I have an oil furnace, controlled by a thermostat that only has 2 wires going to it, but I want to be able to turn on the fan only from the fan switch on the t-stat. There do not appear to be any further connections at the furnace to hook up to, and the fan switch at the furnace (which can be set to auto or on) is wired at 110 volts. So, I ran a new batch of thermostat wire to the thermostat, and I was told to purchase a 110V to 24V transformer (so the t-stat can handle the 24V signal) and a relay - which I went out and bought. I thought I had an idea of how to wire it all up (from the fan switch on the furnace at 110V, to the 2 terminals on the transformer, out of the 24v side of the transformer to two wires on the 5-wire relay (used black and brown if that means anything to you), out of the relay (both wires are yellow), to the thermostat (using the power and fan, or G, connections). But it's not working. What did I do wrong, or how should I make this work?

Thanks!

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. It may be wired to have the control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan.

You should not need the second transformer, return it if you can. Connect all the wires at the thermostat as you would expect red to R or RC, white to W, green to G. Back at the furnace, connect the red wire to the secondary terminal one of the original wires was connected, and the white wire to where the other wire was connected. Now remove the switch for the fan, and wire the contacts of the relay to where the switch was. Run a blue wire from the other contact of either the gas valve or the transformer to one terminal of the relay coil. Run the green wire from the thermostat to the other terminal of the coil. If the 5 wire relay has C,NCC, and NOC, use the C and NOC contacts for the wires that went to the switch. If there was a third wire at the switch for Auto, wire it to the NCC contact.

Post back if my answer doesn't seem to match your furnace and thermostat, giving me the details.

Well, some of that makes sense :) :confused: I guess...

There's a transformer on the furnace now, with 2 wires (white and red) which go to the thermostat for heat calls (red to the R contact on the t-stat, white to the W). Those are the only 2 wires connected to the thermostat, and obviously are also the only 2 secondary terminals on that transformer.

So, obviously I need a third wire (green) connected to the green (fan) terminal on the thermostat. But I don't know what to connect it to at the furnace end. Any other wires I need to worry about at the thermostat? I don't think so - I don't have air conditioning or anything else, just heat.

So, I have a fan switch on the furnace (110 volts), and another transformer (not yet hooked up) and a relay (also not hooked up). The new transformer, of course, has 2 contacts the 110v side, and 2 at the load (24v) side. The relay has 3 wires at one end (blue, brown and black - they're labelled C, NCC and NOC I think, I just don't rememeber which is which), and two wires at the other end (both are yellow).

From your explanation (I've drawn a diagram to help me understand), I hook up the C and NOC wires on the relay to the 110V contacts at the fan switch on the furnace (is that correct?). What do the yellow wires on the relay connect to (I was told that the blue wire on the relay does not get used)? What do I wire to the new transformer? From your explanation, one of the yellow wires on the relay goes to the green (fan) terminal on the thermostat... but I'm confused about where the new transformer fits in.

I hope what I've explained makes sense to you... thanks for your help so far!

Perhaps my first answer had too much expert speak in it. I clearly understood it, but it may have confused you. The control circuits for furnaces and other things commonly use 24 volt circuits, the voltage coming from a transformer wired into the furnace. One wire, often red goes from the transformer to the thermostat. Another wire, often white, goes back to a solenoid operated valve that controls the gas. It also operates a relay to start the fan. However, not directly, but through limit switches or timers to start the fan a little after the gas comes on and keep it going a little after the gas shuts off. Both the fan and gas valve must have a third wire, often blue, from their other terminal going back to the other terminal of the transformer to complete the circuit. When the switch in the thermostat closes calling for heat, power flows from one terminal of the transformer to the thermostat, and back to the gas valve, through its coil and back to the other terminal of the transformer. It also flows from the white wire to the switches and relays controlling the fan, through the fan relay coil, and back to the transformer.

Your furnace has a switch that bypasses the limit and timer switches to feed 115 volt directly to the fan. You can replace that switch with a relay and turn on the fan from the fan switch built into the thermostat. All you need to do is replace the switch with a relay with one side of its coil connected to the G terminal of the thermostat, and the other side to the other terminal of the transformer, the 2 yellow wires, either one, either place. It the switch has 3 terminals, SPDT, you need a 5 wire relay. The hot 115 V wire goes to the common terminal, likely the black wire. The wire directly to the fan goes on the NOC terminal, and the wire that goes to the heat controls goes on the NCC terminal.

A second transformer is not only unneeded, it make things harder to wire up. Thermostats are set up to switch power for one transformer to heat, fan, and air conditioning. To use a second transformer, you must carefully wire them up so there are in phase. Much easier to just use one.

Post back if you still can't figure it out.

Thanks again for the reply; I just read it now but I'd already sat down and carefully thought about how to wire it, and my results matches yours :) .

However, after trying it out, I've discovered that my existing two thermostat wires (red and white) do not simply connect to the transformer; they connect to two terminals at the furnace, which must be wired to some other relay. Obviously there is 24v between those two terminals (I tested to make sure). When I wire one yellow wire on the relay to the fan, and the other to the red (24v power) contact, then jumper the red and green, nothing happens. If I jumper green and white, I can hear the relay switching. So I moved the yellow relay wire from the red to the white, and the opposite happens - nothing between white and green, but a jumper from green to red results in the relay switching. So it would appear that the 24v power is always at the terminal that is NOT connected to the relay :eek:

So, obviously there is more to those contacts than just a transformer. I'll have to 'dig around' some of the internal wiring to find out where the furnace relay is, and where the transformer is, so I can just 'tap into' the 24v power side of the transformer and avoid going through any other relays etc. which may be conflicting with my new relay.

I'll get this figured out yet! :confused: :D