HVAC Missing 'C' wire

[Lenp]

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!

[dacflyer]

was it not possible to replace the cable with a more conductor cable ?
usually you could tie the new to the old, and just pull it thru. i rarely ever seen them tacked or nailed down..

[Externet]

Hi Len.
Been dealing with HVAC hiccups in three rental properties in the last two months; so I have some fresh memories...
The "C" (common) wire is the grounded secondary terminal of the 120-to-24 VAC transformer. If you have a way to run a wire from any furnace chassis -perhaps metal duct/pipe- to the thermostat, it will avoid the added gadget.
I believe the extra "C" wire to the thermostat allows to not deplete its battery, getting operation energy from the 24VAC transformer.
Hope it helps somehow...

[Lenp]

Many thermostst cables are stapled inside finished walls and ceilings so it is not always a simple task to pull.one out and another in. The cost of this device is quickly exceeded by wire and the labor expense. If you DYI the circuit, the cost Is much less. Nope, I make nothing from the sale of these

Taking the concept a bit farther, away from the HVAC application, you could get 3 different control functions on one wire. Using +DC and -DC you can operate 2 differernt relays. Add AC and you can turn on both relays. Interconnect the contacts and relay1 is signal 1, relay 2 is signal 2 and series contacts of #1 & #2 is signal 3. If you hard code that into binary then the sky is the limit.

Nope, not high tech, but it illustrates what can still be done with 'obsolete' technology