What should I do if the heating or cooling system does not respond?

First, press the System button to verify the correct mode is selected, and ensure your set temperature is higher than room temperature for heat or lower for cool. Also check that the circuit breaker is not tripped, the power switch at the system is on, and the furnace door is closed securely. After checking these items, wait up to 5 minutes for the system to respond. (Page 12)

From the official Honeywell TH5110D Home Programmable Thermostat manual, page 12

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