# Passive Solar Air Heating Experiment



## Windy in Kansas (Jun 16, 2002)

Wednesday I made a simple framework of wood and fastened two strips of flashing roll tin onto it with air gaps for heat to escape from. Of course I painted the tin flat black before mounting it. 

I hung the 8.6 square foot heat attractor in the south windows of my tiny basement access porch.

Normally the south windows and sunlight raise the temperature about 4 degrees over outside temperature as there is no ceiling insulation. With the new solar device I was surprised to find a 14Âº difference just a little while ago.

This gives me thoughts that a homebuilt collector might be even more efficient than I thought, even if made with tin instead of flashing aluminum which seems hard to come by in my area. 

Experiment--successful. Maybe I should also add some decorative punched aluminum panels to my house windows.


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## Jim-mi (May 15, 2002)

Good for you.....It works doesn't it.......

For some years now I have been using a 5' x 4' wood framed sheet of rippled Al with the *black chrome* coating . . in my south facing door wall......yes it does work.


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## Windy in Kansas (Jun 16, 2002)

Yesterday while I was washing windows I realized that I still had a mini-blind mounted in one south window. I promptly took it down, outside, and spray painted it flat black.

When reinstalled I turned the slats at about 45Âº which is almost our proper solar pitch and was surprised at how much heat even plastic mini-blinds could give off. Not much but enough you can feel it with your hand. With the angle set at about 45Âº it stills lets plenty of light through as well. Wish I had aluminum mini-blinds on all of my east, south, and west windows to use this time of year. 

So far my experiments haven't cost me anything as I purchased the paint with a gift card and had everything else on hand.


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## fishhead (Jul 19, 2006)

I've wondered if it might be possible to build some stand alone solar heaters and vent them into the house. My house faces south and even in the winter gets some decent sun.


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## Windy in Kansas (Jun 16, 2002)

It would be. If you think of these symbols ]\\\ as being a house wall with a solar panel box leaned against it with the actual heat panel being in the middle, then an old design was that cooler household air would flow behind the panel and go to the bottom while heated air would up flow taking the warmer air into the house. It was a passive window unit that transferred air through the window so that no holes into the house had to be cut. 

Hey, wait a minute, Mother Earth News is where I read about those. Here is a link now that I've done a search.
http://www.motherearthnews.com/Green-Homes/1977-09-01/Mothers-Heat-Grabber.aspx


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## Jim-mi (May 15, 2002)

Can't remember where I've seen those, but yes folks have made up the "whole" absorber box and mounted it on a south facing wall........with holes -tubes?- through the wall for the air to pass.
And another was a unit to mount outside\beneath a window.(using the window opening for the pass through air ducks) It looked a bit weird.


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## RVcook (Mar 29, 2008)

Like this one...

http://www.motherearthnews.com/Green-Homes/1977-09-01/Mothers-Heat-Grabber.aspx

RVcook


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## SolarGary (Sep 8, 2005)

Hi,
There are a couple of the outside solar air heating collectors on this page:
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm
Down the page quite a ways.

You could also build a lean-to style sunspace against the house, and harvest heat from this. These can be very cheap and effective.

Don't want to rain on anyone's parade, building a solar collector inside of a window won't collect any more heat than the window collects by itself -- in fact it may collect less because you have the hot absorber right next to the glazing and it looses heat through the glazing. Just letting the sun shine through the window and into the room is about as good as it gets. The sun may bounce around a little, but nearly all of it will eventually be absorbed in the room.

Gary


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## Explorer (Dec 2, 2003)

Gary, I have low E windows and was planning on doing this window collector. You are saying it will not help even if the metal is about an inch from the current window on the outside with a new clear vinyl covering another inch away? Or do I have a different animal?


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## Windy in Kansas (Jun 16, 2002)

Gary I understand your statement but the darker color of the solar device may gain much more than the sunlight hitting light colored curtains or whatever.

From my first post,"Normally the south windows and sunlight raise the temperature about 4 degrees over outside temperature as there is no ceiling insulation. With the new solar device I was surprised to find a 14Âº difference just a little while ago."

A 10Âº gain is significant in my normal over device observation. Daily observations also show that it continues to out produce over just bouncing off of whatever. A good reason to do experiments in my case.


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## SolarGary (Sep 8, 2005)

Hi Windy, Explorer,

This is the way I understand it -- 

The amount of incoming sun that gets through the window is the same whether you have a collector absorber plate right behind the window or an open room behind the window -- its going to be about 85% for most common windows. So, the energy input is the same either way.

