For traditional battery-powered Honeywell thermostats (such as older non-smart digital or programmable models), you can tell new batteries are required when a low-battery warning icon flashes on the LCD screen, the display dims or goes completely blank, or the thermostat stops triggering heating and cooling cycles. Smart thermostats like the T9 and T10 do not use batteries and rely entirely on hardwired 24V C-wire power, eliminating the need for battery replacements.
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