My unit is about 11 years old. Perhaps three years ago due to my allergies I had a 5-in filter accessory thing installed. The filters I was buying were Merv 11. I had no complaints. This time when I place my order they came in as a Merv 12. Should I be concerned about using these or is it going to force the air handler to work extra hard or anything...I know I've heard you have to be careful when you get up into the higher mervs... 15 16 etc but I don't know if there's a significant difference between 11 or 12.