Atwood Hydro Flame furnace shoots cold air then shuts off

It would sound like the sail switch isn't working properly. I suggest you put in a model number and do a google check on troubleshooting your model.

Generally what is supposed to happen is the fan starts blowing, then the sail switch says that is enough air flow and lets the gas valve open.

There is lots of info on the net, but your model number would really help.