Continuation from Can I use any of my existing oil control terminals (TTFFAA) for a C wire?


Wanted to finally say this is SOLVED with help from @JPhi1618 (see conversation below). If you have a 2 wire system you might likely need a relay. I bought this (https://www.amazon.com/Venstar-ACC0436-2-Wire-24VAC-Thermostats/dp/B00755BZZC/ref=asc_df_B00755BZZC/?tag=hyprod-20&linkCode=df0&hvadid=167116234959&hvpos=&hvnetw=g&hvrand=7451184044459738647&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9001854&hvtargid=pla-313874654619&psc=1#customerReviews) AC Transformer and relay. Using the diagrams provided by the Amazon reviewer "GK" and JPhi1618's diagrams below I was able to successfully install a Nest Thermostat on a two wire heat only system. I had a initial scare where it did not function but I interchanged the two AC connections and ensured all the wire nut connections were secure and it functioned quickly after. Thanks to all for everyone's help and I hope this post can help someone in the future.


I have an older two wire heating system (R, W). I have 5 wires going from my furnace to my thermostat (only the two are used). After learning that my oil control does not have a C terminal I was recommended to buy a transformer to be able to create a C wire for my WiFi thermostat. I bought a plug in transformer online and I thought I could plug in the transformer, attach one lead it to the unused blue wire near my furnace (to create C) and attach the other lead to another unused wire, yellow. I read that from here you plug in the blue wire into C and the yellow into Rc. However, my Nest backplate does not have a Rc, only an R. Hours of troubleshooting have gone by and I'm not having any luck.

Am I going about this the right way (in that I can plug in the transformer, connect the leads to existing wires which go to the thermostat?)

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  • I take it you have a Nest E? Dec 2, 2021 at 3:14
  • No just the standard Nest with the mirror display
    – Teuszie
    Dec 2, 2021 at 3:52
  • I didn't know there was a Nest thermostat without an Rc terminal. So you have a furnace with no C terminal and a thermostat with no Rc terminal. That's a royal pain. Nest documentation suggests you TRY running without a C wire. If the thermostat can leech enough power off the R-W connection to keep the battery charged without the furnace detecting it is doing so, you'll be ok. The only way to know is to try. Just install the thing without a C wire and let it run for a while. If the battery doesn't die and the home temp is managed correctly, you're good.
    – jay613
    Dec 2, 2021 at 20:33
  • 1
    If that doesn't work out you could install a relay or just exchange the thermostat for one of the other Nest ones that have an Rc terminal.
    – jay613
    Dec 2, 2021 at 20:34

2 Answers 2


To start, read this excellent answer about how a thermostat works

This is a good diagram to focus on:

smart thermostat diagram

This is how a thermostat works for a typical HVAC system. The 24v transformer supplies power to the thermostat on the R terminal and the thermostat connects the R wire to W for heat, Y for cool and G for the fan. Whats important is that if you have voltage on the R terminal, that voltage is connected to W. The smart thermostat also uses a C wire so it can power itself (R to C is 24v).

The issue that you (and everyone else with a boiler or oil heat system) has is that they need the thermostat to be a "dry contact" that doesn't use or supply any voltage on the two wires. In fact, your particular controller explicitly states that the thermostat should not supply voltage to the T wires under any circumstance.

thermostat warning

That means that a normal smart thermostat will not work because it needs power on the R terminal to function and it will pass that voltage on to the W wire when it calls for heat.

So, what can we do about that? Well, there happens to be an easy way to turn a voltage contact into a dry contact, and its actually in the diagram above. See how the C and W wires go back to the furnace and power a relay? That's what you need to happen. You can power the coil side of a 24v relay with your smart thermostat and use the normally off contacts on the relay to switch the T terminal wires on your controller.

There are "fan controllers" that are a combination of a 24v transformer and a relay that can be used for this, or you could easily use a separate 24v transformer and relay.

The Honeywell R8239A1052 Fan Center is one such control, but there are a few different companies that make them. Since they are sold as a fan control, they are designed for the thermostat wires (R and W) to be connected to R and G.

If wiring your own relay, here is a sample diagram.

Possible Relay set up

Click to embiggen

  • Hi JPhi1618 I appreciate the explanation and I think I'm understanding more about what needs to be done. I edited my question and included a few diagrams of what I currently what, what I thought would work, and what I understand I need from your answer. Can you confirm I need a relay for the smart thermostat to work given that I need to keep the TT contacts on the furnace dry? I've seen people post online that the 2nd option works for them but obviously everyone's system varies.
    – Teuszie
    Dec 2, 2021 at 17:48
  • 1
    The second option would probably work, sure, but you may also backfeed 24v into one of the T terminals. Adding a relay will totally isolate the thermostat and the controller so you don't have to worry about that. The wire connections in the third diagram are not correct, my next comment will address that.
    – JPhi1618
    Dec 2, 2021 at 18:02
  • A typical relay will have 4 wires or terminals. Two terminals are for the coil, or the control side of the relay, and the other two terminals will be for the switch side (and in this case we need common and "normally open" or N.O.). The two T terminals from your controller will get connected to the Common and NO terminals of the relay. The C wire from the 24v transformer will connect to the thermostat and one of the relay coil terminals and the W wire from the thermostat goes to the other coil terminal.
    – JPhi1618
    Dec 2, 2021 at 18:05
  • @Teuszie, I added a diagram for relay wiring
    – JPhi1618
    Dec 2, 2021 at 18:22
  • Note that the transformer, relay, and controller can all be in the same place with only the three needed wires going to the thermostat (R, W, and C).
    – JPhi1618
    Dec 2, 2021 at 18:31

You may be making it harder than it is.

There must be a 24 volt source inside that furnace somewhere! And it will be the same transformer that is feeding the R wire, so you know it will be phased correctly. The problem with sticking random transformers on furnaces is if you get them out of phase, you'll have 48 volts between yours and the other transformer!

Look around inside the furnace for an existing transformer. Every furnace transformer has 120V power going to it on 2 terminals, but it also has "R" and "C" coming from it on the other 2 terminals.

You can identify the R wire in your furnace. Follow it around and see if it leads to the transformer. If it does, note the other wire, C, and follow it back to the furnace control board or whatever.

That cheapie transformer appears not to be UL Listed (I could be wrong), so it is not safe or legal to use anyway, so back to Amazon it goes. Most electrical gear on Amazon is dangerous trash. I didn't make it that way.

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