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Why is my furnace blowing cold air?

Short answer

It may be the fan setting (set it to Auto), a dirty filter, or an ignition problem. If it keeps happening, call a tech.

Rules differ by state. This is general information, not legal or professional advice.

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Official sources

Energy Saver (U.S. Dept. of Energy)energy.gov
CDC: carbon monoxide safetycdc.gov
Ready.gov: extreme heat and winter weatherready.gov