Homegrown solar attic ventilation

2 Posts
Jun 29, 2009 04:44 pm
Homegrown solar attic ventilation

*** Sorry, I meant to post this in the Technical category, not site discussion.  ***

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.

As for location, 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.

With no inside attic access, that means that I can do one of three things:
1. purchase some 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 using 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 pretty good looking candidate was a Comair Patriot® 24Vdc 6.8" Round Fan, 24 Watt, 233 CFM.  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.

** 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.
« Last Edit: Jun 29, 2009 07:29 pm by Todd Lyons »

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