"I have two large, over 100 lb, dogs that stay outdoors in a kennel when I am at work. I have been using these heaters for the last 5 years. They work great. I recommend that you size the heater to cover half of the floor area of the dog house to allow the dog to cool off, if needed.
The only thing I dislike is that the heaters only last two winters, on average. At $85 with shipping and tax, it isn't cheap."