Start with the exact model
Follow the owner's instructions for battery type and replacement. Record how long a fresh set lasts, any low-power code, and whether the trouble began with installation or appeared after years of normal use. Note whether the screen, Wi-Fi, or heating control fails first.
For certain Nest models, Google identifies reduced battery life as a possible sign of inconsistent system power and describes a C wire or compatible accessory as a potential correction. It does not say that every short-lived battery is caused by the same fault. Nest power symptoms and options.
A thermostat that needs regular battery replacement by design is different from one whose battery is supposed to stay charged through its installed power supply. Do not treat those two cases as a brand-wide failure.
Ask for a power check with the system running
Give the installer the model and the symptom record. Ask whether the equipment and thermostat are compatible, whether the approved power arrangement was installed, and whether the supply remains suitable during heating and cooling operation.
A wire present at C does not establish a correct or reliable circuit. Honeywell Home's power-up guidance makes the equipment-side connection and acceptable voltage part of its product-specific checks. That is work for a qualified person, not a reason to probe a live furnace yourself. Honeywell Home power requirements.
Choose the smallest supported correction
If the finding is an inadequate power arrangement, compare a supported accessory or cable correction with a compatible thermostat choice. If power and setup are correct, manufacturer support may need to evaluate the thermostat itself. Ask what evidence supports replacing it and what the applicable warranty covers.
After work, confirm stable power and the functions you use, not just a brighter display. Your next step is to request a model-specific power diagnosis with the battery history and exact error message. Stop relying on repeated battery changes as the permanent fix.