What to check when a furnace cannot keep up in very cold weather
A falling indoor temperature can come from airflow, controls, the building load or an equipment fault.
Confirm the thermostat setting, check the filter and make sure registers, returns and exterior vents are clear. If the indoor temperature keeps falling, the furnace cycles abnormally or you see an error indication, arrange service rather than repeatedly resetting it.
Safe checks first
Confirm the thermostat is in HEAT mode and its displayed temperature and setpoint make sense. Check a user-serviceable filter. Open supply registers and uncover return-air grilles. From the ground, inspect exterior vent openings for snow or ice.
Cold weather changes the load
A furnace may run much longer during severe cold because heat is leaving the building faster. Open doors, large exhaust fans, wind exposure, insulation and duct losses can all affect comfort. Long runtime alone does not prove the furnace is failing.
What to tell the shop
Tell the technician whether the furnace runs continuously or starts and stops, whether every room is affected, whether the thermostat temperature is falling, and whether there are new sounds, smells or error lights. Those details are more useful than repeatedly changing the setpoint.
Stop for a safety concern
If you smell gas, see smoke or a CO alarm sounds, leave and call 911 from outside. For non-emergency loss of heat, call the shop; after-hours rates and availability apply.