Heating symptoms and their usual causes
- Heater blowing cold air
- On a furnace this usually means the burners are not staying lit, so the blower runs while the heat exchanger cools. On a heat pump it can be normal defrost operation or a system that has lost its heating capacity.
- Heating will not turn on
- Control power problems dominate: a blown low voltage fuse, an open safety switch, a thermostat without power, or a door switch left open after filter service.
- System starts then shuts off after a few minutes
- Classic limit trip. Restricted airflow from a loaded filter or closed vents lets the heat exchanger overheat, and the high limit shuts the burners down to protect it. It can also be a fouled flame sensor.
- Loud bang at startup
- Delayed ignition, where gas accumulates before it lights. This should be corrected promptly because it stresses the heat exchanger.
- Some rooms never get warm
- Duct design, leakage, and balance rather than the furnace itself. Attic duct systems in this region commonly lose meaningful heat before it reaches the room.