My upstairs is very warm. I've shut the thermostat off, and it still reads about 75 degrees. It's a separate zone from downstairs, which is running fine. Do I need to replace the thermostat? Is there a problem at the furnace? Is there a fuse of some sort which may have gone bad? This entire system is only 4 years old, and has been running beautifully since install. I've had it serviced every year. The system is setup with Taco ciruculators and subsequent electronics in an attached box on the furnace. Could dust play some role in this issue? This is an oil/hot water system with baseboards. Any help would be very helpful.