Cute Fuzzy Bunny said:
I looked at a couple of solar gable fans (I'd like to move a little air through the attic when its 100+ and the suns beating down)...but they're looking like $400-500+ for a unit, and I'm not liking the idea that much.
Listen, before I talk solar geek, let me pre-empt the incoming scrum made up of the rest of the board.
You've spent how much on cars this year, and you're complaining about a $500 solar fan?!?
OK, I'm done. Let's talk specs.
Cute Fuzzy Bunny said:
I see in todays home depot circular a "10 watt solar boost panel for gable or roof fan" for $168. It doesnt say if it comes WITH the fan, and I doubt it does, but the gable fan is only $52. For $200 I might give it a whirl. Thing is I note the words "boost" and the 10 watt vs other more expensive standalone solar units coming with a 20 watt panel.
You think a 10 watt panel is enough to run a gable fan fast enough to do any good?
I also have a little 12"x12" panel left over that charged a big honkin battery that went with a motion spotlight thats now broken. Not sure how much juice that turns out or if it could be hooked in with the 10 watt panel to throw in an extra watt or two.
Here's the
specs on the 850 cfm fans that we purchased in Nov 2000. Note that we paid roughly $550 per fan back then plus another $200 installation labor, so it looks like CPI hedonics are working for you again. With what I know now, I'd haul out my own reciprocating saw and a tube of roof caulk and skip the labor charges.
Electricity cost 15 cents/KWHr when our fans were installed. Our 10-watt panels ran for at least 10 hours per day for 0.1 KWHr/day, or a minimum of 1.5 cents per day of free electricity per fan. I'm not sure how much a similar-capacity AC-powered house fan costs or how much juice it consumes but if you have numbers we could crunch them.
The fan works best when it's sucking hot air out the top of a roof. (On really hot days the air is probably rising fast enough to do most of the work anyway.) A gable is easier to install than jackhammering through your roof tiles, but if the gable isn't at the very peaky peak of the roof then you're going to have a thermal layer that never goes away.
From my linked website specs I'm guessing that the 850 cfm fan was designed to be driven by a 10-watt panel, so you should be able to buy the booster and hook it to a DC motor for the smaller-capacity fan. (That gable fan is designed to run on DC and not AC, right?) If the fan is over 850 cfm then you'd probably want to step up to 20 watts. If you connect the fan and it's not running in the correct direction you only have to switch the wires.
The Cyclone fan motors that we got in Nov 2000 have bad bearings and eventually sound clanky. I don't know how widespread that problem was, but the DC motors used after 2003 have been fine (we replaced one then). So you'll want to know how old their stock is.
Personally I'd go with the more expensive fan with the 20-watt panel. It's probably the 1275 cfm version that'll suck a golf ball through a garden hose. We have one 850 cfm fan in the enclosed attic above a 30'x20' garage to cool the attic down below 100 degrees, and that's with the reflective foil insulation turning away a lot of the sunlight on a composition roof. We have a second 850 cfm fan sucking hot air at the top of a stairwell by a cathedral ceiling (uninsulated) over a 20'x20' room, and when our south-facing composition roof above that room wasn't shaded by a bunch of panels the fan couldn't keep up. We even have a passive (venturi) vent over the void above our kitchen, and depending on this summer's weather I may go into that void, insulate it, and replace the passive vent with another 850 cfm fan. With your tile roof (depending on whether there's reflective insulation under your tiles) you're looking at two or maybe even three of those big suckers.
I don't know what your gable fan & booster panel configuration is, but hopefully the wiring connecting the panel to the fan is as short as possible and insulated with cross-linked polyethylene (XLP). Other wiring insulation will only last through one or two years of UV & heat.
If you go with the integrated 20w panel/1275 cfm fan unit, there's no exposed wiring and the panel frame covers the fan exhaust to keep out the rain (and the critters). But then you have to mount it on top of the roof, preferably facing south, and that raises the cutting-through-the-tiles issue.