Notice when the cool air occurs

Does the air feel cool briefly, between heating cycles, or throughout a call for heat? A fan set to On can circulate air even when the equipment is not actively heating. Check the thermostat’s normal user settings and read the operating instructions before deciding that the furnace has failed. Hand-feel at a register is not a reliable measurement of system performance.

Compare room temperature with the setting

Note whether the house reaches the requested temperature and whether the issue affects every room. A single uncomfortable room may involve air distribution or building conditions as well as equipment. Keep accessible registers and returns clear. Tell the technician if a filter change, thermostat replacement or renovation happened just before the issue began.

Avoid internal troubleshooting

Persistent cold airflow during a heating call needs professional assessment. Do not adjust gas valves, clean ignition components, bypass limits or repeatedly reset a fault. Have the model number and a description of the symptoms ready when calling Comfort Nest to discuss the next step. For gas smell, a CO alarm or smoke, leave the area and seek emergency help rather than investigating.