Burn down x burn up: Qual a diferença?

Gostaria de entender melhor a diferença entre os phasal verbs burn down x burn up -

burn down-http://www.macmillandictionary.com/dict ... /burn-down
[INTRANSITIVE/TRANSITIVE] to destroy a building or something large with fire, or to be destroyed in this way
The entire house burned down in 20 minutes.
Burn up
INTRANSITIVE/TRANSITIVE] if something burns up, or if it is burned up, fire completely destroys it
The spacecraft has a heat shield to prevent it burning up when it re-enters the Earth's atmosphere.

http://dictionary.cambridge.org/diction ... ?q=burn+up
burn up something (DESTROY) - › to destroy something with fire:
A huge fire burned up the building.