I powered one of my TBZ48 units with a 150Ma 27VAC transformer.
I do not have a power wire coming from my boiler to power the Tstat and did not want to rely on a battery. The caveat is that you not have a battery installed in the device when using external power. The hardest part was finding a low amperage power supply. You can use a readily available 300Ma supply but the extra amperage is unneccessary and wastes energy.
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