Ah yes, it seems that the built-in stuff has restrictions, but there are external boxes that can be added: * https://www.zehnderamerica.com/our_products/comfow

Ah yes, it seems that the built-in stuff has restrictions, but there are external boxes that can be added: * https://www.zehnderamerica.com/our_products/comfowell-silencer-and-casings/ https://www.zehnderamerica.com/our_products/comfowell-silenc... Which allow MERV 16 and even carbon filters: * https://www.zehnderamerica.com/product-category/comfowell/ https://www.zehnderamerica.com/product-category/comfowell/ * https://www.youtube.com/watch?v=OrG7oG8Tvp8&t=14m35s https://www.youtube.com/watch?v=OrG7oG8Tvp8&t=14m35s