class MainPage(Page):
    def __init__(self, *args, **kwargs):
        Page.__init__(self, *args, **kwargs)

    self.heating_btn = tk.Button(self, text="Thermostat OFF", command=self.heatCallback)
    self.heating_btn.grid(row=0, column=0)

    def heatCallback(self):
        if h.isRunning:        # error is here, new instance is created in HeatingController class (heating_instance) and I am using wrong instance here to check if it’s running
            print(“Heating already started”)
        else:
            h.startHeating()


class HeatingController(Thread):
    def __init__(self, main_instance):
        Thread.__init__(self)
        self._running = False
        self.main_instance = main_instance
   
    @property
    def isRunning(self):
        return self._running

    @isRunning.setter
    def isRunning(self, value):
        self._running = value
    

    def run(self):
        # some code
        # more code
        self._running = False

    def startHeating(self):
        if self._running:
            print("Heating already started")
        else:
            heating_instance = HeatingController(self.main_instance)
            heating_instance.isRunning = True
            heating_instance.start()



if __name__ == "__main__":
    main = MainPage(root)
    h = tempController.HeatingController(main)
    GPIO.add_event_detect(raspberryGPIO.startButton, GPIO.RISING, callback=run_instance.startCycleCallback, bouncetime=2000)