The higher the temperature the faster the drying but also the more damage will be done to the food. As high as 140 and it is being cooked. For something like pineapple this is fine but for delicate things like basil that is not ok.
If you live in a dry climate the heater might not be necessary at all but it will save time.
I don't know exactly why your pineapple didn't turn out but here are a couple notes:
1- Things that are thick or very moist take much longer than usual. 6 hours is not a long time. Obviously pineapple is both. Try letting it go overnight.
2- Really really sugary things are so hygroscopic that they have a hard time drying completely and staying dried.
3- Commercial dried fruits are usually candied and chemically altered before being dried in equipment that I can't reproduce. This shouldn't discourage you by any means but don't expect your product to always be exactly like commercial dried fruit.