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Hello, click here please... the 20 to 30 volt rating is not for the battery, it is for the load on the contacts only. The battery is used just to store the programming.
Load on the contacts is 24 vac standard, 30 volts max.
the battery however is a standard AA or AAA duracel battery, it is not 30 volts.... Those are available almost everywhere,
You can pry the front of the thermostat from its wall mount sub base and see the batteries inside... just replace them.
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