12V DC to 24V DC converter

[Hlynur Tomasson]

Does anyone on the list have schematic or info how to build 12V DC to 24V step
up converter.
Friend of mine has a small automotive gas heater. This gas heater has
electric control box & small fan that draws approx. 0.7 amp & 24V DC as of the 
data plate. He is to use this heater in his 12v power source Travel Van.
Thanks in advance,
H.Tomasson
Pls. Reply to email hlynurt at islandia.is