This is a well made tube heater that can be fixed or free-standing. It warms up rapidly and so far has kept my 8x6 greenhouse above 8 degrees a night. The only snag is the thermostat is not very accurate. So I have wired it up via a frost stat I had spare for more accurate control.
First bought these in feb 2013 and still on last ball (other 2 lost ). Dog loves them. If your buying for a pup a nice tip is to keep a close watch and as soon as you see your dog trying to destroy remove ball for a couple of hours. Works for older dogs too, But takes longer.