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When do I change a furnace filter?

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  • When do I change a furnace filter?

    Our Carrier furnace Owner's Manual says the Carrier Merv 8 air filter P/N FILXXCAR0016 is factory-specified and they should be replaced 2 times a year.
    I normally replace them when they are quite black/dark on the underside. During the cold months of the year this is about every 8-9 weeks.
    The technician who just inspected our furnace said that since these filters have a "large surface area" they should not have to be replaced very often--perhaps once a year but that kind of generic answer doesn't sound like good advice.
    Exactly how should I determine when the filter be replaced?
    Thank you.