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If you want to reduce the back EMF spikes caused by switching an inductive load, just put a 1N4007 diode across the heating element as shown in the attached image.
To increase the frequency, either:
1) Reduce the value of C2,
and/or
2) Reduce the values of R3 and P1, whilst maintaining about a 1:20 ratio, i.e P1=500k, R3=24K, or P1=100K, R3=4.7K etc...
But as others have stated this is not necessary. I'd still put the extra diode in though as it offers protection for the switching transistor too.
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