If your heater has stopped working because the filter is clogged, and you can't get to the store for a replacement filter, here's a temporary solution that can help you out: use your vacuum cleaner to clean the dirty side of the filter (figure A). You can also slap the filter on a sidewalk or other hard surface to dislodge solid debris.
When you reinstall the filter, make sure it's facing the proper direction: an airflow indicator will show which direction the air should flow (figure B).
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