This thermostat controls a 90+ Single Stage Category IV Furnace and a R-22 Split System Air Conditioner. We also have a vanEE Model #1601606 Ventilation system on it. Our system is delivering cold air back out through a cold air return register in a master bedroom with the hot register shut. The outside temperature is currently -10F. The air conditioner has been shut down and tarped for months. Can you please tell me what steps I can take to identify the problem and correct this before I call in a tradesman on a holiday weekend? Thanks.

Why is the hot register shut?