If you have a dark absorber right next to the window, that absorber will absorb nearly all the incoming sun (black paint is good for about 95% absorption). This will get the absorber satisfyingly hot. But, there is a down side to the hot absorber in that it will radiate and convect some heat back out the glazing and not to the room. So, while most of the absorber heat goes to the room, some is lost back out the window.

If there is no absorber, the sun comes through the window, strikes some object in the room. Depending on the object some of the light is absorbed and heats the object and some if reflected. If its reflected it will hit some other object in the room, and some is absorbed and some reflected. This may continue on for a few bounces depending how reflective the room objects are, but eventually nearly all the light is absorbed. A little bit of the light might actually get reflected back out the window, but this has been studied carefully, and the percentage getting back out is very small, even if the windows are large.

So, with an absorber behind the window, you end up heating the absorber's small amount of thermal mass to a high temperature, but if you don't have the absorber, the sun ends up heating the large thermal mass of the room objects just a bit. The heat gain to the room is the same either way.
The advantage of the window is that the cooler room objects don't lose as much heat back out the window as a hot absorber plate right next to the window glass.

I agree that if you had light colored curtains on the window, that would definitely lower the efficiency because it would reflect a percentage the sunlight right back out the glass. But, you can just open the curtains during the sunny periods -- this way you get heat and light?

Not sure whats going on with the greater room temperature rise with the absorber. Can you measure the temperature in 3 or 4 places around the room and make sure its not a local warming that you are measuring? It seems to me that the window with or without absorber plate should raise the overall room temperature about the same or maybe a small advantage to the window alone?

As far as I know, the only real downsides of windows as collector are that 1) they lose a lot of heat at night. You can reduce this with thermal shades or night insulation. And 2) they may have glass in them that does not transmit as much light as you would like. If the sticker is still on the window, the SHGC gives a pretty good idea what how good the transmission is. 

I think the MEN Heat Grabber gadget should work well. I've thought that a better arrangement might be to surround the window with a collector that goes the full height of the wall and is (say) twice the width of the window. Then work out a way to use the window to let cool room air into the bottom of the collector, and collect hot air from the top of the collector. This might give you several times the heat gain of the window alone. It would also be less likely to interfere with landscaping etc because its flush with the wall. As long as the siding is a medium or dark color, it would absorb pretty well. The trick seems to be to work out a way to pick up the air from the top and get cool air to the bottom.

Judging by the length of the message, I'm the one who should be called Windy.

Gary


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## Jim-mi (May 15, 2002)

Explorer I can answer your question reguarding low E glass. 
I got flustrated when I put some of my black chrome coated absorber plates in back of the Anderson casement windows in the bedroom . .(south side) . . They would barely get warm.
duh......finely realized that the glass is low E . . .duh . . 
Yet in front of the door wall (standard glass) the absorber plate will get so hot you wont keep your finger on it for more than a second.

One of these days when its not snowing I'll measure the temp with my Fluke temp probe . . .to see just how hot it gets.


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## Windy in Kansas (Jun 16, 2002)

I just finished taking readings from moving the digital thermometer around in the porch to test for temperature variations. The heat seems to be pretty even. After your post Gary my first thoughts were that the heat from the collector is rising to the ceiling and coming down the opposing wall near where the freestanding thermometer was which would give a false reading for certain. Moved it around and get nearly identical reading. The porch is basically 5' X 11" and the collector is basically 2' X 5' (2-10" strips of tin x 62"). 

I can't account for the difference other than the black paint but your information does make perfect sense. I have been having the faded medium blue curtains open to allow sunlight in but of course as with most curtains they don't expose 100% of the window opening but instead obscure the edge.

I wish that I had some sort of thermometer to read the heat of the tin.


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## Explorer (Dec 2, 2003)

Thanks Jim-mi. I still plan on trying it out. I live is a Spanish Colonial style house (stucco) and my first attempt at a frame of 2"x6" was way too heavy. Next time I go to town I plan to buy some 1"x6" cedar boards for the sides and bottom. I have the rest of the need material. Everything will be on the outside of my windows 3"x5" with two 16" metal strips in the center (running from side to side). I will open the window to use only when the temp in the enclosed space is up.


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## ekjns (May 31, 2002)

Clever idea!


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