DIYbanter - View Single Post - Preheating water by running pipes through attic?

sno

wrote:
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

When I blew the hot air out of attic (Georgia....dark
green roof) on any kind of clear day could get a 20 degree rise in
house above outside.....on a bright day could easily get 90 degree
inside no matter what outside temp...had a differential thermostat
that turned on box fan when attic temp rose above inside temp...
usually turned on around 0900 and turned off around 1800....and
could usually coast all night with morning temp in low 60's....

hope helps...have fun....sno

hope helps...have fun.....sno