Well said JWS, and you hit the nail on the head at what i was thinking about with doubling the voltage, it's been a long while since i've cracked a book on basic electricity so i defaulted to the old standard p=VI, which is power, when dealing with heat the measurements are in Joules which is basically watts/sec.
Looking on the net, this answer was the best i could find about it.
The amount of heat produced by an electric current flowing in a conductor is given by:
Q = I2*R*t
( with 120v) 8amps * 15ohms * 10 sec= 1200joules
(with 240v ) 16amps * 15ohms * 10 sec= 2400joules
where Q is heat; I is current; R is resistance of the conductor; t is time
one can say the the resulting work done by the electric current is heat. This gives rise to a second formula:
W = V*I*t
where V is volts and W is work.
120v * 8amps * 10sec = 9600joules
240v * 8amps * 10sec = 19200 joules
you do get more heat with 240
if you can give me more info or if I am incorrectly understanding this please let me know !
Thanks for challenging me Steve