@nprofile1q... I have whole house filters. I should change them every 6 months (often last 9 months) but I stopped the subscription a year ago because I suspected that my last cat would die before my next order and I would no longer need the very expensive carbon filters.

I neglected to set a reminder for myself to order new ones 6 months ago.

Too many people never change them or buy the super cheap ones which are worthless. Not changing with proper filters can void your HVAC warranty.