Sorry, technically what you said originally about
designing around how much you want/need is the correct approach to be honest - that's what is done with industrial systems.
But in residential you can only fit certain sized fans, filters and so on.... so you are limited to a degree in what you can accomplish at a certain cost point/design point or off-the-shelf components.
Let’s say you have a furnace with a design point flow rate of 1,200 CFM and switch from a MERV 11 filter to a MERV 13 filter because you want a higher degree of filtration. This adds more pressure because the filter media is much finer – so it’s harder to ‘suck the air’ through the filter. The volume flow of 1,200 CFM can be maintained, but your furnace fan just simply works a little harder… It needs to spin faster to move the same volume of air, and to spin faster, it uses more energy.
However, there is a point where the resistance can be to high and your fan just cannot overcome the added pressure – in this case the volume flow decreases. This is dependent on your fan - known as the ‘fan curve’ which is a simple chart that shows the pressure versus volume flow relationship. But I don’t think most furnace fans have this in their manuals and such anyway.
To overcome this issue and maintain the same volume flow (say 1,200 CFM), you can do one of three things; increase the fan motor horse power, switch or find lower resistance filters at the same MERV rating or increase the overall surface area of filtration media. Doing any one of these three things will maintain the same volume flow.
The problem is that you might not be able to find and fit in a larger or more powerful fan/motor, you might not be able to find lower resistance filters of the same MERV rating and you might not be able able to increase the media surface area because of deign constraints. Furnaces come ‘as-is’ in packages to keep manufacturing costs low. They are designed more so to heat and cool and filtration is sort of secondary in a sense.
So that is why based on those three specific options I suggested the MERV 11 – because at any point in time you can put in any standard MERV 12, 13,14+ filters if you choose to do so and if your fan can handle the added pressure and you don;t mind the extra electrical costs. The HEPA by-pass I suggested passing on because, well why would you want 70% of your airflow to be unfiltered? Sometimes they say just put in both systems, so a HEPA by-pass and a 5” deep pre-filter on the main system – but what is the point of that? Is 30% HEPA filtered air going to make a significant difference? Not a chance.
So the MERV 11 is the best option because you can easily upgrade it 'of-the-shelf' in the future, and it's initially going to be the cheapest option - But double check and make sure the MERV 11 kit uses standard 16x25x5 inch filters.
Hopefully this helps somewhat.
Post Edited (Canada Mark) : 11/8/2015 1:06:25 PM (GMT-7)