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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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  • The A contacts are not what you want. There seems to be a 24V source there because it uses a 24V thermostat. But none of the T/F/A terminals is the 24V Common. The Common will be inside the metal box where those terminals are mounted. It may not be accessible, definitely not safely and perhaps not at all. Can you remove the cover, and provide clear photos of the inside of that box? – jay613 Nov 29 '21 at 21:03
  • Hi @jay613 thanks for the reply. I edited my post to include a photo of the wires coming out of the oil control box. The box itself is rather difficult to open. – Teuszie Nov 29 '21 at 21:48
  • A dry contact is a switch that doesn't supply any voltage. It just connects two wires like a light switch. For control systems like this, there is another type of contact that will supply voltage from an internal transformer. The 24vac rating on the dry contact just means that its been tested to safely handle that power rating or anything lower. – JPhi1618 Dec 02 '21 at 15:32

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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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  • As a third option can I buy a new cad cell relay / oil ignition control with a common terminal? – Teuszie Nov 29 '21 at 22:35
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    You'd have to contact Carlin to ask them that. The obvious replacements seem to be identical, no extra terminals. It would be more expensive and more laborious. to swap the whole controller. If you're going to install gear and rewire at the burner, why not just install a transformer for the stat? You can tap off the mains power already available there, you can do it safely, it'll cost about $30 and take an hour. To replace the controller with a dissimilar one is a lot more involved. And you ought then to hire an expert to run through a set of post-install safety checks. – jay613 Nov 29 '21 at 23:32