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Below is my furnace and I need to change the filter however I am unsure which way is the airflow direction. Red or Blue?

enter image description here

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Remember that the air needs to be filtered BEFORE it enters the air handler and furnace. – longneck Mar 11 '14 at 12:51
up vote 4 down vote accepted

Red, here's a diagram that shows airflow in a furnace: enter image description here

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