I have a Taco SR503-4 Three Zone Switching Relay. The current thermostats use batteries and two wires, R & W. I don't see a Common to connect the C-wire. Where should I connect common for the ecobee3 thermostats?
The Taco SR503-4 Zone Switching Relay has a built-in 24V transformer that is only sufficient in rating to supply voltage to the Zone Control. Battery powered thermostats do not steal power from this control, therefore the 1 amp rating will be sufficient.
But with the power requirements of one or more ecobee3 thermostats this transformer will be insufficient in capacity. As a result another external transformer will be needed to power the thermostats and an isolation relay per zone to connect to the R & W terminals of each zone.
This wiring diagram illustrates how each zone would be wired to each R & W zone input of the zone control. The isolation relay must have a set of normally open contacts and a 24V control coil. The normally open contacts as illustrated will connect to the R & W or T and T connections of the boiler zone control.
Refer to this related article for further information: https://ecobee.zendesk.com/hc/en-us/articles/227874667-ecobee3-Heat-only-2-wire-boiler-furnace-installations