First confirm what is actually running and what temperature the system is delivering. “Not heating” can mean lukewarm supply air, cold radiators, or simply failure to reach the thermostat target.
Check filter and airflow on forced-air systems
A severely loaded filter, blocked returns, or many closed registers can restrict distribution and contribute to protective shutdowns.
Heat-pump air may feel only warm
Heat pumps often deliver lower-temperature air than combustion furnaces. Judge whether the home maintains setpoint and whether brief defrost cycles return to normal operation.
Compare radiators by zone
A radiator cold at the top may contain air; a completely cold unit may have a valve or circulation problem. If every radiator is cold, look upstream at the central system.
Notice whether the equipment stops too soon
Short cycles, error codes, new noise, or repeated restarts can prevent adequate heat delivery. Do not bypass limit switches or safety controls.
Stop for fuel or electrical warning signs
Gas odor, a carbon-monoxide alarm, smoke, strong electrical odor, or repeated breaker trips require shutdown or evacuation as appropriate and qualified help.
If the system runs but can no longer maintain a temperature it previously reached, that loss of performance is meaningful evidence for service.
Sources
- U.S. Department of Energy — Home Heating Systems — Heating-system operation, efficiency, controls, and maintenance.
- U.S. Consumer Product Safety Commission — Carbon Monoxide Information Center — Carbon-monoxide hazards, alarms, fuel-burning appliances, and emergency response.