Aprilaire 8126A Energy Recovery Ventilator, 24V Ventilation Control System Heat/Cool/Humidity Will not ventilate if the outdoor temperature is Above 100&deg; F or Below 0&deg; F Only ventilates if a call for heat is received by your central heating system when the outdoor temperature is between 0&deg; F and 20&deg; Will not ventilate if the outdoor air will raise indoor relative humidity above 55% Has cycle times ranging from every one to four hours, with ventilation times lasting from 0 to 60 minutes