The fire kept at Yazd is only the highest of three recognised grades in Zoroastrian practice, and tradition holds it has been burning, in one location or another, for well over a thousand years.
Three grades of fire
Zoroastrian tradition recognises a hierarchy of consecrated fires: the Atash Dadgah, a modest household or local grade; the Atash Adaran, combining fires from four different sources; and the Atash Bahram, the highest grade, consecrated by combining sixteen different fires gathered from various sources, including fire produced by lightning, a process this entry's companion article describes in more ritual detail. Yazd's fire holds this highest Atash Bahram status.
A fire that has moved cities
Tradition holds the specific fire now housed in Yazd has burned continuously since around 470 CE, having been relocated between several cities in Iran over the centuries as Zoroastrian communities themselves moved or as political circumstances required, before settling in its current temple in Yazd in 1934, making the flame's claimed continuity a matter of transferred custody rather than an unbroken presence in one fixed building.
A minority faith's most protected flame
As one of the very few Atash Bahram fires still maintained anywhere in the world, the Yazd fire carries disproportionate significance for a global Zoroastrian community that has shrunk to a small fraction of its historical size, making its continued maintenance a matter of active concern rather than settled routine.