We rent a house with a Carrier Series 180 furnace and a Carrier Performance Programmable Tstat. It has developed a problem where despite the temperature reading, it occasionally will not kick on. The tstat is set to hold 68. Occasionally the temp will drop to the low 60s, over night and the furnace will not kick on and the tstat becomes non-funcitonal, meaning it will not turn on when we increase the temperature control. However, when we turn the SSU switch off and then turn it back on, the furnace will almost immediately fire up. It will operate normally for a day or so, and then the cycle repeats, rather, fails to work automatically. I am somewhat confused as to whether it is a faulty tstat or there is something wrong with the furnace. When I shut off power to the furnce, it also shutsoff power to the tstat. How can I check to see if it's the tstat that need replacing?