When do I change a furnace filter?

[repairsalot]

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.

[FatimaandAhmed]

It sounds like you’re doing a good job staying on top of your furnace maintenance. While the Carrier manual suggests replacing the MERV 8 filter twice a year, your approach of changing it every 8-9 weeks during the colder months might actually be spot on, especially if the filter appears quite dark by then. Filters can get clogged faster depending on factors like dust levels, pets, and how often the furnace runs.

The technician’s advice about the filter having a "large surface area" is true to some extent, as larger filters can hold more debris and might not need replacing as frequently. However, it's important to trust your observations—if the filter looks dirty or airflow seems reduced, it’s time to replace it. Sticking to your regular checks and replacing the filter as needed based on its condition and the furnace's performance is a good strategy.