Assessing the energy and indoor-PM_2.5-exposure impacts of control strategies for residential energy recovery ventilators

The energy recovery ventilator (ERV) is a type of mechanical equipment that provides ventilation into the building while reducing the energy required to condition the ventilation air. A computer modelling approach was used to assess energy consumption (the energy used to run the air-conditioning and ERV) and indoor PM_2.5 exposure concentrations from both indoor and outdoor sources in Hong Kong high-rise residential flats under various ERV control strategies. Three varying inputs, including ventilation mode (energy-exchange, bypass or a hybrid of both), filter efficiency and ventilation rate, were used to develop different ERV control strategies. The estimated energy consumption and indoor PM_2.5 exposure concentration were monetised using per-occupant cost functions, in order to allow a direct combination between the two. Results show that the ERV switching between energy-exchange and bypass modes according to outdoor weather conditions saves 43% of the annual energy cost compared with that operating in bypass mode, while the ERV operating in energy-exchange mode saves 28%. Integrating an air filter with a high PM_2.5 removal efficiency into the ERV causes a reduction in the annual exposure cost with increased ventilation. By combining energy and exposure costs, this study has been able to determine the most cost-effective ventilation rate of the ERV, which costs an occupant approximately HK$ 2692 over the course of a year. Varying the ventilation rate as a function of outdoor temperatures, outdoor humidity ratios or outdoor enthalpy further reduces the combined cost compared with operating the ERV at the most cost-effective ventilation rate.