Hi, I have a Honeywell-TH8110U1003-VisionPro-Thermostat. One day the screen went off, and I changed the battery. However, the heater now can not be turned off. I temporarily shut off the circuit breaker, and when I checked the back of the thermostat, I found a burn mark.
What can be done to fix this problem?

If there is a burn mark, the unit is probably bad. However, I can't imagine a burn from a battery. Most burn marks I have seen are from lightning and possible related power surges.

Thank you for the reply. When you mentioned the unit is probably bad, were you referring to the thermostat or the HVAC ? If it is the thermostat (the model number is discontinued), can I replace it with a NEST thermostat?

Thermostat is likely the culprit. You need to go to the Nest website and check interchange. Simple test, with power off, remove the thermostat face. Move switch to AC. Connect the red and white wires. Turn power on. Does unit start?