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Passive solar air heater(s)

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Posted by 2Hirondelles on January 19, 2009, 3:52 pm
Am in planning stages for installing two of these units on the SE end
of our cottage. (Just so we know what we're talking about, it's the
'Simple Solar Heater' design by Gary Reysa that appeared in Mother
Earth News and Home Power.)  The location is a climate essentially the
same as N. Maine.

The SE end of the house is the furthest from the woodstove, and
therefore the coolest.  The SE wall has a tiny (5x7) sewing room in
the centre, with a window.  The remaining wall space either side is
blank and unobstructed by shading, and seems a good option for
installing one of these units on each of two 11' sections of wall.
The intake/outlet vents would end up being in each of two bedrooms,
one 11 x 16, the other 11 x 11.  The doors of these bedrooms give onto
the central hallway.

1)  I'm wondering how much the heat produced by these units will move
on it's own from each of the bedrooms to the central hallway, and how
to decide to size the units. Essentially, I am concerned about
possibly overheating these bedrooms if the heat doesn't circulate on
it's own.  There is the possibility of installing a ceiling fan in the
central hallway, if that would help.

What we are looking for is to reduce our use of baseboard heating when
the house is occupied and to increase
the temperature however we can when it is not.

Posted by nicksanspam on January 20, 2009, 2:24 pm

Gary's heater is 8'x20', with 5 ft^2 of vents at the top and bottom.

You might put 8'x8' heaters on those sections.

Gary measured his heater's output, but I like to calculate things. If
0.9x250x160 = 36K Btu/h of (full) sun enters 160 ft^2 of R1 glazing with
90% solar transmission on a 30 F day with a 70 FF room and average heater
temp T and (T-30)160/R1 + 16.6x5(2(T-70)^1.5sqrt(8) leaves (using an
empirical chimney formula), T = 70+((255-T)/4.15)^(2/3) = 82 F, after
some iteration, ie 1150 cfm of 70 F room air enters the heater and leaves
at 94 F, with a 27.7K Btu/h output and a 69% solar collection efficiency.

An 8'x8' version might produce 64/160x27.7 = 11.1K Btu/h. Some heat
will leave through the walls of the room, but if they were perfectly
insulated and you left a 3'x7' door from the T (F) room to a 70 F hall
open and 11.1K = 16.6x10.5sqrt(3.5)(2(T-70))^1.5, T = 70+(11.1K/922)^(2/3)
= 75.2, after some iteration. Not too hot.

On an average 14.8 F day in Caribou, Maine, 780 Btu/h-ft^2 falls
on a south wall. If 0.9x160x780/6h = 18.7K Btu/h enters over 6 hours
and (T-14.8)160/R1 + 16.6x5(2(T-70)^1.5sqrt(8) leaves, T = 75.7 F,
with an 8979 Btu/h output. (R2 ThermaGlas Plus twinwall polycarbonate
glazing with 80% solar transmission would probably work better, with
simpler edge sealing.) With an open door and perfect wall insulation,
the room temp would be 74.6.

Sounds like it wouldn't, in a one-story house. A slow ceiling fan
in the room could be interesting, under a massy ceiling. With some
loss of solar collection efficiency, you might store 30K Btu/day at
120 F in 600 pounds of water in an 8'x8'x2" plastic film pillow on
an overhead tray with foil underneath. A room temp thermostat with
an occupancy sensor could run the fan as needed to keep the room
exactly 70 F when it's occupied.

If the heater output is small compared to the house heating requirement,
you don't need to store solar heat, just mix it around the house.


Posted by Morris Dovey on January 21, 2009, 1:39 pm
 nicksanspam@ece.villanova.edu wrote:


This is good advice - and not a bad rule of thumb.

 From a practical viewpoint, storage doesn't doesn't offer much return
on investment until your panels produce significantly /more/ heat than
your house heating requirement.

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by Morris Dovey on January 21, 2009, 3:00 am
 2Hirondelles wrote:

A variable-speed ceiling-hugging fan in each of the bedrooms would
probably provide better results - and might provide greater comfort
year-round. You might even consider a thermostatic control (with the
fans left on a low speed setting) for use when the cottage is unoccupied.

If the panels face SE, then you may want to consider some means of
blocking the sun during the warm part of the year. Otherwise the panels
will provide morning heat year-round.

Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

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