We live in a world where, for the most part, theoretical ideals can only be approached rather than fully realized.
For example, imagine a building with heat loss characteristics such that the rate of heat loss always matched the rate of internal heat generation from sunlight, people and equipment. Such a building would not require any heating system. Some of the new Passivhaus buildings approachthis condition, but still can’t guarantee that internal gains alone will always be sufficient to maintain comfort.