A blower that keeps running well after the house has reached temperature is unsettling mostly because it's unfamiliar; homes are used to hearing the furnace kick on and off in a predictable rhythm. A blower that won't stop is usually not an emergency, but it is worth understanding rather than ignoring, since it can point to a few different things depending on how it's behaving.
Check the thermostat fan setting first
Before assuming anything is wrong, check whether the thermostat's fan setting is set to "On" instead of "Auto." On the "On" setting, the blower runs continuously regardless of whether the furnace is actively heating, which is normal behavior, not a malfunction. This is the single most common cause of a fan that seems to run nonstop, and it's a simple switch to check before anything else.
A stuck or failing limit switch
If the fan setting is correctly on "Auto" and the blower still won't shut off, the limit switch is a common culprit. This safety switch monitors the temperature inside the furnace cabinet and keeps the blower running until it cools down to a safe level, so if it's stuck or misreading the temperature, the blower can keep running long after the burner has shut off.
A relay or control board issue
On furnaces with an electronic control board, a stuck relay can send a continuous signal to the blower motor even when it shouldn't be running. This is a less common cause than the fan setting or limit switch, but it's part of what we check during a diagnostic if the simpler explanations don't hold up.
When this points to a bigger airflow problem
Occasionally a blower that won't shut off is a symptom of the furnace overheating due to restricted airflow, a clogged filter, blocked vents, or a duct problem, that's tripping the limit switch repeatedly as a protective response. If you've also noticed weak airflow from the vents, a burning smell, or the furnace short-cycling before this started, mention that when you call, since it changes what we check first.
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