I am sure many of us egg owners have come across this. The conventional gravity furnace
has no fan. Most of the heat resides at ceiling level and at floor level it's considerably cooler. Would be nice to get the air circulating but a electrically driven fan uses battery
power and runs all the time.
Is there such a thing as a temperature sensitive fan that requires no external power? It turns itself on when some magical part inside heats up and shuts off when that part cools. There is something similar that's made to sit on a wood burning stove but those temperatures are much much higher.
Things that make you go, "hummmmm?"