Window Box  

A simple practical application of a hot box is the window box. Although it may be cold outside in the dead of winter there’s still plenty of heat available from sunlight. A window box is a simple passive method of trapping this heat. If you have a south facing window with unobstructed sunlight a window box could make an interesting project.

Cold Air from the lower part of the house is pulled into the lower part of the window box when a rising column of hot air is displaced from the upper part of the window box. Trapped heat due to the greenhouse effect is responsible for this rising column of hot air.  

Why does hot air rise?
Why does cold air fall?

ANS: Cold air vibrates at a slower rate than hot air and occupies less space. It’s denser than hot air and it separates from according to the laws of gravity. This is why cold air falls and hot air rises.

The frame of the window box is made from 1x 6 boards. 1x3 and 1x2 boards may be used to support the blackened sheet metal absorber plate as well as the glass lid. The bottom of the window box is sealed with a sheet of plywood. Each window box will be a little different depending on the size of the window it’s attached to. Be sure to give your window box multiple coats of oil base paint so that it can hold up to wet weather.

  Here is another view of the window box:  

Notice that the top and bottom of the window box is not finished. You’ll need an attachment mitered at about 45⁰ on the top and bottom of this frame to allow air to flow into and out of the house. On a sunny day you could measure the temperature of the air entering and leaving the window box. It will be difficult to estimate the volume of air being exchanged but you’ll know soon enough if the window box is helpful source of heat. Be sure to seal the openings of the window box at night so that daytime heat collected will not be lost at night. 

Do you think a window box can provide a significant amount of heat for your house?  Why? Why not?  
How much heat could a window box contribute over the period of three hours if the the difference between the input air temp and the output air temp averaged 30 F and the flow rate of the air averaged 5 CFM? 

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