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I, like many other, similar users on this site, am trying to install a WiFi-enabled thermostat. Hopefully there isn't a prize for oldest system.

My current system setup is:

  • Oil furnace (Heat only / No AC) with a Carlin 50200-02 control
  • An 18/5 cable runs from the thermostat to the furnace but only W and R are used on the thermostat
  • The common (C) wire in the 18/5 is not connected to the thermostat and it's not connected to the furnace

As a C wire requires 24V continuous power I am curious whether any of the existing terminals (TTFFAA) on my oil control can be used as common? For example, I don't know what dry contacts are for alarms -- can either of these be used as common?

Happy to supplement more photos as necessary.

Attached images: enter image description here enter image description here enter image description here (via https://tggcdn.azureedge.net/tggakeneoassets/productinstallationsheets/4/4/9/9/44994cf84565eb41e966561d182813018d7d626a_CARLININST00001.pdf) enter image description here

Teuszie
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Based on photos, comments, and documentation I think you do not have access to "C" for this controller. There probably is a C contact but it's inside the control box that is not designed to be opened. None of the available terminals outside the box is the 24V common.

If you have air conditioning the smart thermostat could be powered by that all year round through the Rc contact. Otherwise, to use a smart stat you'll have to buy a separate transformer or 24V wall wart to power it. If you have two spare wires in your thermostat cable you could place the transformer near the burner. Otherwise you could power it from an outlet near the thermostat. Remove the R/Rc jumper and connect the new supply to Rc and C in the stat, just to power it.

Edit: Adding a thought about what might be inside the box. It's important to note that if there is 24V circuitry inside this control box, it was not rated or designed to deliver power to anything. It might be just powerful enough to ensure reliable detection of an open or closed thermostat, and no more. In systems with an exposed 24V transformer, used as a power source for thermostats, zone valves, etc there is also a max current rating that you can compare to the demands of those devices. In yours, there isn't one and there is a chance, perhaps small, that a smart thermostat would exceed the capacity of whatever is inside the box. For that reason I doubt any replacement controller from the manufacturer would serve the purpose you want. Rather than make a replacement controller with an added 24V power source, why not just install a 24V power source?

jay613
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