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Posted by Steve Kraus on May 27, 2007, 6:26 pm
I think the power drawn by the fan would be better spent feeding the
central AC.
Suppose your fan draws 400W and on a hot day would likely run continuously
from late morning into evening (depending on thermostat settings). Suppose
your central AC draws 4kW total (10 times as much). Running the fan cools
the attic and reduces the heat load on the house a bit (depending on the
insulation above the ceiling). Every hour of attic fan would buy 6 minutes
of central air (one tenth of an hour). This is just armchair
reconnoitering but somehow I think that 6 more minutes of real air
conditioning will help me more than an hour of trying to keep the ceiling a
little cooler.
That's not to say that ample attic ventilation is not a good thing. It
is...as long as you don't have to expend power to do it.
There has also been discussion in this forum in the past about fires caused
by faulty attic fans. I had one lock up because unbeknownst to me it had
sleeve bearings. Motor was sitting there cycling on and off on its own
thermal protection. I vowed I would not put it back but then relented and
did (after polishing and relubing the bearings). For some reason I can't
understand the fan itself slipped down the motor shaft--no idea why, the
set screw was plenty tight--and caught on the supports, locking it from
rotation. Again, saved only by the thermal protector. That was the last
straw and I disconnected it. Makes a nice passive vent.
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