Donatello had fixed an old heater when he was nine years old, to warm their lair during the cold winters. The temperature of the sewer sometimes dropped below freezing level and that was always dangerous for Splinters cold-blooded sons.

Growing up all four of them would fall asleep huddled together in a nest of old blankets; the cold and lack of food had caused them to hibernate almost every year when the weather turned colder. 

Every year Splinter dreaded the winter months, not knowing how long his sons would sleep, or if they would be okay when they woke up.