If I start a session with replacing my filament and the nozzle temperature is 180* then I start a print, wh does the algorithm take the temperature down to 150* instead of just going up from where it is?
Turn on Serial Logging. Look for a M104 or M109 with an S150. (You can also use the Terminal Tab, but sometimes it goes really fast.) Then look thru the Gcode file for it and/or the OctoPrint Scripts for it. If it doesn't exist there, it's a function of the printer.
To give you a good answer, we likely will need a bit more info. But maybe this will help.
Sounds like you might have your printer profile in your slicer set up to get you hot end just hot enough to started but not so hot to have it drip while your printer does some other things getting ready for your print.
@CmdrCody is pointing out the 2 Gcode commands that set the hot end temp. The difference between the 2 are that one (M109) will make all other activities wait for the proper temperature to be achieved. This can be a cool down or a heat up. This is not needed when you have just changed the filament or like me that initial heat up to 145-150 because I know that the bed will take longer to heat up than the nozzle. I don't want to wait for the hotend to get to temp.. I will trust that it will move in the right direction so I issue the other command (M104). This command sets the hotend temp but does not wait to make sure it gets there before moving to the next commands. You really need to make sure that you do issue a M109 prior to printing though.. or you might try to print before the hot end is ready.
In my case, I send commands configured in the slicer like this.
M104 S145
M190 S[bed_temperature_initial_layer_single]
Do other stuff here like bed level or whatever is needed... or call macro PRINT_START
M109 S[nozzle_temperature_initial_layer]
Note that we set the hotend to 145 but don't wait.. then we do other stuff and then we set the hot end temp and make it wait before running any of the gcode from the slicer.
Hope that helps. If not, please turn on your serial.log and supply your Systeminfo Bundle. And any other info that might help about your printer. Maybe just open a get help request and fill in the template. Be sure to have turned on your serial.log and run a print before you gather the Systeminfo Bundle.