Over the summer I replaced my older non-programmable thermostat with a new Honeywell RTH6300B thermostat. It worked well all winter long. It continues to function fine, but the temperature it shows is not accurate. It is registering about 4-5 degrees higher than the actual room temperature(based on multiple thermometers).

This is an older home, about 75 years old, and so the forced air heating system was retrofitted to the house (before we bought it). In putting the system in the installers created a chase, running from our basement up to the attic (3 floors) where they ran the ductwork, also in the chase is the house's chimney (used to vent the gas furnace and a gas water heater). The thermostat is actually mounted on the wall of this chase. Is it possible that that is causing the reading to be inaccurate?

So, does it seem like the thermostat is just broken? Do thermostats need to be calibrated? Could the location of the thermostat be the problem? What do you suggest? Do I move the thermostat? If Honeywell thermostats are able to be calibrated, how do I do it? The manual doesn't give instructions for this.

Any help would be appreciated!