MERV 11 with Carbon Pre-Filters

You might be able to do this but it would be through an alternative compliance path, and you would have to demonstrate definitively that your filtration is as good as or better than MERV 13. Some data or evidence would be required, in other words.

I agree with Tristan - I think the key here is confirming the type of filtration at issue. My understanding (or lack thereof) is that there may be a difference between the effectiveness of a filtration process in removing particles and the SIZE of the particles being removed. MERV refers to the latter, so your system may be very effective at removing particles, just not particles as small as MERV 13. Worth exploring further with your filter vendor or HVAC team.

A MERV 13 filter will be effective (% wise) at removeing particles of different sizes...here's a nonsense example: particle size: 1 2 3 4 5 % effective: 80 75 20 65 90 under specific lab test constraints/conditions/proceedures You would have to demonstrate that your filter combination will be as good as or better than the baseline MERV numbers. The manufacturers would most likely have to give you this information or find someone who could.