Article 31101 of alt.solar.thermal: Path: news.misty.com!not-for-mail From: nicksanspam@ece.villanova.edu Newsgroups: alt.home.repair,alt.solar.thermal Subject: Re: Preheating water by running pipes through attic? Date: 9 Aug 2008 16:16:01 -0400 Organization: Villanova University Lines: 53 Message-ID: References: <9a3d0efd-f0c7-41fa-b771-9350c88b0546@m44g2000hsc.googlegroups.com> NNTP-Posting-Host: acadia.ece.villanova.edu X-Trace: max.inside.misty.com 1218312963 27581 153.104.44.130 (9 Aug 2008 20:16:03 GMT) X-Complaints-To: abuse@misty.com NNTP-Posting-Date: Sat, 9 Aug 2008 20:16:03 +0000 (UTC) Xref: news.misty.com alt.home.repair:1038402 alt.solar.thermal:31101 hallerb@aol.com wrote: >"Percival P. Cassidy" wrote: >> My brother-in-law and his wife are planning to retire and build a custom >> home with as many energy-saving and eco-friendly features as possible. >> Since they're likely to be stuck with HOA rules about exterior >> appearance, solar panels on the roof are probably out, but they were >> wondering about simply running water pipes through the roof space. I'd put a $35 used car radiator with its 12 V fans under the ridge and make the south roof transparent. It seems to me there's a federal law that prohibits HOAs from outlawing this form of renewable energy. >> Does this have possibilities? You might get 5 Btu/h-F per $2 foot of fin-tube, vs 1000 for a car radiator, which might also circulate some warm attic air through the house on a winter day, with a couple of motorized dampers. >A newer home should have a well ventilated attic, building codes call >for attic no more than 15 degrees warmer than outside air temp. Which code? Section R806.2 (Roof Ventilation--minimum area) of the 2006 International Residential Code (used in PA, NJ, and lots of other states) says an attic can have 1/300 of its floor area as ventilation if upper vents have 80% of that and vents at least 3' below them have 20%. So my 24'x32' attic might have a total vent area of 24x32/300 = 2.56 ft^2 with 0.512 ft^2 of low vents. In full sun on a still day the roof might absorb about 24x32x250 = 19.2K Btu/h of sun and lose heat to outdoor air with a 24x32xU.5 = 384 Btu/h-F thermal conductance, with an equivalent circuit like this, viewed in a fixed font: T 1/384 | --- ------www-------|-->|---| | --- | 125 F I --- - | | - One empirical chimney formula says I = 16.6Asqrt(H)T^1.5. A = 0.512 ft^2 and H = 3' make I = 14.72T^1.5, which makes T = 0.0383(3261-T^1.5). T = 91 F on the right makes T = 91.7 on the left. Repeating makes T = 91.3, so the air in an IRC-code attic could be 91 degrees warmer than outdoor air. Nick