Suburban Furnace - Thermostat Wiring Question

[JohnD]

Yesterday, while rerouting and untangling wires to make space for my solar/lithium conversion, I disconnected a green/black wire from the blue wire going into the furnace and untangled it from a mess of other wires. When I went to reconnect it, I realized that there is a blue wire and a blue/white wire going to the furnace, and I hadn't been very careful to examine that before disconnecting. I came home and looked up the generic wiring for Suburban furnaces, and found that the thermostat goes to the blue and blue/white wires. The odd thing is that the thermostat wires were both connected to the same furnace input. The thermostat wires were stripped, twisted together, and connected to one of the blue blue/white wires, and the other blue was not connected to anything. Our furnace has always worked, so I'm wondering if I'm missing something. I'm pretty certain that the thermostat wire should be separated and one should go to each of the blue wires. What are others thoughts?

In the picture, you can see that the thermostat wire casing has never been separated, so I'm not just suffering from a "senior moment." Residue on the blue/white wire is just tape.

[howson]

Based on the GD Parts Lookup site I assume the furnace is an SFV-35Q. I found the SFV-35Q installation manual here: https://www.google.com/url?sa=t&rct=...lcVdMXzTO9BGRN

The yellow and red wires, as shown in your picture, make sense based on the furnace's schematic. The yellow is -12v (or "ground") so it's tied to that solid white wire (the supply side) which on a GD trailer is normally solid white.

The Red wire is +12vDC, so the red/white is the supply side coming from the trailer.

Why there are two wires marked "Blue" so close together on the schematic escapes me...and I see nothing with an external connection that helps to understand the purpose for the black and green wires. (All of the black and green wires shown look like internal connections on the board.)





What thermostat is in the trailer? I found the thermostat on the parts lookup site but the wiring shown in your picture doesn't cross to the wires shown.

Sorry...not much help.

Howard

[colan]

The thermostat wires from the furnace are basically an out and in wire. One sends 12volts out, the other when seeing 12volts tells the furnace to run.

When bench testing a furnace connecting the two blue wires together is a substitute for the thermostat. Connect the wires the furnace thinks the thermostat is calling for heat.

The confusing part is when there is no connection to one of the wires, the 12volt out, typically labeled 12+. What the manufacturer is doing is trying to save running two wires from the furnace to the control box in the roof mounted A/C. Instead since the 12+ wire is just a source of 12V they pick up the voltage from the power source to the A/C control board.

With the duplex wire it looks like someone lost site of the concept, run ONE wire.

[howson]

The thermostat wires from the furnace are basically an out and in wire. One sends 12volts out, the other when seeing 12volts tells the furnace to run.

When bench testing a furnace connecting the two blue wires together is a substitute for the thermostat. Connect the wires the furnace thinks the thermostat is calling for heat.

The confusing part is when there is no connection to one of the wires, the 12volt out, typically labeled 12+. What the manufacturer is doing is trying to save running two wires from the furnace to the control box in the roof mounted A/C. Instead since the 12+ wire is just a source of 12V they pick up the voltage from the power source to the A/C control board.

With the duplex wire it looks like someone lost site of the concept, run ONE wire.

So schematic I've cobbled together below is correct for the blue wires?

[JohnD]

The thermostat wires from the furnace are basically an out and in wire. One sends 12volts out, the other when seeing 12volts tells the furnace to run.

When bench testing a furnace connecting the two blue wires together is a substitute for the thermostat. Connect the wires the furnace thinks the thermostat is calling for heat.

The confusing part is when there is no connection to one of the wires, the 12volt out, typically labeled 12+. What the manufacturer is doing is trying to save running two wires from the furnace to the control box in the roof mounted A/C. Instead since the 12+ wire is just a source of 12V they pick up the voltage from the power source to the A/C control board.

With the duplex wire it looks like someone lost site of the concept, run ONE wire.

That kind of makes sense. I was wondering how the duplex wires wouldn't short, but if its just connected to the +12, that would explain it. Next time I get to the RV, I'll activate the thermostat and get a voltage reading.