal distribution with a standard deviation of 12%, a minimum reduction of 20% and a maximum of 56%. A higher reduction effectiveness of 80% was observed in the occupied house that had electric baseboard heating and in which an independently ducted HRV was installed.  There was an overall trend of higher reduction effectiveness for continuous than periodic fan operation for houses in which more than two HRV settings were assessed.  Well designed and installed HRV systems were effective at reducing the moderate indoor radon concentrations measured in these houses that lacked a balanced mechanical ventilation system. In one house, both a passive radon depressurization system and an HRV were required to reduce the indoor radon concentration below the recommended mitigation threshold of 200 Bq/m³ for existing housing in Canada.