In engineering speak - there needs to be "thermal mass" to hold the heat. The tin warmers can't concentrate enough focused heat to keep the water ice free. Here is an inexpensive alternative to the cookie tin heater:
I made a coop water heater that seems to be working very well. Today it is 0F outside and it was 20F in the coop. I bought a standard base cleat light socket from the hardware store and screwed it onto the center of a board about 12"x12". I attached a length of lamp cord to the cleat socket and added a plug end (hardware store). I swept away all the deep litter from the floor where I wanted to set the heater. Set down the board, and centered a 6" thick concrete block over the socket. Make sure there is no combustible bedding or straw in the open cell of the block. I bent the edges of a sheet of galvanized flashing to create a water shedding lid for the block. I set my plastic waterer on top. I changed the bulb to 75W when the temps dropped, but a 40W was keeping the water ice free just fine. All the extra heat keeps the concrete block radiating warmth all day. I think I spent $5 on the entire setup.
http://www.backyardchickens.com/forum/uploads/65938_img_0569.jpg http://www.backyardchickens.com/forum/uploads/65938_img_0571.jpg http://www.backyardchickens.com/forum/uploads/65938_img_0572.jpg http://www.backyardchickens.com/forum/uploads/65938_img_0566.jpg
Edited by duluthmom - 2/15/11 at 6:54am