You read it correctly; the Wood magazine article appears very revealing,
however:

I didn't see in the article any record of what MERV rating was used with the
box fan, as tested. As discussed in the article's sidebar, it should make a
big difference in performance, as well as in price and longevity of the
filters. Ordinary, el-cheapo furnace filters won't remove much fine dust
(i.e., won't do much for your lungs) and MERV-13 and above furnace filters
are in the $15 price range. The article fails to mention anything about how
long these last before becoming so clogged as to restrict air flow CFM. For
that matter, the article fails to mention replacement cost or life of the
high efficiency filters and pre-filters used in the commercial air filter
units.

The apparent dramatic cost/benefit consideration favoring the box fan /
furnace filter combo, fairly begs for a follow-on article devoted to it.

I suspect that ideal would be a filter sandwich comprised of a low MERV
pre-filter to get the coarse dust and prolong the clog-free life of the
expensive high MERV filter under it (i.e., closest to the fan inlet).
Perhaps a simple ribbon tied to the outlet screen would suffice to provide
an indication of reduced airflow as the filter(s) start to clog. It would
sure be nice to see an article specifically on various box fan and filter
combinations to help sort out these considerations.

The Wood magazine article also failed to identify the make and model of the
box fan used in the test. Home Depot, Lowe's and Wal-Mart typically feature
cheap box fans for $10-12 in the spring. Their design seems to changes a
little each year. Anybody seen an article comparing makes and models of box
fans for CFM and static pressure performance which could be significantly
affected by blade shape and fan housing design as well as fan blade RPM.
Call me cynical, but I can't help suspecting that box fan blades are
designed more for ease and cost of molding than for performance efficiency.

David Merrill


wrote in message
ups.com...
I just wanted to verify that I read those charts correctly....the
regular box fan with a furnace filter duct taped to it out performed
like 95% of the dedicated air filtration systems??? I've been thinking
about buying one because I've been using an old attic exhaust fan with
some furnace filters but maybe what I have is doing the job!

Greg