Adding a C Wire using a 24V AC Transformer to a 2 Wire Heating System for Nest

Question

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

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?)

Answer 1

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

This is a good diagram to focus on:

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.

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.

Click to embiggen

Answer 2

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.