I've never asked them about it. It's just their long-standing habit or tradition in that house to use no heat other than one kotatsu, despite the fact there are between three and four generations of folks living in the house as a general thing. They also don't use any air conditioning whatsoever in the summer.
Come to think of it....maybe they're trying to run everybody off.
I once knew a guy from Canada who lived here in my town for a few years. One of his students had an empty house, which was being kept in the family for a grown daughter to return to the area and move into several years hence. Meanwhile, it was sitting empty so the owners let the Canadian guy have it for his home and classroom for dirt cheap rent. Just to do a guy a favor, and to keep the house occupied and kept up.
Unfortunately, he was a narcissistic whiny-assed piece of *cough* who thought himself to good to undergo the deprivations the primitive and backwards Japanese put up with. He had grown up with central heating and saw no reason he should have to come home to a cold house....so he left the kerosene stoves burning while he was gone. These were the old types, with no self-timers to shut them off, and without convection fans, so that the heat largely went straight up. They were the type you could set a kettle on and boil water. He also had a bad habit of placing the stoves almost directly against the wall, because Japanese hallways are too narrow and he shouldn't be inconvenienced by having to leave a safe space around the stove.
You can see where this story is going, no doubt.
He came home one evening and found nothing left of the house but a pile of ashes.
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