HVAC Missing 'C' wire
Posted: Fri Nov 09, 2018 5:55 pm
I was recently faced with installing a programmable HVAC thermostat, but the new thermostat need the 'C' (common) wire and that was not in the existing cable. I searched and found a gadget called Add-A-Wire that adds 1 extra wire to either a 4 or 5 wire existing thermostat cable. It's about $25 and looks simple to install.
Not having one in hand to look at, I found the Add-A-Wire installation instructions. There is a 'Diode' module that is installed at the thermostat and another at the HVAC system cabinet. Ah Ha! The Heat and Cool thermostat connections go to the diode module but only one wire leaves the module.
Voila!. Two control signals, all on one wire. The theory is that the 24 vac for heat is rectified to be+ (or -) and 24 vac for cool is rectified to the opposite polarity and that is sent to a module in the HVAC system. Since heat and cool are mutually exclusive, both will never be on at he same time.
Now, at the system end, two more diodes steer the positive and negative voltage inputs, maybe directly to the system control relays, or they may use the contacts of internal relays to operate the system's control relays.
Either way, the bottom line is that one wire is now spare in the existing cable that can be used as the missing 'C' wire (or whatever you need it to be)
Even though I've used that same concept years ago, and I never mad a dime from it, I must say, ... that's pretty smart!
Not having one in hand to look at, I found the Add-A-Wire installation instructions. There is a 'Diode' module that is installed at the thermostat and another at the HVAC system cabinet. Ah Ha! The Heat and Cool thermostat connections go to the diode module but only one wire leaves the module.
Voila!. Two control signals, all on one wire. The theory is that the 24 vac for heat is rectified to be+ (or -) and 24 vac for cool is rectified to the opposite polarity and that is sent to a module in the HVAC system. Since heat and cool are mutually exclusive, both will never be on at he same time.
Now, at the system end, two more diodes steer the positive and negative voltage inputs, maybe directly to the system control relays, or they may use the contacts of internal relays to operate the system's control relays.
Either way, the bottom line is that one wire is now spare in the existing cable that can be used as the missing 'C' wire (or whatever you need it to be)
Even though I've used that same concept years ago, and I never mad a dime from it, I must say, ... that's pretty smart!