My existing thermostat is heat only (gas boiler) and has a 2-wire connection. To install a C-Wire ecobee support said to purchase a Fast-Stat Common Maker for each zone (makes sense), but
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FAST-STAT Common Maker Guide
FSCM00 - Fast-Stat FSCM00 - Common Maker Thermostat Wire Extender (Adds a Common C Connection)
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Fast-Stat Common Maker Thermostat Wire Extender (Adds a Common C Connection) FSCM00 - The Home Depot
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Fast-Stat 1000 and Common Maker - Installation Guide – Simple
Fast-Stat Common Maker Thermostat Wire Extender (Adds a Common C Connection) FSCM00 - The Home Depot