I've been running 2 Nest thermostats (1 upstairs, 1 downstairs) for years that didn't require a common wire. I'm replacing them now with Ecobee3 Lite thermostats. Both thermostats control heat from the furnace and central A/C. Note, the A/C was installed in the house years after it was built. Air handlers are in the attic. Furnace is in the basement. Each thermostat has wires for Y1, G, Rc, W1 and RH. However, my new Ecobee3's require a C wire just to power on. I snaked wire for this purpose from each thermostat, through the walls, down to the furnace. However, I cannot figure out where to connect the C wire in the furnace. I've tried a number of spots with no luck. I'm beginning to wonder if the C wire should actually run to the attic and connect to the air handlers somewhere. See pictures attached. Any idea where the C wire should connect? On the furnace somewhere or could it be near the air handlers in the attic?
You need to be tapping C from your air handler
The Ecobee 3 is like many other smart thermostats in that in a two-transformer system, it assumes that the main 24VAC transformer in the system is at the air handler or otherwise associated with the cooling system. As a result, you need to connect the C wire to the same device that provides the Rc wire to your Ecobee. This is shown in the diagram for a boiler + heat pump system found on page 31 of your Ecobee's installation manual.
The photos of the diagrams are not big enough to zoom into, but typically the C wire is blue. The C wire provides 24V so check your diagram for a terminal labeled as that amount of voltage. Confirm with a multi-meter by touching it to the wire terminal and the ground connection. If I squint at the diagram, it should be the blue with the white stripe wire that's attached on the 8 terminal block (4 Left and 4 Right screws) 3rd down on the left, the terminal is the 2nd one on the bottom below your gauge. A photo of the wiring at your thermostat would also be helpful as it may already be connected but unused.
If you had an mercury thermostat it would have a wire running to it and a return it simply completes a circuit. The C wire is to supply 24 volts so your electronics in the new thermostat are able to run. My transformer on my boiler has a lug marked c. I do not see that there. The instruction for your thermostat should list a 800 number to call, take clear pictures of the wiring diagrams and be ready to send it to the help desk. If you do not have a c wire you can buy an kit to hook one up.