Why your furnace runs but your home stays cold
A furnace can run continuously while the house stays cold for reasons that range from a misset fan switch to a cracked heat exchanger. Some causes, like a clogged filter or a blocked vent, you can fix in minutes. Others require a contractor with diagnostic tools to find and repair.
Why is my furnace running but not heating?
Start with the thermostat.
If the fan setting is switched to ON rather than AUTO, the blower runs constantly, pushing unheated air through the vents even when the furnace is not firing. Switch it to AUTO and give the system a few minutes. Next, pull out the air filter. A filter packed with dust chokes airflow so severely that warm air never makes it into the living space, and the furnace keeps running trying to compensate. If the filter looks gray and dense, replace it. Then walk the house and check every supply and return vent. A vent blocked by furniture or closed by hand starves that room of heat. Finally, go to your electrical panel. The air handler sometimes sits on its own breaker, and a tripped breaker kills the blower while the furnace itself keeps cycling.
Why is my furnace running but house still cold?
If the self-check in the first movement changed nothing, what remains is a class of failures that all look identical from the thermostat but require instruments and access you do not have.
A heat pump low on refrigerant will run continuously without transferring heat, and confirming it means measuring pressures at the refrigerant circuit. A cracked heat exchanger routes combustion gases away from the air stream and can only be confirmed through combustion analysis. A failing inducer motor disrupts airflow through the heat exchanger before warmth ever reaches the ducts. Ductwork with significant leaks or disconnections inside walls bleeds conditioned air into unconditioned space, and the only way to find and measure that loss is with a static pressure test. A system undersized for the house will simply never catch up on a cold day, and proving that requires a full load calculation. Every one of these failures produces the same symptom you started with.
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