Yes it is true. No one with allergies should ever rely on furnace filters to clean the air in their house. Use a merv 6 or 8 filter and look into other options for cleaning the air in your home. A 1" merv 12 filter thrown into your filter rack without you knowing what your ESP is,well it's a recipe for disaster. Your system needs to breath and a merv 12 is more likely than not way to restrictive.

Deeper pleats can have more surface area for airflow; many factors enter into a filters pressure drop.

They should be required to list the pressure drop on all filters at a specific velocity.

Optimally, the filter rack should provide 1-sq.in. of free air area per 2-CFM of airflow through the filter.

With larger tonnages you may need two exterior filter racks.

The linked page explains ductwork & airflow "Best Practices." - udarrell

Here is a link that might be useful: Ductwork & Airflow Practices

I have increased my filter space to 800 sq. inches for a 2.5 ton heat pump running @ 1060 cfm. I could probably use any 1" merv filter out there but stick to a merv 8. Good equipment protection and great airflow.

thanks for the advice. i guess we will have to stick with the merv 8 pleated filters since i can't do anything about my furnace. any suggestions for other means of filtration for our allergies and get cleaner air in the house??? advice always well appreciated!