Sir bttremblay . . . . . .( BOY ! . . . . try saying that fast . . .three times in succession. )
With that heavy of a current rating on the diode . . .it is apparent that it is to be switched / inserted in series with the resistive heating element in order to acquire a half power setting.
AND . . . . since you are using 50/60 cycle AC power, that should be irrelevant, for the polarity of install of that diode.
(Unless you are a very discriminating person and have developed a like taste, for only heating with positive nodes, and negative nodes would be quite distastefull and less than satisfactory to you. )
Thaaaaasssssit . . . . .
73's de Edd . . . .
I put a new engine in my car, but forgot to take the old one out. Now my car goes 500 miles per hour.