If you have lived in a cold climate with dogs during the winter months, you've almost certainly noticed - canines of all ages love to huddle up to any sources of heat when it is frosty outside. And they'll learn the signs and signals quickly, too; when Mom comes home and touches the box on the wall (adjusts the central heating thermostat) or when Dad messes with the box on the floor (switches the space heater on), Bandit and Daisy know that there will soon be welcome, comforting HEAT. And they'll quickly start jockeying for position, both with each other and with their humans; they want to find and secure the best position for both maximum heat exposure as well as maximum closeness to the humans, although maybe the humans take a back seat to the heat source.
The Root of the Behavior
Wherever you live, you can just about bet that your dogs are more likely to want to come back inside after a walk or a quick comfort break outside in the winter months than in the summer months. Although there are certainly dog breeds with double coats that are more-or-less comfortable outside in the winter (good examples would be Huskies, Malamutes, Samoyeds, Akitas, and Chow Chows), an average Beagle, Golden Retriever, or Rottweiler doesn't care to spend all its time outside in the winter. And you shouldn't leave them there, either.
Dogs, just like humans, are warm-blooded creatures, and as such, it is more energy-efficient for both to make our environment warm in the winter. Shivering from a winter chill is one method mammals use to keep their body temperature up in cold weather, but shivering uses up a lot of energy. However, that energy can be saved and put to other use if they can make (or find) a warm place to take shelter. Stoke up a stove, start a fire, or turn on a heater, and you make the environment more comfortable. You also make it possible for yourself and your dog to expend less energy. Moreover, dogs have a higher basal body temperature than humans do (about 102F/39C compared to 98.6F/37C for humans), which means that they prefer and can tolerate higher temperatures than humans can. So if you're setting your central heating thermostat for 68F in the winter, your dog would probably prefer you turn it a bit higher. Like 72F. Or 76F. Or even 80F, if you don't mind. Given this, you might find your dog getting what appears to be dangerously close to heat sources like space heaters, wood stoves, and hot-air registers. If you use any of these means to heat your home in winter, be sure to follow all the manufacturers' safety warnings and (in the case of central heating), ask a qualified HVAC technician what precautions you can take to avoid injury to your dog or damage to your home.
Encouraging the Behavior
Speaking of precautions, there are a few common-sense steps you can begin with. If you use space heaters in your home (say, to heat a single room that you and your dog occupy most of the time), be certain you're using heaters that are certified by UL (in the USA) or CSA (in Canada). Certification by one or both of those organizations assures you that the product has been tested and found to be safe when used as recommended by the manufacturer. Also, check to ensure that your heater has additional safety features like a tip-over switch (which will cut the power if the unit is knocked over for some reason) and overheat cutoffs (which will cut the power if the unit overheats past a safe operating temperature). And it should go without saying that you should never allow your dog to chew on the power cord and that you should immediately dispose of any space heater with a frayed or damaged power cord.
If you use a fireplace for heating, ensure that you have a stable grate or screen that completely covers your fireplace so that stray embers can't pop out and land in your dog's fur. Check to be sure that bumping or jostling won't cause it to tip over. You should never leave a fire unattended in the first place, but you should always have the grate in place just in case you fall asleep by the fire. If you're one of those folks who use a wood, pellet, or heating oil stove to warm your house, here's a creative idea. Consider repurposing the fencing panels from an indoor dog fence or pen to block your dog's direct access to the area where the stove is located (this idea can work for fireplaces, too).
Other Solutions and Considerations
There are things you can do to give positive encouragement to your dog as regards supplemental winter heating. Instead of telling him no, don't sleep next to the fire or the space heater, consider setting up an appealing dog bed at a safe distance and encouraging him to use it. Sweeten the deal with a toy or blanket that you know he really likes; show him that the toy or blanket are in the new bed. You can also use psychological means - positive reinforcement - to help model the behavior you desire. Whenever your dog goes to the new 'safe' bed on his own, give him a treat. With a little persistence, your dog will soon be making use of the new bed on his own and you'll no longer need to monitor him closely.
Dogs love warmth and heat, and in the winter months, they are often much colder than we are. They will want to get as close to your supplemental heating as they can despite the dangers they are unwittingly courting. But with a little forethought and planning, you can forestall the dangers, get your dog on your side and enjoy a much safer and warmer winter.