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I'm currently collecting cans to build one myself. Seems like a great idea, and it's pretty inexpensive to try.
FirstChickens: Here's a link to the instructions I'm going to use on how to build one.
http://www.coloradowindpower.com/page.php?26
taamator: Love to hear how it works out for ya.
With just one fan as they did you create a negative pressure (vacuum) in the heating chamber and although the chamber gets real hot, the fan can't pull the heat out as no return air duct to allowed air back in to replace the hot air coming out...
You really need channel on top and bottom, so you can 'circulate' the air... Look the guys design in the link bellow... Either install the fan on top pulling air out of the collector combined with a return duct on bottom or install a fan on bottom blowing the return air in and a hot air duct on top back into the coop...
http://rimstar.org/renewnrg/can_solar_air_heater_DIY_gs.htm
BTW, my mother supplements her house with a similar setup, her panels are 4 foot by 8 foot, and she has several of them chained together... Instead of pop cans she has 1" copper pipe zig-zaged inside the collectors... Inside her basement is something like a 500-1000 gallon water tank and water is circulated from that tank through the collectors but only when the collectors reach a certain temp... By the end of the day she has hundreds of gallons of HOT (generally in excess of 180° even in the winter) water in that tank, at night that hot water is circulated though radiators to heat the house overnight... She also has a separate collector that is hooked up to her hot water heater and circulated for near free hot water to shower and wash with... It's not a total replacement for her furnace but it helps a lot, and delays when she has to actually turn on the furnace in mildly cool weather...
Is your mother's setup DIY, or something commercial? I know the commercial units are VERY efficient, as they're vacuum purged, so there's basically no heat loss. Something like the OP is building could be interesting (and could work quite well), but isn't going to have nearly the gain that a commercial unit does (not that it's needed to warm a chicken coop).