My current furnace has Rh and W1 to the thermostat. I'd like to add a C for my new networked stat, but I'm not sure where to connect it.

From my research it sounds like I should be able to just connect a new wire to one of the sides of the transformer, but I feel like it might be more appropriate to connect to the Taco zone valve where the transformer, aquastat, and thermostat come together currently (terminal 1 is W1, terminal 2 is white from the transformer and white from the aquastat, terminal 3 is aquastat red, and Rh is joined to transformer red).

So, can I add a Common wire for the new stat by connecting directly to one side of the transformer? Or joining one of the terminals of the zone valve?

For reference, the aquastat is a Honeywell L8124 C 1102.

Thanks.

(I can post pictures of the valve, transformer, aquastat if it helps)