A HVAC tech just replaced our Compressor, Coil and Furnace (brand new Carrier units). We are seeing the same problem that we experienced for three years with our old system. While the furnace is running, for reasons we don't understand, the thermostat suddenly blacks out. The furnace continues to run but we can not make any adjustments because the thermostat display is blank. After a short period the Thermostat again begins to display and access to all functions returns.

This has happened with three different thermostats and two different furnaces. What can the problem be?

If the furnace is overheating, the main limit control will interrupt low voltage control power until it cools and resets. Look for a status LED behind a clear viewer disc on the front panel or blower access panel. Clues are there. If blinking while t-stat display is blank, observe and note pattern. Look for service legend inside access panels or on circuit board for description of status being displayed.