At 11:21 PM 12/10/2012, Mike South wrote:
>Synthesizing these comments with what I read in the archive, I think the
>softer stuff is preferable because you don't want too much friction or they
>will be hard to close.  One message in particular speculated that for the
>ones that blow down, they are activated by differential force of air flow
>or something like that, and possibly won't even be able to function with
>something on there that's too stiff.

All the flaps need soft (compliant) foam.  The free-swinging ones IMO
don't need any at all; the original stuff was extremely thin and may
be hard to duplicate.  There is a tiny loss of fan efficiency from
them not sealing tight, but I did not experience any whistling
resulting from them not having new foam on them.  They do make a
slightly louder clack if you suddenly turn the fan from off to full,
but to me that just makes it easier to verify that they're working
correctly.  OTOH to the extent they fail to close properly the fan
operation will be seriously compromised or rendered null.  The other
flaps only need seals to stop airflow from their respective outlets
and to prevent whistling at small openings.  I think both those
functions are important.

The issue with compliance isn't friction but compressibility: if the
foam is thick the flaps have to compress it near the hinges in order
to get the outer edge to make contact at all.

Someone mentioned thin closed-cell polyethylene packing foam -- I've
used that (probably doubled) at least once with good results.

Yours,
David