35 to 40 degrees with PVC or terra cotta. Don't use metal unless it's stainless. If your building a chute to the bin from the outside make sure you have it as high as possible. a foot or two one way or the other is the difference between gettin an extra ton in. If the chute is low, that is as high as you can get the coal. You can stack it up a little around the outside.... well you get the idea.
Personally I prefer just a plain window, the larger the better. A detachable one. I can fill a basement bin up to the rafters about five to 10 feet back into the house without going into the house or touching a shovel as long as there is a big enough window and bin. 8) Many customers think they are making things easy but in the end they make it harder. In an ideal setup I can be in and out in about 10 minutes with a full load.