I was thinking about my sister's rabbit today, who she keeps indoors. Her house doesn't always keep a stable temperature, as it is mostly wood-heated. It is getting warmer out, so I suspect there won't be any issues for the rabbit come spring/summer, but I wonder nonetheless - is there anything she can do to make sure the rabbit stays at a good temperature if the house gets really cold?
John has great points in his answer (+1).
Domestic rabbits are more tolerant of cold temperatures than heat. I reached out to the chief medical officer of Rabbit Wranglers to get more information before posting this answer. In general, inside of a home, rabbits are unlikely to encounter temperatures that are too cool. See related question for temperature ranges.
But in situations where there is lack of power, or a sick bunny needs supplemental heat, there are some options. With all of these it is important that the rabbit is closely monitored or has the ability to move away from the heat source.
- Hot water in a thick plastic bottle (Gatorade) inside of a wool sock or thick towel.
- Rice packets heated in the microwave
- Warm towel just out of the dryer
- Snugglesafe heating disks
- There also some rabbit safe (cord enclosed in metal) electric heating pads.
- Share your body heat, snuggle with the bunny under a blanket
- In very extreme conditions without power, the rabbit's litter can compost to create supplemental heat.
Any of those solutions should be used in a bunny proof area, where the bunny can freely move to or away from the heat source. They should have access to hay and water at all times.
NOTE on composting heat sources
Compost systems are complex and beyond the scope of this answer to fully address. Effective systems can be designed with a volume as small as 10 gallons. An animal (or person) next to or sitting on top of the compost system can be warmed by the waste heat.
I suspect you're worrying more than you need to on this score. Wild rabbits handle Canadian winter conditions without issue, based on the bunny tracks all over my yard, and this was a harsher winter than most. While domestic rabbits aren't the same as wild ones, they're not so vastly different that you couldn't draw some conclusions, especially since they still form a winter coat and have some fat to help insulate.
However, just to give some experience I had... We kept our rabbits and guinea pigs in our basement, which was also my office, and the basement is unfinished with poor heating. When I'm in there, I use a good space heater, but I don't run it when I'm not. Net effect, the room could get very cold in the winter and our rabbits and guinea pigs never appeared to have issues and all of them lived to the far end of the standard lifespans. Bear in mind that this included guinea pigs and they're tropical animals.
Also, consider that many "pet" rabbits (and I don't agree with this) are kept outdoors 24/7, even in Northern Ontario. That's a level of extreme your sister's house is unlikely to reach.
Nevertheless, if you want to provide some additional heating, then the easiest way is a space heater. If it is a fan-based one, you don't want to blow the air over the bun, it's not really necessary. Indirect, perhaps wall bounced, is probably better. Your other option is a more passive heat source such as a portable oil radiator, which I quite like. Either of these can dry out the air, though, so you may want to couple that with a humidifier.