Homegrown solar power gable mount attic ventilation

2 Posts
Jul 1, 2009 08:17 pm
Homegrown solar power gable mount attic ventilation

I am reposting this in the correct forum topic.

I watched the video of the gable fan install and it is very straightforward, however, I have a major and a minor issue I have to work around:  I do not have attic access (major) and this side of the house does not have a gable (minor).  I want to avoid cutting any studs, and in all cases I have to install a new gable on the side I want to install this.  There is a gable on the northern side of the house, but south side gets hottest, so I want to put the forced ventilation on that side.

My stats: I have a 2 story 1700 sq ft house located in southern California.  As an example, yesterday the temperature was right at 100 F in my town.  On those hot days, the upstairs stays about 10-15 degrees warmer than the downstairs due to superheated buildup in the attic.  Additionally, I hope to see some reduction electric bills.  The roof is tile, not asphalt shingle, and I don't have the knowledge or tools to properly poke a hole in the roof and get it sealed properly against the weather.

With no inside attic access, that means that I can do one of three things:
1. purchase a ready made kit, assemble everything (fan to a custom square bracket to a face piece for mounting, and a gable) and mount it all from the outside.  The biggest limiting factor will be it all has to fit between the studs spaced 16" apart (effectively 14" empty space). **
2. purchase some ready made kit, use the supplied flange mount and mount it from the outside with some creative use of brackets and sheet metal screws (but I think that's going to be a bit hokey), then mount a gable on the outside. **
3. purchase the pieces separately myself.  It looks like I can save a little money buying a separate 20 Watt panel ($245) and fan ($15).  I also gain some flexibility with mounting/sizing by being able to choose the fan.

Modern fans seem to be electrically commutated and so have a minimum input voltage before it will start spinning.  Will underpowering the fan harm it during the early morning and late evening hours?  Is there some fan that just is better to use with the variable voltage supplied by solar panels?  At worst I figured I could grab a 7812 and a 12v reed relay to control power to the fan which should keep it from pulling in until around 13 volts (a few well placed diodes could up that voltage to a more comfortable 15 or 16 volts too).  A properly sized power resistor on the NC contacts of the relay should keep the supply loaded enough so that voltage won't dip too much when the fan load is applied.  A reasonable looking candidate was a Comair Patriot® 24Vdc 6.8" Round Fan, 24 Watt, 233 CFM.  (Interestingly, that's nowhere near the stated CFM of the packaged 20W units that are sold on this site.)  I do realize that I won't get but 20 or 21 volts out of the panel so I will get reduced CFM in reality.  What I don't know is if that ~ 200 CFM realistic volume is enough to cool the attic of my house enough to be noticed when going to bed at night.

Any suggestions for a fan?  Why is it so impossible to find a 24VDC fan with the same CFM ratings as the prepackaged units claim?

I'd also like to get a 25 or 30 watt panel, but am unsure that it will result in any kind of enhanced ventilation.

Am I better off just buying a prepackaged gable unit?  My limiting factor is not ability, it's physical access to the inside of the attic.

** I'm assuming that the studs are 16" spaced.  If not, I'm going to complain loudly to Lennar for code violations. :-) If it's less than 16", well, I'm gonna have to rethink this.
 

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