CIBSE has advised that thermal wheels be turned on in winter to ensure there is adequate outdoor airflow to dilute any indoor viral contaminant in the heating season.
In an update to Covid-19 Ventilation Guidance, CIBSE said the heat-recovery function was integral to system design in terms of airflow and meeting heating demand.
With the wheel turned off in cooler weather, CIBSE said there may be less outdoor air entering the building and lower airflow rates, leading to a potential reduction in dilution of any indoor virus. This is deemed a greater risk for viral transmission than the potential viral transfer across the thermal wheel.
CIBSE added that thermal wheels should be switched off outside of the heating season, or bypassed, because there is a risk of air leakage and moisture transfer between the supply and exhaust airstreams.