Fire Alarm Circuit using transistor
You've labeled your 'thermistor' RT1 as a KTY81. That device is not actually a thermistor - it is a 'silicon temperature sensor' http://www.nxp.com/documents/data_sheet/KTY81_SER.pdf  

Have you actually used this particular device in your real circuit?  

If you've used a 'normal' NTC thermistor instead then this would explain the behaviour of your circuit since the KTY81 is a PTC device.