Can the TH8320U work with heat-only systems?
Yes, the TH8320U can work with heat-only systems including two-wire systems, power to open and close zone valves (Series 20), normally open zone valves, warm air, hot water, high-efficiency furnaces, and gravity systems. It also supports heat-only with fan configurations.
From the official Honeywell T8320U Programmable Thermostat manual, page 1
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