900 cfm will move through 8" easily. Not to mention if the home is not very wit tight if will be even less important.

Ah! The MUA filter box conundrum. If you wander the paths of the internet looking at filter data, you may find that the pressure drop at the flow rate you want is high enough to back-draft combustion appliances (as low as 0.03 inches w.c.). A 4-inch pleated 24 x 24-inch filter may may be OK for a 900 cfm hood that likely is only pulling 600 cfm with such an MUA configuration; YMMV. If the filter pressure loss at the flow rate you will achieve (with the MUA filter pressure loss, baffle losses, transition losses, duct losses, etc.) -- you may notice that this is a recursive calculation -- there are a few candidate solutions:

o Don't use combustion appliances (other than the stove)
o Give combustion appliances their own path for (unfiltered) MUA
o Add a fan to the path and find a way to control it.

The last is the most complicated. The alternative is to only filter out larger bugs and other critters, but not use a high efficiency filter.

So first I would try hunt down a potential filter that at 600 - 900 cfm has a pressure loss of only 0.03 or less inches.

kas

P.S. The air flow that the filter has to be evaluated for is your hood actual flow as noted above, plus that of any bathroom fans or other exhaust devices that will also add negative pressure to the house. A refinement would be to measure the house leakage as a function of pressure to include that in the calculation.

This post was edited by kaseki on Sat, Nov 15, 14 at 11:01