My townhouse in the northeast has a ridge vent plus soffit vents. It also
happens to have a roof fan not too far from the ridge. I have the fan's
thermostat set at 110 degrees. On a perfectly sunny, and relatively calm,
spring day in the mid 60s or higher, the fan will turn on early in the
afternoon and not ever shut off until toward the end of the afternoon.
(there are no trees to block the sunlight until later in the afternoon).

I've heard that having a roof fan near the ridge is less than ideal, because
they tend to draw air in from the closest source which is the ridge vent.
When I put my hand up to the soffit vent, by sticking my hand out the
window, it did feel like the fan was helping suck air in through the soffit
(a little bit) relative to when the fan was off, but we're not talking much
flow here. When it was calm outside, it seemed a small piece of Kleenex
would tend to stick to the soffit more when the fan was on vs off, although
the Kleenex would still tend to stick somewhat even without the fan. Bottom
line is that I believe the fan is sucking some extra air in from the soffits
relative to when the fan is off, but it did not seem like very much.

1) Do you think that this fan is likely helping enough to justify using the
electricity? On a perfectly sunny spring day, it is not shutting off until
the end of the afternoon, and the thermostat is the kind that cause would
shut it off if it were to cool down a only by a couple of degrees. At
best, this fan may be preventing it from getting even hotter than 110
degrees. I suppose I could try taking temperature measurements, but 'm
trying to avoid going up there when it 110 degrees.

2) One other question: this roof fan's motor is somewhat noisy, and I can
feel vibrations if I put my hand on the walls of the living area below the
attic? Could these vibrations from the fan's motor potentially cause any
harm to the roof, etc, or are they harmless?

Anyway, It's too bad that the ridge/soffit system alone isn't enough to
prevent my attic from reaching 110 degrees or higher on a perfectly sunny
spring day. What do you think?

Thanks,

Jay