I disagree that. It is not efficient simple reason that miners will not earn money back because difficult level rising all the time and when you use example 1000w keep miners running you will get only 1000w heat. If you use air source heat pump you will get 2000-6000W heat per one 1KW so it is much more efficient for heating than running miners.

So you have to spend 2-6times more electricity when using miners compared heat pump or other efficient heater to heat house for same temperature. Keep that mind when doing calculations.