To be quite frank we have overrated the relays in this temp controller. We use 30amp relays inside the box. So technically the relays can handle almost 7kw. With that said we did not do testing over 3600watts so you would be on your own.
If you really wanted to use the RAPT temp controller for higher loads my guess is that it would work fine but you would really have to remove the current cables and replace them with higher amperage cables and this would probably work but it's just not something we have done or tested ourselves so it's difficult to give advice on suitability. If you do make this modification it would be great to hear how you go and if you can report back to the forum this would be great as I am sure others would also be keen to hear how you go.
Here is the datasheet for the relays that we use:
https://datasheetspdf.com/pdf-file/1350755/Ningbo/JT105F-1/1
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