I would like to add a "C" / Common wire hookup to my hvac system and get a fancy new thermostat. I've read How can I add a "C" wire to my thermostat? and still have some questions.

First of all, I have an Apollo Hydroheat system. Unusual system I gather. My house has two zones (upstairs and downstairs), both thermostats are connected the same way with four wires:

R - Red, G - Green, Y - Yellow, W - White

Luckily for me, the cable connecting the hvac to the thermostat has unused wires (orange, brown, blue, etc), so I shouldn't need to actually run any new cables.

I found an installation manual for an Apollo Hydronic air handler that has a wiring diagram. The PDF of the Apollo installation manual is here and the wiring diagram is on page 8.

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Any thoughts as to whether or not it would be easy to add a live "C" wire? Do I need an external transformer? Can I literally just get a transformer, plug it into a wall outlet, and connect the "C" wire to that?


1 Answer 1


In the wiring diagram in the PDF you linked, the BR (brown) wire connected to the secondary side of the transformer would be your "C" wire. It connects inside the cabinet to the pump and fan relays, and comes out of the cabinet to the A/C condensor control (along with the "Y" wire from the thermostat).

This post on RadioThermostat's forum discusses a similar issues (no affiliation, except I have one of their thermostats).

  • Awesome, thank you. I will see if I can actually understand the wiring when I get to the transformer! Appreciate you taking the time to look at the PDF diagram.
    – Raolin
    Commented Jul 2, 2012 at 3:50
  • I just wanted to say that TomG's advice was perfect. I easily hooked up an unused blue wire to the BR (brown) wire coming from the transformer and my new "C" wire works perfectly. Thanks so much for the help, I would not have ever been able to figure that out.
    – Raolin
    Commented Aug 17, 2012 at 5:28

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