Why is my furnace blower running continuously?

A continuously running furnace blower is usually caused by incorrect thermostat settings, a faulty limit switch, or a stuck relay. The most common culprit is the thermostat fan setting being switched to "ON" instead of "AUTO."

Check your thermostat first - if the fan setting is on "ON," it will run continuously regardless of heating cycles. Switch it to "AUTO" so the blower only runs when heating or cooling is needed. If the thermostat is already set correctly, you likely have a mechanical or electrical issue that requires professional attention.

The limit switch is a safety device that can cause continuous blower operation if it fails. When the furnace overheats, the limit switch is designed to keep the blower running to cool down the heat exchanger. If this switch is stuck or malfunctioning, the blower won't stop. This often happens alongside restricted airflow from a dirty filter, blocked vents, or closed dampers.

A stuck blower relay is another common cause, where the electrical component that controls the blower motor gets stuck in the "on" position. This is an electrical issue that requires replacement of the relay or control board. In Ottawa's climate, continuous blower operation can also indicate problems with heat pump systems trying to maintain temperature during extreme cold snaps.

Safety considerations include checking that your furnace isn't overheating - feel the air coming from vents to ensure it's not excessively hot. A continuously running blower uses more electricity and can indicate underlying problems that may lead to carbon monoxide issues if the heat exchanger is compromised.

Start by checking your thermostat fan setting and replacing your air filter if it's dirty. If the problem persists, the issue likely requires diagnosis by a licensed HVAC technician, as it involves electrical components or safety switches that shouldn't be handled by homeowners. For professional furnace diagnosis in Ottawa, consider getting a free consultation to identify the exact cause and prevent potential safety hazards.

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